OUTLINES

OF

DAIRY BACTERIOLOGY

A CONCISE MANUAL FOR THE USE OF STUDENTS IN DAIRYING

BY

H. L. RUSSELL

DEAN OF THE COLLEGE OF AGRICULTURE, UNIVERSITY OF WISCONSIN

EIGHTH EDITIONTHOROUGHLY REVISED

MADISON, WISCONSINH. L. RUSSELL1907

Copyrighted 1905BYH. L. RUSSELL

STATE JOURNAL PRINTING COMPANY, Printers And Stereotypers, Madison, Wis. 

Transcriber's note:

For Text: A word surrounded by a cedilla such as ~this~ signifies thatthe word is bolded in the text. A word surrounded by underscores like_this_ signifies the word is italics in the text. For numbers andequations, underscores before bracketed numbers in equations denote asubscript. 

Minor typos have been corrected. 

PREFACE. 

Knowledge in dairying, like all other technical industries, has grownmainly out of experience. Many facts have been learned by observation, but the _why_ of each is frequently shrouded in mystery. 

Modern dairying is attempting to build its more accurate knowledge upona broader and surer foundation, and in doing this is seeking toascertain the cause of well-established processes. In this, bacteriologyis playing an important rôle. Indeed, it may be safely predicted thatfuture progress in dairying will, to a large extent, depend uponbacteriological research. As Fleischmann, the eminent German dairyscientist, says: "The gradual abolition of uncertainty surrounding dairymanufacture is the present important duty which lies before us, and itssolution can only be effected by bacteriology. "

It is therefore natural that the subject of Dairy Bacteriology has cometo occupy an important place in the curriculum of almost every DairySchool. An exposition of its principles is now recognized as an integralpart of dairy science, for modern dairy practice is rapidly adopting themethods that have been developed as the result of bacteriological study. The rapid development of the subject has necessitated a frequentrevision of this work, and it is gratifying to the writer that theattempt which has been made to keep these Outlines abreast ofbacteriological advance has been appreciated by students of dairying. 

While the text is prepared more especially for the practical dairyoperator who wishes to understand the principles and reasons underlyinghis art, numerous references to original investigations have been addedto aid the dairy investigator who wishes to work up the subject morethoroughly. 

My acknowledgments are due to the following for the loan ofillustrations: Wisconsin Agricultural Experiment Station; CreameryPackage Mfg. Co. , Chicago, Ill. ; and A. H. Reid, Philadelphia, Pa. 

 H. L. Russell. University of Wisconsin. 

CONTENTS. 

CHAPTER I. Structure of the bacteria and conditions governingtheir development and distribution 1

CHAPTER II. Methods of studying bacteria 13

CHAPTER III. Contamination of milk 19

CHAPTER IV. Fermentations in milk and their treatment 62

CHAPTER V. Relation of disease-bacteria to milk 82

 Diseases transmissible from animal to man through diseased milk 84

 Diseases transmissible to man through infection of milk after withdrawal 94

CHAPTER VI. Preservation of milk for commercial purposes 102

CHAPTER VII. Bacteria and butter making 134

 Bacterial defects in butter 156

CHAPTER VIII. Bacteria in cheese 160

 Influence Of bacteria in normal cheese processes 160

 Influence of bacteria in abnormal cheese processes 182

CHAPTER I. 

STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT ANDDISTRIBUTION. 

Before one can gain any intelligent conception of the manner in whichbacteria affect dairying, it is first necessary to know something of thelife history of these organisms in general, how they live, move andreact toward their environment. 

~Nature of Bacteria. ~ Toadstools, smuts, rusts and mildews are known toeven the casual observer, because they are of evident size. Theirplant-like nature can be more readily understood from their generalstructure and habits of life. The bacteria, however, are so small, thatunder ordinary conditions, they only become evident to our unaidedsenses by the by-products of their activity. 

When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discoveredthese tiny, rapidly-moving organisms he thought they were animals. Indeed, under a microscope, many of them bear a close resemblance tothose minute worms found in vinegar that are known as "vinegar-eels. "The idea that they belonged to the animal kingdom continued to holdground until after the middle of the nineteenth century; but with theimprovement in microscopes, a more thorough study of these tinystructures was made possible, and their vegetable nature demonstrated. The bacteria as a class are separated from the fungi mainly by theirmethod of growth; from the lower algae by the absence of chlorophyll, the green coloring matter of vegetable organisms. 

~Structure of bacteria. ~ So far as structure is concerned the bacteriastand on the lowest plane of vegetable life. The single individual iscomposed of but a single cell, the structure of which does not differessentially from that of many of the higher types of plant life. It iscomposed of a protoplasmic body which is surrounded by a thin membranethat separates it from neighboring cells that are alike in form andsize. 

~Form and size. ~ When a plant is composed of a single cell but littledifference in form is to be expected. While there are intermediatestages that grade insensibly into each other, the bacteria may begrouped into three main types, so far as form is concerned. These arespherical, elongated, and spiral, and to these different types are giventhe names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural, _cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and acorkscrew serve as convenient models to illustrate these differentforms. 

[Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_, represent different types as to form: _a_, coccus, _b_, bacillus, _c_, spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus orcluster coccus; _f_ and _g_, different forms of bacilli, _g_ showsinternal endospores within cell; _h_ and _i_, bacilli with motile organs(cilia). ]

In size, the bacteria are the smallest organisms that are known toexist. Relatively there is considerable difference in size between thedifferent species, yet in absolute amount this is so slight as torequire the highest powers of the microscope to detect it. As an averagediameter, one thirty-thousandth of an inch may be taken. It is difficultto comprehend such minute measurements, but if a hundred individualgerms could be placed side by side, their total thickness would notequal that of a single sheet of paper upon which this page is printed. 

~Manner of Growth. ~ As the cell increases in size as a result of growth, it elongates in one direction, and finally a new cell wall is formed, dividing the so-called mother-cell into two, equal-sized daughter-cells. This process of cell division, known as _fission_, is continued untilgrowth ceases and is especially characteristic of bacteria. 

~Cell Arrangement. ~ If fission goes on in the same plane continually, itresults in the formation of a cell-row. A coccus forming such a chain ofcells is called _strepto-coccus_ (chain-coccus). If only two cellscohere, it is called a _diplo-coccus_ (twin-coccus). If the second celldivision plane is formed at right angles to the first, a _cell surface_or _tetrad_ is formed. If growth takes place in three dimensions ofspace, a _cell mass_ or _sarcina_ is produced. Frequently, these cellaggregates cohere so tenaciously that this arrangement is of value indistinguishing different species. 

~Spores. ~ Some bacteria possess the property of forming _spores_ withinthe mother cell (called _endospores_, fig. 1g) that are analogous infunction to the seeds of higher plants and spores of fungi. By means ofthese structures which are endowed with greater powers of resistancethan the vegetating cell, the organism is able to protect itself fromthe effect of an unfavorable environment. Many of the bacilli formendospores but the cocci do not. It is these spore forms that make itso difficult to thoroughly sterilize milk. 

~Movement. ~ Many bacteria are unable to move from place to place. Theyhave, however, a vibrating movement known as the _Brownian_ motion thatis purely physical. Many other kinds are endowed with powers oflocomotion. Motion is produced by means of fine thread-like processes ofprotoplasm known as _cilia_ (sing. _cilium_) that are developed on theouter surface of the cell. By means of the rapid vibration of theseorgans, the cell is propelled through the medium. Nearly all cocci areimmotile, while the bacilli may or may not be. These cilia are sodelicate that it requires special treatment to demonstrate theirpresence. 

~Classification. ~ In classifying or arranging the different members of anygroup of living objects, certain similarities and dissimilarities mustbe considered. These are usually those that pertain to the structure andform, as such are regarded as most constant. With the bacteria thesedifferences are so slight that they alone do not suffice to distinguishdistinctly one species from another. As far as these characters can beused, they are taken, but in addition, many characteristics of aphysiological nature are added. The way that the organism grows indifferent kinds of cultures, the by-products produced in differentmedia, and effect on the animal body when injected into the same arealso used as data in distinguishing one species from another. 

~Conditions favoring bacterial growth. ~ The bacteria, in common with allother living organisms are affected by external conditions, eitherfavorably or unfavorably. Certain conditions must prevail beforedevelopment can occur. Thus, the organism must be supplied with anadequate and suitable food supply and with moisture. The temperaturemust also range between certain limits, and finally, the oxygenrequirements of the organism must be considered. 

~Food supply. ~ Most bacteria are capable of living on dead, inert, organicmatter, such as meats, milk and vegetable material, in which case, theyare known as _saprophytes_. In contradistinction to this class is asmaller group known as _parasites_, which derive their nourishment fromthe living tissues of animals or plants. The first group comprise by farthe larger number of known organisms which are concerned for the mostpart in the decomposition of organic matter. The parasitic groupincludes those which are the cause of various communicable diseases. Between these two groups there is no sharp line of division, and in somecases, certain species possess the faculty of growing either asparasites or saprophytes, in which case they are known as _facultative_parasites or saprophytes. 

The great majority of bacteria of interest in dairying belong to thesaprophytic class; only those species capable of infecting milk throughthe development of disease in the animal are parasites in the strictsense of the term. Most disease-producing species, as diphtheria ortyphoid fever, while parasitic in man lead a saprophytic method of lifeso far as their relation to milk is concerned. 

Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen, together with a limited amount of mineral matter. The nitrogen andcarbon are most available in the form of organic compounds, such asalbuminous material. Carbon in the form of carbohydrates, as sugar orstarch, is most readily attacked by bacteria. 

Inasmuch as the bacteria are plant-cells, they must imbibe their foodfrom material in solution. They are capable of living on solidsubstances, but in such cases, the food elements must be renderedsoluble, before they can be appropriated. If nutritive liquids are toohighly concentrated, as in the case of syrups and condensed milk, bacteria cannot grow therein, although all the necessary ingredients maybe present. Generally, bacteria prefer a neutral or slightly alkalinemedium, rather than one of acid reaction; but there are numerousexceptions to this general rule, especially among the bacteria found inmilk. 

~Temperature. ~ Growth of bacteria can only occur within certaintemperature limits, the extremes of which are designated as the_minimum_ and _maximum_. Below and above these respective limits, lifemay be retained in the cell for a time, but actual cell-multiplicationis stopped. Somewhere between these two cardinal temperature points, andgenerally nearer the maximum limit is the most favorable temperature forgrowth, known as the _optimum_. The temperature zone of most dairybacteria in which growth occurs ranges from 40°-45° F. To somewhat aboveblood-heat, 105°-110° F. , the optimum being from 80°-95° F. Manyparasitic species, because of their adaptation to the bodies ofwarm-blooded animals, generally have a narrower range, and a higheroptimum, usually approximating the blood heat (98°-99° F). The broadergrowth limits of bacteria in comparison with other kinds of life explainwhy these organisms are so widely distributed in nature. 

~Air supply. ~ Most bacteria require as do the green plants and animallife, the free oxygen of the air for their respiration. These are called_aerobic_. Some species, however, and some yeasts as well possess thepeculiar property of taking the oxygen which they need from organiccompounds such as sugar, etc. , and are therefore able to live and growunder conditions where the atmospheric air is excluded. These are knownas _anaerobic_. While some species grow strictly under one condition orthe other, and hence are _obligate_ aerobes or anaerobes, others possessthe ability of growing under either condition and are known as_facultative_ or optional forms. The great majority of milk bacteria areeither obligate or facultative aerobes. 

~Rate of growth. ~ The rate of bacterial development is naturally very muchaffected by external conditions, food supply and temperature exertingthe most influence. In the neighborhood of the freezing point but littlegrowth occurs. The rate increases with a rise in temperature until atthe _optimum_ point, which is generally near the blood heat or slightlybelow (90°-98° F. ), a single cell will form two cells in 20 to 30minutes. If temperature rises much above blood heat rate of growth islessened and finally ceases. Under ideal conditions, rapidity of growthis astounding, but this initially rapid rate of development cannot bemaintained indefinitely, for growth is soon limited by the accumulationof by-products of cell activity. Thus, milk sours rapidly at ordinarytemperatures until the accumulation of acid checks its development. 

~Detrimental effect of external conditions. ~ Environmental influences of adetrimental character are constantly at work on bacteria, tending torepress their development or destroy them. These act much more readilyon the vegetating cell than on the more resistant spore. A thoroughknowledge of the effect of these antagonistic forces is essential, forit is often by their means that undesirable bacteria may be killed out. 

~Effect of cold. ~ While it is true that chilling largely preventsfermentative action, and actual freezing stops all growth processes, still it does not follow that exposure to low temperatures willeffectually destroy the vitality of bacteria, even in the vegetativecondition. Numerous non-spore-bearing species remain alive in ice for aprolonged period, and recent experiments with liquid air show that evena temperature of -310° F. For hours does not effectually kill allexposed cells. 

~Effect of heat. ~ High temperatures, on the other hand, will destroy anyform of life, whether in the vegetative or latent stage. The temperatureat which the vitality of the cell is lost is known as the _thermal deathpoint_. This limit is not only dependent upon the nature of theorganism, but varies with the time of exposure and the condition inwhich the heat is applied. In a moist atmosphere the penetrating powerof heat is great; consequently cell-death occurs at a lower temperaturethan in a dry atmosphere. An increase in time of exposure lowers thetemperature point at which death occurs. 

For vegetating forms the thermal death point of most bacteria rangesfrom 130°-140° F. Where the exposure is made for ten minutes which isthe standard arbitrarily selected. In the spore stage resistance isgreatly increased, some forms being able to withstand steam at 210°-212°F. From one to three hours. If dry heat is employed, 260°-300° F. For anhour is necessary to kill spores. Where steam is confined underpressure, a temperature of 230°-240° F. For 15-20 minutes suffices tokill all spores. 

~Drying. ~ Spore-bearing bacteria like anthrax withstand drying withimpunity; even tuberculous material, although not possessing sporesretains its infectious properties for many months. Most of the dairybacteria do not produce spores, and yet in a dry condition, they retaintheir vitality unimpaired for considerable periods, if they are notsubjected to other detrimental influences. 

~Light. ~ Bright sunlight exerts on many species a powerful disinfectingaction, a few hours being sufficient to destroy all cells that arereached by the sun's rays. Even diffused light has a similar effect, although naturally less marked. The active rays in this disinfectingaction are those of the chemical or violet end of the spectrum, and notthe heat or red rays. 

~Influence of chemical substances. ~ A great many chemical substances exerta more or less powerful toxic action of various kinds of life. Many ofthese are of great service in destroying or holding bacterial growth incheck. Those that are toxic and result in the death of the cell areknown as _disinfectants_; those that merely inhibit, or retard growthare known as _antiseptics_. All disinfectants must of necessity beantiseptic in their action, but not all antiseptics are disinfectantseven when used in strong doses. Disinfectants have no place in dairywork, except to destroy disease bacteria, or preserve milk foranalytical purposes. Corrosive sublimate or potassium bichromate aremost frequently used for these purposes. The so-called chemicalpreservatives used to "keep" milk depend for their effect on theinhibition of bacterial growth. With a substance so violently toxic asformaldehyde (known as formalin, freezene) antiseptic doses are likelyto be exceeded. In this country most states prohibit the use of thesesubstances in milk. Their only function in the dairy should be to checkfermentative or putrefactive processes outside of milk and so keep theair free from taints. 

~Products of growth. ~ All bacteria in their development form certain moreor less characteristic by-products. With most dairy bacteria, theseproducts are formed from the decomposition of the medium in which thebacteria may happen to live. Such changes are known, collectively, asfermentations, and are characterised by the production of a large amountof by-products, as a result of the development of a relatively smallamount of cell-life. The souring of milk, the formation of butyric acid, the making of vinegar from cider, are all examples of fermentativechanges. 

With many bacteria, especially those that affect proteid matter, foul-smelling gases are formed. These are known as putrefactive changes. All organic matter, under the action of various organisms, sooner orlater undergoes decay, and in different stages of these processes, acids, alkalies, gases and numerous other products are formed. Many ofthese changes in organic matter occur only when such material is broughtin direct contact with the living bacterial cell. 

In other instances, soluble, non-vital ferments known as _enzyms_ areproduced by the living cell, which are able to act on organic matter, ina medium free from live cells, or under conditions where the activity ofthe cell is wholly suspended. These enzyms are not confined to bacteriabut are found throughout the animal and plant world, especially in thoseprocesses that are concerned in digestion. Among the better known ofthese non-vital ferments are rennet, the milk-curdling enzym; diastaseor ptyalin of the saliva, the starch-converting enzym; pepsin andtrypsin, the digestive ferments of the animal body. 

Enzyms of these types are frequently found among the bacteria and yeastsand it is by virtue of this characteristic that these organisms areable to break down such enormous quantities of organic matter. Most ofthese enzyms react toward heat, cold and chemical poisons in a mannerquite similar to the living cells. In one respect they are readilydifferentiated, and that is, that practically all of them are capable ofproducing their characteristic chemical transformations underanaesthetic conditions, as in a saturated ether or chloroformatmosphere. 

~Distribution of bacteria. ~ As bacteria possess greater powers ofresistance than most other forms of life, they are to be found morewidely distributed than any other type. At the surface of the earth, where conditions permit of their growth, they are found everywhere, except in the healthy tissues of animals and plants. In the superficialsoil layers, they exist in myriads, as here they have abundance ofnourishment. At the depth of several feet however, they diminish rapidlyin numbers, and in the deeper soil layers, from six to ten feet or more, they are not present, because of the unsuitable growth conditions. 

The bacteria are found in the air because of their development in thesoil below. They are unable to grow even in a moist atmosphere, but areso readily dislodged by wind currents that over land areas the lowerstrata of the air always contain them. They are more numerous in summerthan in winter; city air contains larger numbers than country air. Wherever dried fecal matter is present, as in barns, the air containsmany forms. 

Water contains generally enough organic matter in solution, so thatcertain types of bacterial life find favorable growth conditions. Waterin contact with the soil surface takes up many impurities, and is ofnecessity rich in microbes. As the rain water percolates into the soil, it loses its germ content, so that the normal ground water, like thedeeper soil layers, contains practically no bacterial life. Springstherefore are relatively deficient in germ life, except as they becomeinfected with soil organisms, as the water issues from the soil. Watermay serve to disseminate certain infectious diseases as typhoid feverand cholera among human beings, and a number of animal maladies. 

While the inner tissues of healthy animals are free from bacteria, thenatural passages as the respiratory and digestive tracts, being in moredirect contact with the exterior, become more readily infected. This isparticularly true with reference to the intestinal tract, for in theundigested residue, bacterial activity is at a maximum. The result isthat fecal matter contains enormous numbers of organisms so that thepossibility of pollution of any food medium such as milk with suchmaterial is sure to introduce elements that seriously affect the qualityof the product. 

CHAPTER II. 

METHODS OF STUDYING BACTERIA. 

~Necessity of bacterial masses for study. ~ The bacteria are so extremelysmall that it is impossible to study individual germs separately withoutthe aid of first-class microscopes. For this reason, but little advancewas made in the knowledge of these lower forms of plant life, until theintroduction of culture methods, whereby a single organism could becultivated and the progeny of this cell increased to such an extent in ashort course of time, that they would be visible to the unaided eye. 

This is done by growing the bacteria in masses on various kinds of foodmedia that are prepared for the purpose, but inasmuch as bacteria are souniversally distributed, it becomes an impossibility to cultivate anyspecial form, unless the medium in which they are grown is first freedfrom all pre-existing forms of germ life. To accomplish this, it isnecessary to subject the nutrient medium to some method ofsterilization, such as heat or filtration, whereby all life iscompletely destroyed or eliminated. Such material after it has beenrendered germ-free is kept in sterilized glass tubes and flasks, and isprotected from infection by cotton stoppers. 

~Culture media. ~ For culture media, many different substances areemployed. In fact, bacteria will grow on almost any organic substancewhether it is solid or fluid, provided the other essential conditions ofgrowth are furnished. The food substances that are used for culturepurposes are divided into two classes; solids and liquids. 

Solid media may be either permanently solid like potatoes, or they mayretain their solid properties only at certain temperatures like gelatinor agar. The latter two are of utmost importance in bacteriologicalresearch, for their use, which was introduced by Koch, permits theseparation of the different forms that may happen to be in any mixture. Gelatin is used advantageously because the majority of bacteria presentwider differences due to growth upon this medium than upon any other. Itremains solid at ordinary temperatures, becoming liquid at about 70° F. Agar, a gelatinous product derived from a Japanese sea-weed, has a muchhigher melting point, and can be successfully used, especially withthose organisms whose optimum growth point is above the melting point ofgelatin. 

Besides these solid media, different liquid substances are extensivelyused, such as beef broth, milk, and infusions of various vegetable andanimal tissues. Skim-milk is of especial value in studying the milkbacteria and may be used in its natural condition, or a few drops oflitmus solution may be added in order to detect any change in itschemical reaction due to the bacteria. 

[Illustration: FIG. 2. A gelatin plate culture showing appearance ofdifferent organisms in a sample of milk. Each mass represents abacterial growth (colony) derived from a single cell. Different formsreact differently toward the gelatin, some liquefying the same, othersgrowing in a restricted mass. _a_, represents a colony of the ordinarybread mold; _b_, a liquefying bacterium; _c_, and _d_, solid forms. ]

~Methods of isolation. ~ Suppose for instance one wishes to isolate thedifferent varieties of bacteria found in milk. The method of procedureis as follows: Sterile gelatin in glass tubes is melted and cooled downso as to be barely warm. To this gelatin which is germ-free a drop ofmilk is added. The gelatin is then gently shaken so as to thoroughlydistribute the milk particles, and poured out into a sterile flat glassdish and quickly covered. This is allowed to stand on a cool surfaceuntil the gelatin hardens. After the culture plate has been left fortwenty-four to thirty-six hours at the proper temperature, tiny spotswill begin to appear on the surface, or in the depth of the culturemedium. These patches are called _colonies_ and are composed of analmost infinite number of individual germs, the result of the continuedgrowth of a single organism that was in the drop of milk which wasfirmly held in place when the gelatin solidified. The number of thesecolonies represents approximately the number of germs that were presentin the milk drop. If the plate is not too thickly sown with these germs, the colonies will continue to grow and increase in size, and as they do, minute differences will begin to appear. These differences may be in thecolor, the contour and the texture of the colony, or the manner inwhich it acts toward gelatin. In order to make sure that the seeding innot too copious so as to interfere with continued study, an_attenuation_ is usually made. This consists in taking a drop of theinfected gelatin in the first tube, and transferring it to another tubeof sterile media. Usually this operation is repeated again so that theseculture plates are made with different amounts of seed with theexpectation that in at least one plate the seeding will not be so thickas to prevent further study. For transferring the culture a loop made ofplatinum wire is used. By passing this through a gas flame, it can besufficiently sterilized. 

[Illustration: FIG. 3. Profile view of gelatin plate culture; _b_, aliquefying form that dissolves the gelatin; _c_ and _d_, surfacecolonies that do not liquefy the gelatin. ]

To further study the peculiarities of different germs, the separatecolonies are transferred to other sterile tubes of culture material andthus _pure cultures_ of the various germs are secured. These culturesthen serve as a basis for continued study and must be planted and grownupon all the different kinds of media that are obtainable. In this waythe slight variations in the growth of different forms are detected andthe peculiar characteristics are determined, so that the student is ableto recognize this form when he meets it again. 

These culture methods are of essential importance in bacteriology, as itis the only way in which it is possible to secure a quantity of germs ofthe same kind. 

~The microscope in bacterial investigation. ~ In order to verify the purityof the cultures, the microscope is in constant demand throughout all thedifferent stages of the isolating process. For this purpose, it isessential that the instrument used shall be one of strong magnifyingpowers (600-800 diameters), combined with sharp definition. 

[Illustration: FIG. 4. Pure cultures of different kinds of bacteria ingelatin tubes. _a_, growth slight in this medium; _b_, growth copious atand near surface. Fine parallel filaments growing out into mediumliquefying at surface; _c_, a rapid liquefying form; _d_, agas-producing form that grows equally well in lower part of tube as atsurface (facultative anaerobe); _e_, an obligate anaerobe, that developsonly in absence of air. ]

The microscopical examination of any germ is quite as essential as thedetermination of culture characteristics; in fact, the two must go handin hand. The examination reveals not only the form and size of theindividual germs, but the manner in which they are united with eachother, as well as any peculiarities of movement that they may possess. 

In carrying out the microscopical part of the work, not only is theorganism examined in a living condition, but preparations are made byusing solutions of anilin dyes as staining agents. These are of greatservice in bringing out almost imperceptible differences. The art ofstaining has been carried to the highest degree of perfection inbacteriology, especially in the detection of germs that are found indiseased tissues in the animal or human body. 

In studying the peculiarities of any special organism, not only is itnecessary that these cultural and microscopical characters should beclosely observed, but special experiments must be carried out alongdifferent lines, in order to determine any special properties that thegerm may possess. Thus, the ability of any form to act as a fermentativeorganism can be tested by fermentation experiments; the property ofcausing disease, studied by the inoculation of pure cultures intoanimals. A great many different methods have been devised for thepurpose of studying special characteristics of different bacteria, but afull description of these would necessarily be so lengthy that in a workof this character they must be omitted. For details of this natureconsult standard reference books on bacteriological technique. 

CHAPTER III. 

CONTAMINATION OF MILK. 

No more important lesson is to be learned than that which relates to theways in which milk is contaminated with germ life of various kinds; forif these sources of infection are thoroughly recognized they can inlarge measure be prevented, and so the troubles which they engenderovercome. Various organisms find in milk a congenial field fordevelopment. Yeasts and some fungi are capable of growth, but moreparticularly the bacteria. 

~Milk a suitable bacterial food. ~ The readiness with which milk undergoesfermentative changes indicates that it is well adapted to nourishbacterial life. Not only does it contain all the necessary nutritivesubstances but they are diluted in proper proportions so as to renderthem available for bacterial as well as mammalian life. 

Of the nitrogenous compounds, the albumen is in readily assimilableform. The casein, being insoluble, is not directly available, until itis acted upon by proteid-dissolving enzyms like trypsin which may besecreted by bacteria. The fat is relatively resistant to change, although a few forms are capable of decomposing it. Milk sugar, however, is an admirable food for many species, acids and sometimes gases beinggenerally produced. 

~Condition when secreted. ~ When examined under normal conditions milkalways reveals bacterial life, yet in the secreting cells of the udderof a healthy cow germ life is not found. Only when the gland is diseasedare bacteria found in any abundance. In the passage of the milk fromthe secreting cells to the outside it receives its first infection, sothat when drawn from the animal it generally contains a considerablenumber of organisms. 

[Illustration: FIG. 5. Microscopic appearance of milk showing relativesize of fat globules and bacteria. ]

~Contamination of milk. ~ From this time until it is consumed in one formor another, it is continually subjected to contamination. The major partof this infection occurs while the milk is on the farm and the degree ofcare which is exercised while the product is in the hands of the milkproducer is the determining factor in the course of bacterial changesinvolved. This of course does not exclude the possibility ofcontamination in the factory, but usually milk is so thoroughly seededby the time it reaches the factory that the infection which occurs hereplays a relatively minor rôle to that which happens earlier. The greatmajority of the organisms in milk are in no wise dangerous to health, but many species are capable of producing various fermentative changesthat injure the quality of the product for butter or cheese. To be ableto control abnormal changes of an undesirable character one must knowthe sources of infection which permit of the introduction of theseunwelcome intruders. 

~Sources of infection. ~ The bacterial life that finds its way into milkwhile it is yet on the farm may be traced to several sources, which maybe grouped under the following heads: unclean dairy utensils, fore milk, coat of animal, and general atmospheric surroundings. The relativeimportance of these various factors fluctuates in each individualinstance. 

~Dairy utensils. ~ Of first importance are the vessels that are used duringmilking, and also all storage cans and other dairy utensils that come incontact with the milk after it is drawn. By unclean utensils, actually_visible_ dirt need not always be considered, although such material isoften present in cracks and angles of pails and cans. Unless cleansedwith especial care, these are apt to be filled with foul and decomposingmaterial that suffices to seed thoroughly the milk. Tin utensils arebest. Where made with joints, they should be well flushed with solder soas to be easily cleaned (see Fig. 6). In much of the cheaper tin ware onthe market, the soldering of joints and seams is very imperfect, affording a place of refuge for bacteria and dirt. 

Cans are often used when they are in a condition wholly unsuitable forsanitary handling of milk. When the tin coating becomes broken and thecan is rusty, the quality of the milk is often profoundly affected. Olson[1] of the Wisconsin Station has shown that the action of rennet isgreatly impaired where milk comes in contact with a rusty iron surface. 

[Illustration: FIG. 6. ]

With the introduction of the form or hand separator a new milk utensilhas been added to those previously in use and one which is veryfrequently not well cleaned. Where water is run through the machine torinse out the milk particles, gross bacterial contamination occurs andthe use of the machine much increases the germ content of the milk. Every time the separator is used it should be taken apart and thoroughlycleaned and dried before reassembling. [2]

~Use of milk-cans for transporting factory by-products. ~ The generalcustom of using the milk-cans to carry back to the farm the factoryby-products (skim-milk or whey) has much in it that is to be deprecated. These by-products are generally rich in bacterial life, more especiallywhere the closest scrutiny is not given to the daily cleaning of thevats and tanks. Too frequently the cans are not cleaned immediately uponarrival at the farm, so that the conditions are favorable for rapidfermentation. Many of the taints that bother factories are directlytraceable to such a cause. A few dirty patrons will thus seriouslyinfect the whole supply. The responsibility for this defect should, however, not be laid entirely upon the shoulders of the producer. Thefactory operator should see that the refuse material does not accumulatein the waste vats from day to day and is not transformed into a more orless putrid mass. A dirty whey tank is not an especially good objectlesson to the patron to keep his cans clean. 

It is possible that abnormal fermentations or even contagious diseasesmay thus be disseminated. 

Suppose there appears in a dairy an infectious milk trouble, such asbitter milk. This milk is taken to the factory and passes unnoticed intothe general milk-supply. The skim-milk from the separator is of courseinfected with the germ, and if conditions favor its growth, the wholelot soon becomes tainted. If this waste product is returned to thedifferent patrons in the same cans that are used for the fresh milk, theprobabilities are strongly in favor of some of the cans beingcontaminated and thus infecting the milk supply of the patrons. If theorganism is endowed with spores so that it can withstand unfavorableconditions, this taint may be spread from patron to patron simplythrough the infection of the vessels that are used in the transportationof the by-products. Connell has reported just such a case in a Canadiancheese factory where an outbreak of slimy milk was traced to infectedwhey vats. Typhoid fever among people, foot and mouth disease andtuberculosis among stock are not infrequently spread in this way. InDenmark, portions of Germany and some states in America, compulsoryheating of factory by-products is practiced to eliminate this danger. [3]

~Pollution of cans from whey tanks. ~ The danger is greater in cheesefactories than in creameries, for whey usually represents a moreadvanced stage of fermentation than skim-milk. The higher temperature atwhich it is drawn facilitates more rapid bacterial growth, and theconditions under which it is stored in many factories contribute to theease with which fermentative changes can go on in it. Often thisby-product is stored in wooden cisterns or tanks, situated below ground, where it becomes impossible to clean them out thoroughly. A custom thatis almost universally followed in the Swiss cheese factories, here inthis country, as in Switzerland, is fully as reprehensible as any dairycustom could well be. In Fig. 7 the arrangement in vogue for thedisposal of the whey is shown. The hot whey is run out through thetrough from the factory into the large trough that is placed over therow of barrels, as seen in the foreground. Each patron thus has allottedto him in his individual barrel his portion of the whey, which he issupposed to remove day by day. No attempt is made to clean out thesereceptacles, and the inevitable result is that they become filled with afoul, malodorous liquid, especially in summer. When such material istaken home in the same set of cans that is used to bring the fresh milk(twice a day as is the usual custom in Swiss factories), it is no wonderthat this industry is seriously handicapped by "gassy" fermentationsthat injure materially the quality of the product. Not only is the abovedanger a very considerable one, but the quality of the factoryby-product for feeding purposes, whether it is skim-milk or whey, isimpaired through the development of fermentative changes. 

[Illustration: FIG. 7. Swiss cheese factory (Wisconsin), showingcareless way in which whey is handled. Each patron's share is placed ina barrel, from which it is removed by him. No attempt is made to cleansethese receptacles. ]

~Improved methods of disposal of by-products. ~ The difficulties whichattend the distribution of these factory by-products have led todifferent methods of solution. One is to use another separate set ofreceptacles to carry back these products to the farm. This method hasbeen tried, and while it is deemed impracticable by many to handle twosets of vessels, yet some of the most progressive factories reportexcellent results where this method is in use. 

Large barrels could be used for this purpose to economize in wagonspace. 

Another method that has met with wider acceptance, especially increameries, is the custom of pasteurizing or scalding the skim-milkimmediately after it is separated, so that it is returned to the farmerin a hot condition. In factories where the whole milk is pasteurized, further treatment of the by-product is not necessary. In most factoriessteam, generally exhaust, is used directly in the milk, and experiencehas shown that such milk, without any cooling, will keep sweet for aconsiderable number of hours longer than the untreated product. It isnoteworthy that the most advanced and progressive factories are the onesthat appreciate the value of this work, and although it involves sometime and expense, experience has shown the utility of the process inthat a better grade of milk is furnished by the patrons of factorieswhich follow this practice. [4] The exclusion of all danger of animal orhuman disease is also possible in this way. 

~Cleaning dairy utensils. ~ The thorough cleaning of all dairy apparatusthat in any way comes in contact with the milk is one of the mostfundamental and important problems in dairying. All such apparatusshould be so constructed as to permit of easy cleaning. Tinware, preferably of the pressed variety, gives the best surface for thispurpose and is best suited for the handling of milk. 

Milk vessels should never be allowed to become dry when dirty, for driedparticles of milk residue are extremely difficult to remove. In cleaningdairy utensils they should first be rinsed in lukewarm instead of hotwater, so as to remove organic matter without coagulating the milk. Thenwash thoroughly in hot water, using a good washing powder. The bestwashing powders possess considerable disinfecting action. [5] Strongalkalies should not be used. After washing rinse thoroughly in clean hotwater. If steam is available, as it always is in creameries, cans andpails should be turned over jet for a few moments. While a momentaryexposure will not suffice to completely sterilize such a vessel, yetmany bacteria are killed in even a short exposure, and the cans dry morethoroughly and quickly when heated by steam. 

Not only should the greatest care be paid to the condition of the cansand milk-pails, but all dippers, strainers, and other utensils that comein contact with the milk must be kept equally clean. Cloth strainers, unless attended to, are objectionable, for the fine mesh of the clothretains so much moisture that they become a veritable hot-bed ofbacterial life, unless they are daily boiled or steamed. 

The inability to thoroughly render vessels bacteria-free with theconveniences which are generally to be found on the farm has led in somecases to the custom of washing and sterilizing the milk cans at thefactory. 

~Germ content of milk utensils. ~ Naturally the number of bacteria found indifferent milk utensils after they have received their regular cleaningwill be subject to great fluctuations; but, nevertheless, suchdeterminations are of value as giving a scientific foundation forpractical methods of improvement. The following studies may serve toindicate the relative importance of the utensils as a factor in milkcontamination. 

Two cans were taken, one of which had been cleaned in the ordinary way, while the other was sterilized by steaming. Before milking, the udderwas thoroughly cleaned and special precautions taken to avoid raising ofdust; the fore milk was rejected. Milk drawn into these two cans showedthe following germ content:

 No. Bacteria Hours before per cc. Souring. 

 Steamed pail 165 28-1/2 Ordinary pail 4265 23

Harrison[6] has shown how great a variation is in the bacterial contentin milk cans. The utensils were rinsed with 100 cc. Of sterile water andnumerical determinations of this rinsing water made. In poorly cleanedcans, the average germ content was 442, 000; in cans washed in tepidwater and then scalded--the best farm practice--54, 000, and in canscarefully washed and then steamed for five minutes, 880. 

Another method used by the writer is to wash the utensil with 100 cc. Sterile wash water, using a sterile swab to remove dirt. Then repeat theprocess twice or more with fresh rinsing waters, making plate culturesfrom each. The following data were obtained from three suchdeterminations:

No. Bacteria in different washings. Total No. I. II. III. Bacteria. 7, 800, 000 1, 450, 000 49, 000 9, 299, 000 283, 000 43, 400 35, 000 361, 4001, 685, 000 105, 000 61, 400 1, 851, 400

~Infection of milk in udder cavity. ~ A frequently neglected butconsiderable factor of infection is that which is attributable to thebacteria which are present in the udder and which are removed in largenumbers during the milking process. An examination of the fore milk, i. E. , the first few streams from each teat, and that which is subsequentlywithdrawn, generally reveals a very much larger number of organisms inthe fore milk. [7] Not infrequently will this part of the milk when drawnunder as careful conditions as possible, contain several score thousandorganisms per cc. If successive bacterial determinations are made atdifferent periods of the milking, as shown in the following experiment, a marked diminution is to be noted after the first portion of the milkis removed:

 _Bacterial content at different periods of milking. _

 Fore 200th 2000th 4300th 6500th Strippings. Milk. Cc. Cc. Cc. Cc. Expt. 1 6, 500 1, 700 475 220 75 5Expt. 2 8, 100 1, 650 400 240 50 10

By some observers it has been claimed that it is possible to secureabsolutely sterile milk in the strippings but this is rarely so. It isquite probable that such reported results are due to the fact that toosmall quantities of milk were used in the examinations and so erroneousconclusions were drawn. This marked diminution in numbers indicates thatthe larger proportion of the organisms found in the fore milk arepresent in the lower portion of the udder and milk ducts. Whenconsideration is given to the structure of the udder, it is readilyapparent that infection will be greater here than above. 

[Illustration: FIG. 8. Sectional view of udder showing teat with milkduct connecting exterior with the milk cistern. Milk sinuses are mostlyshown in cross section interspersed and below the secreting tissue(Moore and Ward). ]

The udder is composed of secreting tissue (_gland cells_) held in placeby fibrous connective tissue. Ramifying throughout this glandularstructure are numerous channels (_milk sinuses_) that serve to conveythe milk from the cells where it is produced into the _milk cistern_, acommon receptacle just above the teats. This cavity is connected withthe exterior through the milk duct in the teat, which is more or lesstightly closed by the circular sphincter muscles, thus preventing themilk from flowing out. The mucous membranes of the milk duct and cisternare naturally moist. The habits of the animal render it impossible toprevent infection of the external opening at the end of the teat andthere is no mechanical reason why bacteria cannot readily find their wayalong the moist lining membrane for some distance. If organisms areadapted to this kind of an environment, ideal conditions exist for theirmultiplication, as moisture, warmth and suitable food supply arepresent. The question arises how far up into this organ is penetrationpossible? Within late years numerous observations have been made on thepresence of organisms in the upper portion of the udder in contact withthe secreting tissue. [8]

These investigations prove that bacteria are distributed throughout thewhole of the udder, although numerically they are much less abundant inthe region of the milk-secreting tissue than in the lower portion. Ward's conclusions are "that milk when secreted by the glands of ahealthy udder is sterile. It may however, immediately becomecontaminated by the bacteria which are normally present in the smallermilk ducts of the udder. "

~Nature of bacteria in fore milk. ~ Generally speaking the number ofdifferent species found in the fore milk is not large, and of thosewhich do appear many occur at only occasional intervals. Moore[9] in theexamination of 9 udders found 20 different forms, and of these only 3species predominated, all of which proved to be micrococci. Streptococcihave also been quite frequently reported. Freudenreich[10] found themost common types to be cocci, belonging to both the liquefying andnon-liquefying class. 

Peptonizing[11] and spore-bearing[12] species have also been reported aswell as gas-producing[13] forms allied to the colon bacillus. Suchfindings are, however, due in all probability to accidental invasion. Most investigators report the absence of the distinctively lactic-acidgroup of organisms. [14]

~Origin of bacteria in udder. ~ There is no question but that many of thetypes of bacteria found in the udder gain access from the outside. Thosebelonging to the spore-bearing, digesting and intestinal types have sucha favorable opportunity for introduction from outside and are sounlikely to have come directly from the body of the animal, that theexternal source of infection is much more probable. Whether thisexplanation answers the origin of the cocci that are so generally foundin the upper portion of the udder is questionable. The statement isordinarily made that the inner tissues of healthy organs arebacteria-free, but the studies of Ford[15] seem to indicate that 70 percent. Of such organs, removed under aseptic conditions from guinea pigs, rabbits, dogs and cats contained living organisms. Others have reportedsimilar results in which cocci have been found[16] very similar to thoseoccurring in the udder. These findings increase the probability that theorigin of this type is from the blood. The persistence of certainspecies in the udder for months as noted by Ward indicates possibilityof growth of some forms at least. Stocking[17] has shown where cows arenot milked clean that the germ content of succeeding milkings is greatlyincreased. 

~Artificial introduction of bacteria into udder. ~ If bacteria are capableof actually developing in the udder proper, it ought to be possible toeasily demonstrate this by the artificial introduction of cultures. In anumber of cases[18] such experiments have been made with varioussaprophytic forms, such as _B. Prodigiosus_, lactic acid bacilli andothers. In no case has it appeared evident that actual growth hasoccurred, although the introduced organism has been demonstrated indiminishing numbers for 5-6 days. Even the common lactic acid germ and ayellow liquefying coccus isolated from the fore milk failed to persistfor more than a few days when thus artificially introduced. This failureto colonize is indeed curious and needs explanation. Is it due tounsuitable environmental conditions or attributable to the germicidalinfluence of the milk?

Various body fluids are known to possess the property of destroyingbacteria and it is claimed by Fokker[19] that this same property wasfound in freshly drawn milk. This peculiarity has also been investigatedby Freudenreich, [20] and Hunziker[21] who find a similar property. 

No material increase in germ content takes place in milk for severalhours when chilled to 40°-70° F. ; on the other hand an actual, butusually not a marked decrease is observed for about 6 hours. Thisphenomenon varies with the milk of different cows. Nothing is known asto the cause of this apparent germicidal action. The question is yet byno means satisfactorily settled, although the facts on which thehypothesis is based are not in controversy. If such a peculiaritybelongs to milk, it is not at all improbable that it may serve to keepdown the germ content in the udder. Freudenreich[22] found that udderswhich were not examined for some time after death showed abundantgrowth, which fact he attributed to the loss of this germicidalproperty. 

The infection of the whole milk can be materially reduced by rejectingthe fore milk, but it is questionable whether such rejection is worthwhile, except in the case of "sanitary" dairies where milk is producedwith as low a germ content as possible. The intrinsic loss in butter fatin the fore milk is inconsiderable as the first few streams contain onlyabout one-fifth the normal fat content. 

~Infection of milk after withdrawal from animal. ~ The germ content of themilk, when it is being drawn from the animal is immediately increasedupon contact with the atmosphere. These organisms are derived from thesurrounding air and the utensils in which the milk is received andstored. The number of organisms which find their way into the milkdepends largely upon the character of the surroundings. Bacteria are sointimately associated with dirt, dust and filth of all kinds thatwherever the latter are found, the former are sure to be present inabundance. 

The most important factors in the infection of the milk after withdrawalare the pollution which is directly traceable to the animal herself andthe condition of the milk utensils. Fortunately both of these sources ofcontamination are capable of being greatly minimized by more carefulmethods of handling. 

~Infection directly from the cow. ~ It is a popular belief that theorganisms found in milk are derived from the feed and water which thecow consumes, the same passing directly from the intestinal tract to themilk by the way of the blood circulation. Such a view has no foundationin fact as bacteria absorbed into the circulation are practically alldestroyed in the tissues by the action of the body fluids and cells. [23]While organisms cannot pass readily from the intestine to the udder, yetthis must not be interpreted as indicating that no attention should begiven to the bacterial character of the material consumed. The watersupply given should be pure and wholesome and no decomposed or spoiledfood should be used. 

The infection traceable directly to the cow is modified materially bythe conditions under which the animal is kept and the character of thefeed consumed. The nature of the fecal matter is in part dependent uponthe character of the food. The more nitrogenous rations with whichanimals are now fed leads to the production of softer fecal dischargeswhich is more likely to soil the coat of the animal unless care istaken. The same is true with animals kept on pasture in comparison withthose fed dry fodder. 

Stall-fed animals, however, are more likely to have their flanks fouled, unless special attention is paid to the removal of the manure. All dairystalls should be provided with a manure drop which should be cleaned asfrequently as circumstances will permit. 

[Illustration: FIG. 9. Showing the bacterial contamination arising fromhair. These hairs were allowed to fall on a sterile gelatin surface. Theadherent bacteria developed readily in this medium, and the number ofbacteria thus introduced into the milk from these hairs can be estimatedby the number of developing colonies. ]

The animal herself contributes materially to the quota of germ lifefinding its way into the milk through the dislodgment of dust and filthparticles adhering to its hairy coat. The nature of this coat is such asto favor the retention of these particles. Unless care is taken theflanks and udder become polluted with fecal matter, which upon drying isdisplaced with every movement of the animal. Every hair or dirt particleso dislodged and finding its way into the milk-pail adds its quota oforganisms to the liquid. This can be readily demonstrated by placingcow's hairs collected with care on the moist surface of gelatin cultureplates. Almost invariably, bacteria will be found in considerablenumbers adhering to such hairs as is indicated in Fig. 9. Dirt particlesare even richer in germ life. Not only is there the dislodgment ofhairs, epithelial scales and masses of dirt and filth, but during themilking process, as at all other times, every motion of the animal isaccompanied by a shower of _invisible_ particles more or less teemingwith bacterial life. 

The amount of actual impurities found in milk is often considerable andwhen it is remembered that about one-half of fresh manure dissolves inmilk, [24] and thus does not appear as sediment, the presence of thisundissolved residue bespeaks filthy conditions as to milking. Fromactual tests made, it is computed that the city of Berlin, Germanyconsumes about 300 pounds of such dirt and filth daily. Renk has laiddown the following rule with reference to this insoluble dirt: If asample of milk shows any evidence of impurity settling on a transparentbottom within two hours, it should be regarded as too dirty for use. 

While the number of organisms here introduced is at all times large, thecharacter of the species is of even greater import. Derived primarilyfrom dirt and fecal matter, it is no wonder that such forms are able toproduce very undesirable fermentative changes. 

~Influence of milker. ~ The condition of the milker is not to be ignored indetermining all possible factors of infection, for when clothed in thedust-laden garments that have been worn all day, a favorable opportunityfor direct contamination is possible. The filthy practice of wetting thehands with milk just before milking is to be condemned. The milker'shands should be washed immediately before milking in clean water anddried. A pinch of vaseline on hands is sometimes used to obtain a firmergrasp and prevents the ready dislodgment of scales. [25] It must also beborne in mind that the milker may spread disease through the milk. Intyphoid fever and diphtheria, the germs often remain in the system forweeks and thus make infection possible. Stocking[26] has shown that theindividual milker exerts a potent influence on the total germ content ofmilk, even where the procedure is quite the same. In sanitary dairiesmilkers are usually clad in white duck suits. 

~Milking by machinery. ~ Several mechanical devices have been invented formilking, some of which have been tested bacteriologically as to theirefficiency. Harrison[27] has examined the "Thistle" machine but found amuch higher germ content than with hand-drawn milk. The recentintroduction of the Burrel-Lawrence-Kennedy machine has led to numeroustests in which very satisfactory results have been obtained. If therubber parts of the milker are thoroughly cleaned and kept in lime watersolution, they remain nearly sterile. When milk is properly handled, thegerm content may be greatly reduced. 

~Reduction in dirt and adherent bacteria. ~ No factor of contamination isso susceptible of improvement as that which relates to the reduction infilth and dirt which gains access during and immediately subsequent tothe milking. The care which is taken to keep the coat of the animalclean by grooming lessens very much the grosser portion of suchcontamination, but with a dry, hairy coat, fine scales and dustparticles must of necessity be dislodged. [28] Ordinarily the patronthinks all evidence of such dirt is removed if the milk is strained, butthis process only lessens the difficulty; it does not overcome it. Various methods are in use, the effectiveness of which is subject toconsiderable variation. Some of these look to the elimination of thebacteria after they are once introduced into the milk; others to theprevention of infection in the first place. 

_1. Straining the milk. _ Most of the visible, solid particles of filth, such as hairs, dirt particles, etc. , can be removed by simple straining, the time-honored process of purification. As ordinarily carried out, this process often contributes to instead of diminishing the germ lifein milk. The strainer cloths unless washed and thoroughly sterilized byboiling harbor multitudes of organisms from day to day and may thusactually add to the organisms present. Various methods have beensuggested for this simple process, but the most practical and efficientstrainer is that made of fine wire gauze to which is added 3-4 layers ofcheese cloth, the whole to set over the storage milk can. 

_2. Filtration. _ In Europe especially, the system of cleaning milk byfiltration through sand, gravel and other substances has been quiteextensively used. These filters are built in sections and the milkpasses from below upward. The filtering substance is washed in hot waterimmediately after use and then steamed and finally baked. While it ispossible to remove the solid impurities in this way, the germ contentcannot be greatly reduced. [29] Cellulose filters have also beensuggested[30] as an improvement over the sand filters. Methods offiltration of this character have not been used under commercialconditions here in this country. 

_3. Clarification in separator. _ Within recent years the custom hasgrown of clarifying milk or removing the visible dirt by passing themilk through a centrifugal separator the cream and skim milk beingremixed after separation. This process naturally removes the solidimpurities as dirt, hairs, epithelial scales and cells, also some of thecasein, making what is known as centrifuge slime. This conglomerate massis incomparably rich in germ life and the natural inference would bethat the bacterial content of the milk would be greatly reduced by thisprocedure. Eckles and Barnes[31] noted a reduction of 37 to 56 per cent. Of the bacteria but others have failed to observe such reductions. [32]This condition is explained by the more thorough breaking up of thebacterial masses in the process, thus apparently not reducing them innumbers. 

It is somewhat surprising that in spite of the elimination of muchorganic matter and bacteria, such clarified milk sours as rapidly as theuntreated product. [33]

The mechanical shock of separation ruptures the clusters of fat globulesand so delays creaming and also lessens the consistency of cream derivedfrom such milk. This practical disadvantage together with the increasedexpense of the operation and the failure to materially enhance thekeeping quality of the product outweigh the advantage which might comefrom removal of solid impurities which can be largely accomplished onthe farm by efficient straining. 

_4. Washing the udder. _ If a surface is moist, bacteria adherent to itcannot be dislodged by ordinary movements. Thus the air oversnow-covered mountains or oceans is relatively devoid of germ life. Themethod of moistening the udder is applied with success to the hairy coatof the animal thus subserving the double purpose of cleaning the animaland preventing in large measure the continual dislodgment of dustparticles. After these parts have been well carded to remove loose hairsand dirt particles, the skin should be thoroughly moistened with cleanwater and then wiped. It has been urged that this procedure lessens theyield of milk but Eckles[34] concludes from experiments that when theanimal becomes accustomed to this treatment, no noticeable change inamount of milk or butter-fat is produced. 

The effectiveness of this method in reducing the actual amount of dirtand filth introduced into the milk as well as the great diminution ingerm life is shown by the instructive experiments of Fraser[35] whofound that the actual amount of dirt dislodged from udders of apparentlyclean animals during the milking process was three and one-half times asmuch as when the cow's udders were washed. From udders visibly pollutedone ounce of such filth was removed in 275 pounds of milk, while fromcows whose udders had been washed, the same amount of dirt wasdistributed through 24, 030 pounds. 

Fraser observed as a result of 420 examinations that the average germcontent of 4-inch culture dishes under clean but unwashed udders was578, while under washed animals it was reduced to 192. From numeroustests made in the writer's laboratory, it is evident that the germcontent of the milk in the pail is increased from 20, 000-40, 000 bacteria_per minute_ during the milking period. By far the larger part of thispollution can be easily prevented by cleaning and dampening the udder. 

_5. Diminishing exposed surface of pail. _ The entrance of organisms intothe milk can be greatly reduced by lessening the area of the milk paildirectly exposed to the dust shower. A number of so-called sanitary orhygienic milk pails have been devised for this purpose. In one case thepail is smaller at the top than bottom, but in most of them the commonform is kept and the exposed area is lessened by means of a cover, themilk being received through a narrower opening. In some cases, strainersare also interposed so as to remove more effectually the coarseparticles. It is necessary to have these covers and strainersconstructed in such a way so they can be easily removed and cleaned. 

[Illustration: FIG. 10. Sanitary milk pails designed to diminish theintroduction of hairs, scales, dirt, etc. , into milk. ]

Stocking tested one of these pails (A, Fig. 10) and found that 63 percent of the dirt and 29 per cent. Of the bacteria were prevented frompassing into the milk. Eckles examined one in which the germ content wasfound to be 3200 per cc. As against 43200 per cc. In a common openpail. This milk did not sour until it was 64 hours old in the first casewhile in the latter it curdled in 43 hours. 

~Air in barn. ~ The atmosphere of the barn where the milking is done mayfrequently contribute considerable infection. Germ life is incapable ofdevelopment in the air, but in a dried condition, organisms may retaintheir vitality for long periods. Anything which contributes to theproduction of dust in the stable and aids in the stirring up of the sameincreases the number of organisms to be found in the air (Fig. 11). Thus, the feeding of dry fodder and the bedding of animals with strawadds greatly to the germ life floating in the air. Dust may vary much inits germ content depending upon its origin. Fraser found the dust fromcorn meal to contain only about one-sixth to one-eighth as much germlife as that from hay or bran. [36] In time most of these dust particlessettle to the floor, but where the herd is kept in the barn, theconstant movement of the animals keeps these particles more or less inmotion. Much can be done by forethought to lessen the germ content ofstables. Feeding dry feed should not be done until after milking. [37] Insome of the better sanitary dairies, it is customary to have a specialmilking room that is arranged with special reference to the eliminationof all dust. In this way this source of infection may be quite obviatedas the air of a clean, still room is relatively free from bacteria, especially if the floor is moistened. It has often been noted that themilk of stall-fed animals does not keep as well as that milked out ofdoors, a condition in part attributable to the lessened contamination. 

[Illustration: FIG. 11. Effect of contaminated air. The number of spotsindicate the colonies that have developed from the bacteria which fellin 30 seconds on the surface of the gelatin plate (3 inches indiameter). This exposure was made at time the cows were fed. ]

~Relative importance of different sources of infection. ~ It is impossibleto measure accurately the influence of the different sources ofinfection as these are continually subject to modification in each andevery case. As a general rule, however, where milk is drawn and handledwithout any special care, the utensils and the animal contribute thelarger proportion of dirt and bacteria that find their way into milk. Where the manner of milking and handling is designed to exclude thelargest number of organisms possible, the bacteria appearing in the foremilk make up the major portion remaining. By putting into practice thevarious suggestions that have been made with reference to diminishingthe bacterial content of milk, it is possible to greatly reduce thenumber of organisms found therein, and at the same time materiallyimprove the keeping quality of the milk. Backhaus[38] estimates thatthe germ life in milk can be easily reduced to one-two thousandth of itsoriginal number by using care in milking. He reports a series ofexperiments covering two years in which milk was secured that averagedless than 10, 000 bacteria per cc. , while that secured under ordinaryconditions averaged over 500, 000. 

[Illustration: FIG. 12. Bacterial content of milk handled in ordinaryway. Each spot represents a colony growing on gelatin plate. Comparewith Fig. 13, where same quantity of milk is used in making culture. Over 15, 000 bacteria per cc. In this milk. ]

Fig. 13 gives an illustration as to what care in milking will do in theway of eliminating bacteria. Fig. 12 shows a gelatin plate seeded withthe same quantity of milk that was used in making the culture indicatedby Fig. 13. The first plate was inoculated with milk drawn under goodconditions, the germ content of which was found to be 15, 500 bacteriaper cc. , while the sample secured under as nearly aseptic conditions aspossible (Fig. 13) contained only 330 organisms in the same volume. 

[Illustration: FIG. 13. Bacterial content of milk drawn with care. Diminished germ content is shown by smaller number of colonies (330bacteria per cc. ). Compare this culture with that shown in Fig. 12. ]

~"Sanitary" or "certified" milk. ~ Within recent years there has been moreor less generally introduced into many cities, the custom of supplyinghigh grade milk that has been handled in a way so as to diminish itsgerm content as much as possible. Milk of this character is frequentlyknown as "sanitary, " "hygienic" or "certified, " the last term being usedin connection with a certification from veterinary authorities or boardsof health as to the freedom of animals from contagious disease. Frequently a numerical bacterial standard is exacted as a pre-requisiteto the recommendation of the board of examining physicians. Thus, thePediatric Society of Philadelphia requires all children's milk thatreceives its recommendation to have not more than 10, 000 bacteria percc. Such a standard has its value in the scrupulous cleanliness thatmust prevail in order to secure these results. This in itself ispractically a guarantee of the absence of those bacteria liable toproduce trouble in children. The number of organisms found in such milksis surprisingly low when compared with ordinary milk. Naturally, thereis considerable fluctuation from day to day, and occasionally the germcontent is increased to a high figure without any apparent reason. Theaverage results though, show a greatly reduced number of organisms. DeSchweinitz[39] found in a Washington dairy in 113 examinations extendingthroughout a year, an average of 6, 485 bacteria per cc. The dailyanalyses made of the Walker-Gordon supply sold in Philadelphia for anentire year, showed that the milk almost always contained less than5, 000 bacteria per cc. And on 120 days out of the year the germ contentwas 1, 000 organisms per cc. Or less. 

From a practical point of view, the improvement in quality of sanitarymilk, in comparison with the ordinary product is seen in the enhancedkeeping quality. During the Paris Exposition in 1900, milk and creamfrom several such dairies in the United States were shipped to Paris, arriving in good condition after 15 to 18 days transit. When milk hasbeen handled in such a way, it is evident that it is much better suitedto serve as a food supply than where it has undergone the fermentativechanges incident to the development of myriads of organisms. 

~Application of foregoing precautions to all milk producers. ~ Milk is sosusceptible to bacterial changes that it is necessary to protect it frominvasion, if its original purity is to be maintained, and yet, from apractical point of view, the use to which it is destined has much to dowith the care necessary to take in handling. The effect of the bacterialcontamination of milk depends largely upon the way in which the productis used. To the milk-man engaged in the distribution of milk for directconsumption, all bacterial life is more or less of a detriment, while tothe butter-maker and cheese-maker some forms are a direct necessity. Itis unnecessary and impracticable to require the same degree of care inhandling milk destined to be worked up into factory products as is done, for instance, in sanitary milk supplies, but this fact should not beinterpreted to mean that the care of milk for factories is a matter ofsmall consequence. In fact no more important dairy problem exists, andthe purer and better the quality of the raw material the better theproduct will be. Particularly is this true with reference tocheese-making. 

Dairymen have learned many lessons in the severe school of experience, but it is earnestly to be hoped that future conditions will not besummed up in the words of the eminent German dairy scientist, Prof. Fleischmann, when he says that "all the results of scientificinvestigation which have found such great practical application in thetreatment of disease, in disinfection, and in the preservation ofvarious products, are almost entirely ignored in milking. "

~Growth of bacteria in milk. ~ Milk is so well suited as a medium for thedevelopment of germ life that it might be expected that allmicroörganisms would develop rapidly therein, and yet, as a matter offact, growth does not begin at once, even though the milk may be richlyseeded. At ordinary temperatures, such as 70° F. , no appreciableincrease is to be noted for a period of 6-9 hours; at lower temperatures(54° F. ) this period is prolonged to 30-40 hours or even longer. Afterthis period has elapsed, active growth begins and continues more or lessrapidly until after curdling. 

The cause of this suspended development is attributed to the germicidalproperties inherent to the milk. [40]

Milk is of course seeded with a considerable variety of organisms atfirst. The liquefying and inert species are the most abundant, thedistinctively lactic acid class occurring sparsely, if at all. As milkincreases in age, germ growth begins to occur. More or less developmentof all types go on, but soon the lactic species gain the ascendency, owing to their being better suited to this environment; they soonoutstrip all other species, with the result that normal curdlinggenerally supervenes. The growth of this type is largely conditioned bythe presence of the milk sugar. If the sugar is removed from milk bydialysis, the liquid undergoes putrefactive changes due to the fact thatthe putrefactive bacteria are able to grow if no acid is produced. 

~Relation of temperature to growth. ~ When growth does once begin in milk, the temperature at which it is stored exerts the most profound effect onthe rate of development. When milk is not artificially cooled, itretains its heat for some hours, and consequently the conditions becomevery favorable for the rapid multiplication of the contained organisms, as is shown in following results obtained by Freudenreich[41]:

 _No. Of bacteria per cc. In milk kept at different temperatures. _

 77° F. 95° F. 5 hrs. After milking 10, 000 30, 000 8 " " " 25, 000 12, 000, 000 12 " " " 46, 000 35, 280, 000 26 " " " 5, 700, 000 50, 000, 000

[Illustration: FIG. 14. Effect of cooling milk on the growth ofbacteria. ]

Conn[42] is inclined to regard temperature of more significance indetermining the keeping quality than the original infection of the milkitself. Milk which curdled in 18 hours at 98° F. , did not curdle in 48hours at 70°, and often did not change in two weeks, if the temperaturewas kept at 50° F. 

Where kept for a considerable period at this low temperature, the milkbecomes filled with bacteria of the undesirable putrefactive type, thelactic group being unable to form acid in any appreciable amounts. Running well water can be used for cooling, if it is possible to secureit at a temperature of 48°-50° F. The use of ice, of course, givesbetter results, and in summer is greatly to be desired. The influence ofthese lowered temperatures makes it possible to ship milk longdistances[43] by rail for city supplies, if the temperature is kept lowduring transit. 

~Mixing night and morning milk. ~ Not infrequently it happens when old milkis mixed with new, that the course of the fermentative changes is morerapid than would have been the case if the two milks had been keptapart. Thus, adding the cooled night milk to the warm morning milksometimes produces more rapid changes in both. The explanation for thisoften imperfectly understood phenomenon is that germ growth may havegone on in the cooled milk, and when this material is added to thewarmer, but bacteria-poor, fresh milk, the temperature of the whole massis raised to a point suitable for the more rapid growth of all bacteriathan would have occurred if the older milk had been kept chilled. 

~Number of bacteria in milk. ~ The number of organisms found in milkdepends upon (1) the original amount of contamination, (2) the age ofthe milk, and (3) the temperature at which it has been held. Thesefactors all fluctuate greatly in different cases; consequently, the germlife is subject to exceedingly wide variations. Here in America, milkreaches the consumer with less bacteria than in Europe, although it mayoften be older. This is due largely to the more wide-spread use of icefor chilling the milk _en route_ to market. Examinations have been madeof various supplies with the following results: Sedgwick and Batchelderfound in 57 tests of Boston milk from 30, 000-4, 220, 000 per cc. Jordanand Heineman found 30% of samples of Chicago milk to range from 100, 000to 1, 000, 000 while nearly one half were from 1-20, 000, 000 per cc. Thegerm content of city milks increase rapidly in the summer months. Park[44] found 250, 000 organisms per cc. In winter, about 1, 000, 000 incool weather and 5, 000, 000 per cc. In hot summer weather. Knox andBassett in Baltimore report 1, 500, 000 in spring and nearly 4, 500, 000 insummer. Eckles[45] studied milk under factory conditions. He finds from1, 000, 000 to 5, 000, 000 per cc. In winter, and in summer from 15-30millions. 

~Bacterial standards for city supplies. ~ It would be very desirable tohave a hygienic standard for city milk supplies, as there is a butterfat and milk-solid test, but the wide spread variation in germ contentand the impracticability of utilizing ordinary bacterial determinations(on account of time required) makes the selection of such a standarddifficult. Some hold, as Park, that such a standard is feasible. The NewYork City Milk commission has set a standard of 30, 000 bacteria per cc. For their certified milk and 100, 000 per cc. For inspected milk. Rochester, N. Y. Has attempted the enforcement of such a standard(limit, 100, 000 per cc. ) with good results it is claimed while Bostonhas placed the legal limit at 500, 000 per cc. Quantitative standardswould seem more applicable to "certified" or sanitary supplies than togeneral city supplies, where the wide range in conditions lead to suchenormous variations that the bacterial standard seems too refined amethod for practical routine inspection. 

~Other tests. ~ Any test to be of much service must be capable of beingquickly applied. The writer believes for city milk inspectors that theacid test would serve a very useful purpose. This test measures theacidity of the milk. There is, of course, no close and directrelationship between the development of acidity and the growth ofbacteria, yet in a general way one follows the other at normaltemperatures. Where the temperature is kept rather low, bacterial growthmight go on without much acid development, but in the great majority ofcases a high degree of acidity means either old milk, in which there hasbeen a long period of incubation, or high temperature, where rapidbacterial growth has been possible. Either of these conditionsencourages germ growth and thus impairs the quality of the milk. 

The rapid determination of acidity may be made in an approximate mannerso as to serve as a test at the weigh-can or intake. The test is bestmade by the use of the well known alkaline tablet which is composed of asolid alkali, and the indicator, phenolphthalein. The tablets aredissolved in water, one to each ounce used. A number of white cups arefilled with the proper quantity of the solution necessary to neutralizesay, 0. 2 per cent. Lactic acid. Then, as the milk is delivered, theproper quantity is taken from each can to which is added the tabletsolution. A retention of the pink color shows that there is notsufficient acid in the milk to neutralize the alkali used; adisappearance of color indicates an excess of acid. The standardselected is of course arbitrarily chosen. In our experience, 0. 2 percent. Acidity (figured as lactic), has proven a satisfactory point. Withcarefully handled milk the acidity ought to be reduced to about 0. 15 percent. The acidity of the milk may be abnormally reduced if milk is keptin rusty cans, owing to action of acid on the metal. 

[Illustration: FIG. 15. Apparatus used in making rapid acid test. Adefinite quantity of the alkali solution and indicator is placed in thewhite tea cup. To this is added the quantity of milk by means of thecartridge measure which would just be neutralized if the acidity was 0. 2per cent. A retention of the pink color shows a low acid milk; itsdisappearance, a high acid milk. ]

~Kinds of bacteria in milk. ~ The number of bacteria in milk is not of somuch consequence as the kinds present. With reference to the number ofdifferent species, the more dirt and foreign matter the milk contains, the larger the number of varieties found in the same. While milk maycontain forms that are injurious to man, still the great majority ofthem have no apparent effect on human health. In their effect on milk, the case is much different. Depending upon their action in milk, theymay be grouped into three classes:

1. Inert group, those producing no visible change in the milk. 

2. Sour milk forms, those breaking up the milk sugar with or without theformation of gas. 

3. Digesting or peptonizing group, those capable of rendering the caseinof milk soluble and more or less completely digested. 

A surprisingly large number of bacteria that are found in milk belong tothe first class. Undoubtedly they affect the chemical characteristics ofthe milk somewhat, but not to the extent that it becomes physicallyperceptible. Eckles[46] reports in a creamery supply from 20 to 55 percent. Of entire flora as included in this class. 

By far the most important group is that embraced under the second head. It includes not only the true lactic acid types in which no gas isformed, but those species capable of producing gases and various kindsof acids. These organisms are the distinctively milk bacteria, althoughthey do not predominate when the milk is first drawn. Their adaptationto this medium is normally shown, however, by this extremely rapidgrowth, in which they soon gain the ascendency over all other speciespresent. It is to this lactic acid class that the favorableflavor-producing organisms belong which are concerned in butter-making. They are also indispensable in cheese-making. 

The third class represents those capable of producing a liquefied ordigested condition on gelatin or in milk. They are usually thespore-bearing species which gain access from filth and dirt. Their highpowers of resistance due to spores makes it difficult to eradicate thistype, although they are materially held in subjection by the lacticbacteria. The number of different kinds that have been found in milk isquite considerable, something over 200 species having been describedmore or less thoroughly. In all probability, however, many of theseforms will be found to be identical when they are subjected to a morecritical study. 

~Direct absorption of taints. ~ A tainted condition in milk may result fromthe development of bacteria, acting upon various constituents of themilk, and transforming these in such a way as to produce by-productsthat impair the flavor or appearance of the liquid; or it may beproduced by the milk being brought in contact with any odoriferous oraromatic substance, under conditions that permit of the directabsorption of such odors. 

This latter class of taints is entirely independent of bacterial action, and is largely attributable to the physical property which milkpossesses of being able to absorb volatile odors, the fat in particular, having a great affinity for many of these substances. This directabsorption may occur before the milk is withdrawn from the animal, orafterwards if exposed to strong odors. 

It is not uncommon for the milk of animals advanced in lactation to havea more or less strongly marked odor and taste; sometimes this is apt tobe bitter, at other times salty to the taste. It is a defect that ispeculiar to individual animals and is liable to recur at approximatelythe same period in lactation. 

The peculiar "cowy" or "animal odor" of fresh milk is an inherentpeculiarity that is due to the direct absorption of volatile elementsfrom the animal herself. This condition is very much exaggerated whenthe animal consumes strong-flavored substances as garlic, leeks, turnipsand cabbage. The volatile substances that give to these vegetables theircharacteristic odor are quickly diffused through the system, and if suchfoods are consumed some few hours before milking, the odor in the milkwill be most pronounced. The intensity of such taints is diminishedgreatly and often wholly disappears, if the milking is not done for somehours (8-12) after such foods are consumed. 

This same principle applies in lesser degree to many green fodders thatare more suitable as feed for animals, as silage, green rye, rape, etc. Not infrequently, such fodders as these produce so strong a taint inmilk as to render it useless for human use. Troubles from such sourcescould be entirely obviated by feeding limited quantities of suchmaterial immediately after milking. Under such conditions the taintproduced is usually eliminated before the next milking. The milk ofswill-fed cows is said to possess a peculiar taste, and the urine ofanimals fed on this food is said to be abnormally acid. Brewers' grainsand distillery slops when fed in excess also induce a similar conditionin the milk. 

Milk may also acquire other than volatile substances directly from theanimal, as in cases where drugs, as belladonna, castor oil, sulfur, turpentine, jalap, croton oil, and many others have been used asmedicine. Such mineral poisons as arsenic have been known to appeareight hours after ingestion, and persist for a period of three weeksbefore being eliminated. 

~Absorption of odors after milking. ~ If milk is brought in contact withstrong odors after being drawn from the animal, it will absorb themreadily, as in the barn, where frequently it is exposed to the odor ofmanure and other fermenting organic matter. 

It has long been a popular belief that milk evolves odors and cannotabsorb them so long as it is warmer than the surrounding air, but fromexperimental evidence, the writer[47] has definitely shown that thedirect absorption of odors takes place much more rapidly when the milkis warm than when cold, although under either condition, it absorbsvolatile substances with considerable avidity. In this test fresh milkwas exposed to an atmosphere impregnated with odors of various essentialoils and other odor-bearing substances. Under these conditions, thecooler milk was tainted very much less than the milk at body temperatureeven where the period of exposure was brief. It is therefore evidentthat an exposure in the cow barn where the volatile emanations from theanimals themselves and their excreta taint the air will often result inthe absorption of these odors by the milk to such an extent as toseriously affect the flavor. 

The custom of straining the milk in the barn has long been deprecated asinconsistent with proper dairy practice, and in the light of the aboveexperiments, an additional reason is evident why this should not bedone. 

Even after milk is thoroughly cooled, it may absorb odors as seen wherethe same is stored in a refrigerator with certain fruits, meats, fish, etc. 

~Distinguishing bacterial from non-bacterial taints. ~ In perfectly freshmilk, it is relatively easy to distinguish between taints caused by thegrowth of bacteria and those attributable to direct absorption. 

If the taint is evident at time of milking, it is in all probability dueto character of feed consumed, or possibly to medicines. If, however, the intensity of the taint grows more pronounced as the milk becomesolder, then it is probably due to living organisms, which require acertain period of incubation before their fermentative properties aremost evident. 

Moreover, if the difficulty is of bacterial origin, it can be frequentlytransferred to another lot of milk (heated or sterilized is preferable)by inoculating same with some of the original milk. Not all abnormalfermentations are able though to compete with the lactic acid bacteria, and hence outbreaks of this sort soon die out by the re-establishment ofmore normal conditions. 

~Treatment of directly absorbed taints. ~ Much can be done to overcometaints of this nature by exercising greater care in regard to the feedof animals, and especially as to the time of feeding and milking. Butwith milk already tainted, it is often possible to materially improveits condition. Thorough aeration has been frequently recommended, butmost satisfactory results have been obtained where a combined process ofaeration and pasteurization was resorted to. Where the milk is used inmaking butter, the difficulty has been successfully met by washing thecream with twice its volume of hot water in which a little saltpeter hasbeen dissolved (one teaspoonful per gallon), and then separating itagain. [48]

The treatment of abnormal conditions due to bacteria has been givenalready under the respective sources of infection, and is also stillfurther amplified in following chapter. 

~Aeration. ~ It is a common belief that aeration is of great aid inimproving the quality of milk, yet, when closely studied, no materialimprovement can be determined, either where the milk is made into butteror sold as milk. Dean in Canada and Storch in Denmark have bothexperimented on the influence of aeration in butter making, but withnegative results. Marshall and Doane failed to observe any materialimprovement in keeping quality, but it is true that odors are eliminatedfrom the milk during aeration. The infection of the milk during aerationoften more than counterbalances the reputed advantage. Especially isthis so if the aeration is carried out in an atmosphere that is notperfectly clean and pure. 

In practice aeration differs greatly. In some cases, air is forced intothe milk; in others, the milk is allowed to distribute itself in a thinsheet over a broad surface and fall some distance so that it is broughtintimately in contact with the air. This latter process is certainlymuch more effective if carried out under conditions which precludeinfection. It must be remembered that aeration is frequently combinedwith cooling, in which case the reputed advantages may not be entirelyattributable to the process of aeration. 

~Infection of milk in the factory. ~ The problem of proper handling of milkis not entirely solved when the milk is delivered to the factory orcreamery, although it might be said that the danger of infection is muchgreater while the milk is on the farm. 

In the factory, infection can be minimized because effective measures ofcleanliness can be more easily applied. Steam is available in mostcases, so that all vats, cans, churns and pails can be thoroughlyscalded. Special emphasis should be given to the matter of cleaningpumps and pipes. The difficulty of keeping these utensils clean oftenleads to neglect and subsequent infection. In Swiss cheese factories, the custom of using home-made rennet solutions is responsible forconsiderable factory infection. Natural rennets are soaked in whey whichis kept warm in order to extract the rennet ferment. This solution whenused for curdling the milk often adds undesirable yeasts and othergas-generating organisms, which are later the cause of abnormal fermentaction in the cheese (See page 186). 

The influence of the air on the germ content of the milk is, as a rule, overestimated. If the air is quiet, and free from dust, the amount ofgerm life in the same is not relatively large. In a creamery or factory, infection from this source ought to be much reduced, for the reason thatthe floors and wall are, as a rule, quite damp, and hence germ lifecannot easily be dislodged. The majority of organisms found under suchconditions come from the person of the operators and attendants. Anyinfection can easily be prevented by having the ripening cream-vatscovered with a canvas cloth. The clothing of the operator should bedifferent from the ordinary wearing-apparel. If made of white duck, thepresence of dirt is more quickly recognized, and greater care willtherefore be taken than if ordinary clothes are worn. 

The surroundings of the factory have much to do with the danger of germinfection. Many factories are poorly constructed and the drainage ispoor, so that filth and slime collect about and especially under thefactory. The emanations from these give the peculiar "factory odor" thatindicates fermenting matter. Not only are these odors absorbeddirectly, but germ life from the same is apt to find its way into themilk. Connell[49] has recently reported a serious defect in cheese thatwas traced to germ infection from defective factory drains. 

The water supply of a factory is also a question of prime importance. When taken from a shallow well, especially if surface drainage from thefactory is possible, the water may be contaminated to such an extent asto introduce undesirable bacteria in such numbers that the normal courseof fermentation may be changed. The quality of the water, aside fromflavor, can be best determined by making a curd test (p. 76) which isdone by adding some of the water to boiled milk and incubating the same. If "gassy" fermentations occur, it signifies an abnormal condition. Indeep wells, pumped as thoroughly as is generally the case with factorywells, the germ content should be very low, ranging from a few score toa few hundred bacteria per cc. At most. 

Harrison[50] has recently traced an off-flavor in cheese in a Canadianfactory to an infection arising from the water-supply. He found the samegerm in both water and cheese and by inoculating a culture intopasteurized milk succeeded in producing the undesirable flavor. Thedanger from ice is much less, for the reason that good dairy practicedoes not sanction using ice directly in contact with milk or cream. Then, too, ice is largely purified in the process of freezing, althoughif secured from a polluted source, reliance should not be placed in themethod of purification; for even freezing does not destroy allvegetating bacteria. 

FOOTNOTES:

[1] Olson. 24 Rept. Wis. Expt. Stat. , 1907. 

[2] Erf and Melick Bull. 131, Kan. Expt. Stat. , Apr. 1905. 

[3] Storch (40 Rept. Danish Expt. Stat. , Copenhagen, 1898) has devised atest whereby it can be determined whether this treatment has beencarried out or not: Milk contains a soluble enzym known as galactasewhich has the property of decomposing hydrogen peroxid. If milk isheated to 176° F. (80° C. ) or above, this enzym is destroyed so that theabove reaction no longer takes place. If potassium iodid and starch areadded to unheated milk and the same treated with hydrogen peroxid, thedecomposition of the latter agent releases oxygen which acts on thepotassium salt, which in turn gives off free iodine that turns thestarch blue. 

[4] McKay, N. Y. Prod. Rev. , Mch. 22, 1899. 

[5] Doane, Bull. 79, Md. Expt. Stat. , Jan. 1902. 

[6] Harrison, 22 Rept. Ont. Agr'l Coll. , 1896, p. 113. 

[7] Moore and Ward, Bull. 158, Cornell Expt. Stat. , Jan. 1899; Ward, Bull. 178, Cornell Expt. Stat. , Jan. 1900. 

[8] Harrison, 22 Rept. Ont. Agr. Coll. , 1896, p. 108; Moore, 12 Rept. Bur. Animal Ind. , U. S. Dept. Ag. , 1895-6, p. 261. 

[9] Moore, Bacteria in Milk, N. Y. Dept. Ag. , 1902. 

[10] Freudenreich, Cent. F. Bakt. , II Abt. , 10: 418, 1903. 

[11] Harrison, 22 Rept. Ont. Agr. Coll. , 1896, p. 108. 

[12] Marshall, Bull. 147, Mich. Expt. Stat. , p. 42. 

[13] Moore and Ward, Bull. 158, Cornell Expt. Stat. , Jan. 1899. 

[14] Burr, R. H. Cent. F. Bakt. , II Abt. , 8: 236, 1902. Freudenreich, l. C. P. 418. Ward, Bull. 178, Cornell Expt. Stat. , p. 277. Bolley (Cent. F. Bakt. , II Abt. , 1: 795, 1895), in 30 experiments found 12 out of 16species to belong to lactic class. Harrison (Trans. Can. Inst. , 7: 474, 1902-3) records the lactic type as most commonly present. 

[15] Ford, Journ. Of Hyg. , 1901, 1: 277. 

[16] Freudenreich, l. C. P. 421. 

[17] Stocking, Bull. 42, Storrs Expt. Stat. , June, 1906. 

[18] Dinwiddie, Bull, 45 Ark. Expt. Stat. , p. 57. Ward, Journ. Appld. Mic. 1: 205, 1898. Appel, Milch Zeit. , No. 17, 1900. Harrison andCumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis. Expt. Stat. , 158, 1904. 

[19] Fokker, Zeit. F. Hyg. , 9: 41, 1890. 

[20] Freudenreich, Ann. De Microg. , 3: 118, 1891. 

[21] Hunziker, Bull. 197, Cornell Expt. Stat. , Dec. 1901. 

[22] Freudenreich, Cent. F. Bakt. , II Abt. , 10: 417, 1903. 

[23] This general statement is in the main correct, although Ford(Journ. Of Hyg. , 1: 277, 1901) claims to have found organisms sparinglypresent in healthy tissues. 

[24] Backhaus, Milch Zeit. , 26: 357, 1897. 

[25] Freudenreich, Die Bakteriologie, p. 30. 

[26] Stocking, Bull. 42, Storrs Expt. Stat. , June 1906. 

[27] Harrison, Cent. F. Bakt. , II Abt. , 5: 183, 1899. 

[28] Drysdale, Trans. High. And Agr. Soc. Scotland. 5 Series, 10: 166, 1898. 

[29] Schuppan, (Cent. F. Bakt. , 13: 155, 1893) claims to have found areduction of 48 per cent. In the Copenhagen filters while in the moreextended work of Dunbar and Kister (Milch Zeit. , pp. 753, 787, 1899) thebacterial content was higher in the filtered milk in 17 cases out of 22. 

[30] Backhaus and Cronheim, Journ. F. Landw. , 45: 222, 1897. 

[31] Eckles and Barnes, Bull. 159 Iowa Expt. Stat. , Aug. 1901. 

[32] Dunbar and Kister, Milch Zeit. , p. 753, 1899. Harrison and Streit, Trans. Can. Inst. , 7: 488, 1902-3. 

[33] Doane, Bull. 88 Md. Expt. Stat. , May 1903. 

[34] Eckles, Hoard's Dairyman, July 8, 1898. 

[35] Fraser, Bull. 91, Ill. Expt. Stat. 

[36] Fraser, Bull. 91, Ill. Expt. Stat. , Dec. 1903. 

[37] Stocking, Bull. 42, Storrs Expt. Stat. , June, 1906. 

[38] Backhaus. Ber. Landw. Inst. Univ. Königsberg 2: 12, 1897. 

[39] De Schweinitz, Nat. Med. Rev. , April, 1899. 

[40] Conn, Proc. Soc. Amer. Bacteriologists, 1902. 

[41] Freudenreich, Ann. De Microg. , 2:115, 1890. 

[42] Conn, Bull. 26, Storrs Expt. Stat. 

[43] New York City is supplied with milk that is shipped 350 miles. 

[44] Park, N. Y. Univ. Bull. , 1: 85, 1901. 

[45] Eckles, Bull. 59, Iowa Expt. Stat. , Aug. 1901. 

[46] Eckles, Bull. 59, Iowa Expt. Stat. , Aug. 1901. 

[47] Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104. 

[48] Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. Of Bot. ). 

[49] Connell, Rept. Of Commissioner of Agr. , Canada, 1897, part XVI, p. 15. 

[50] Harrison, Hoard's Dairyman, March 4, 1898. 

CHAPTER IV. 

FERMENTATIONS IN MILK AND THEIR TREATMENT. 

Under the conditions in which milk is drawn, it is practicallyimpossible to secure the same without bacterial contamination. Theresult of the introduction of these organisms often changes itscharacter materially as most bacteria cause the production of more orless pronounced fermentative processes. Under normal conditions, milksours, i. E. , develops lactic acid, but at times this more commonfermentation may be replaced by other changes which are marked by theproduction of some other more or less undesirable flavor, odor or changein appearance. 

In referring to these changes, it is usually customary to designate themafter the most prominent by-product formed, but it must be kept in mindthat generally some other decomposition products are usually produced. Whether the organisms producing this or that series of changes prevailor not depends upon the initial seeding, and the conditions under whichthe milk is kept. Ordinarily, the lactic acid organisms grow soluxuriantly in the milk that they overpower all competitors and sodetermine the nature of the fermentation; but occasionally the milkbecomes infected with other types of bacteria in relatively largenumbers and the conditions may be especially suitable to the developmentof these forms, thereby modifying the course of the normal changes thatoccur. 

The kinds of bacteria that find it possible to develop in milk may beincluded under two heads:

1. Those which cause no appreciable change in the milk, either in taste, odor or appearance. While these are frequently designated as the inertbacteria, it must not be supposed that they have absolutely no effect onmilk. It is probably true in most cases that slight changes of achemical nature are produced, but the nature of the changes do notpermit of ready recognition. 

2. This class embraces all those organisms which, as a result of theirgrowth, are capable of producing evident changes. These transformationsmay be such as to affect the taste, as in the sour milk or in the bitterfermentations, or the odor, as in some of the fetid changes, or theappearance of the milk, as in the slimy and color changes laterdescribed. 

~Souring of milk. ~ Ordinarily if milk is allowed to stand for several daysat ordinary temperatures it turns sour. This is due to the formation oflactic acid, which is produced by the decomposition of the milk-sugar. While this change is well nigh universal, it does not occur without apre-existing cause, and that is the presence of certain living bacterialforms. These organisms develop in milk with great rapidity, and thedecomposition changes that are noted in souring are due to theby-products of their development. 

The milk-sugar undergoes fermentation, the chief product being lacticacid, although various other by-products, as other organic acids(acetic, formic and succinic), different alcohols and gaseous products, as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts. 

In this fermentation, the acidity begins to be evident to the taste whenit reaches about 0. 3 per cent. , calculated as lactic acid. As theformation of acid goes on, the casein is precipitated and incipientcurdling or lobbering of the milk occurs. This begins to be apparentwhen the acidity is about 0. 4 per cent. , but the curd becomes more solidwith increasing acidity. The rapidity of curdling is also dependent uponthe temperature of the milk. Thus milk which at ordinary temperaturesmight remain fluid often curdles when heated. The growth of the bacteriais continued until about 0. 8 to 1. 0 per cent. Acid is formed, althoughthe maximum amount fluctuates considerably with different lactic acidspecies. Further formation then ceases even though all of the milk-sugaris not used up, because of the inability of the lactic bacteria tocontinue their growth in such acid solutions. 

As this acidity is really in the milk serum, cream never develops somuch acid as milk, because a larger proportion of its volume is made upof butter-fat globules. This fact must be considered in the ripening ofcream in butter-making where the per cent. Of fat is subject to widefluctuations. 

The formation of lactic acid is a characteristic that is possessed by alarge number of bacteria, micrococci as well as bacilli being numerouslyrepresented. Still the preponderance of evidence is in favor of the viewthat a few types are responsible for most of these changes. The mostcommon type found in spontaneously soured milk changes the milk-sugarinto lactic acid without the production of any gas. This type has beendescribed by various workers on European as well as American milks, andis designated by Conn as the _Bact. Lactis acidi_ type. [51] It issubject to considerable variation under different conditions. 

Curiously enough if milk which has been drawn with special care isexamined immediately after milking, the lactic organisms are not usuallyfound. They are incapable of development in the udder itself, as shownby injections into the milk cistern. They abound, however, on hay, indust, in the barn air, on the hairy coat of the animal, and from thesesources easily gain access to the milk. In this medium they find anexceptionally favorable environment and soon begin a very rapid growth, so that by the time milk is consumed, either in the form of milk or milkproducts, they make up numerically the larger portion of the bacteriapresent. 

Another widely disseminated, although numerically less prevalent, typeis _B. Lactis aerogenes_. This type forms gas in milk so that the souredmilk is torn by the presence of gas bubbles. It also grows moreluxuriantly in contact with the air. 

Other types occur more or less sporadically, some of which are capableof liquefying the casein of milk while at the same time they alsodevelop lactic acid. Conn and Aikman refer to the fact that over onehundred species capable of producing variable quantities of lactic acidare already known. It is fair to presume, however, that a carefulcomparative study of these would show that simply racial differencesexist in many cases, and therefore, that they are not distinct species. 

As a group these bacteria are characterized by their inability toliquefy gelatin or develop spores. On account of this lattercharacteristic they are easily destroyed when milk is pasteurized. Theylive under aerobic or anaerobic conditions, many of them being able togrow in either environment, although, according to McDonnell, [52] theyare more virulent when air is not excluded. 

While growth of these lactic forms may go on in milk throughout arelatively wide range in temperature, appreciable quantities of acid arenot produced except very slowly at temperatures below 50° F. [53]

From the standpoint of frequency the most common abnormal changes thatoccur in milk are those in which gases of varying character aredeveloped in connection with acids, from the milk sugar. Other volatileproducts imparting bad flavors usually accompany gas production. Thesefermentations are of most serious import in the cheese industry, as theyare especially prone to develop in the manufacture of milk into certaintypes of cheese. Not often is their development so rapid that theyappear in the milk while it is yet in the hands of the milk producer, but almost invariably the introduction of the causal organisms takesplace while the milk is on the farm. Numerous varieties of bacteriapossess this property of producing gas (H and CO_{2} are most commonalthough N and methane (CH_{4}) are sometimes produced). The more commonforms are those represented by _B. Lactis aerogenes_ and the commonfecal type, _B. Coli commune_. The ordinary habitat of this type is dirtand intestinal filth. Hence careless methods of milk handling invitethis type of abnormal change in milk. 

It is a wide-spread belief that thunder storms cause milk to sourprematurely, but this idea has no scientific foundation. Experiments[54]with the electric spark, ozone and loud detonations show no effect onacid development, but the atmospheric conditions usually incident to athunder storm are such as permit of a more rapid growth of organisms. There is no reason to believe but that the phenomenon of souring iswholly related to the development of bacteria. Sterile milks are neveraffected by the action of electric storms. 

~"Gassy" milks. ~ Where these gas bacteria abound, the amount of lacticacid is generally reduced, due to the splitting up of some of the sugarinto gaseous products. This type of germ life does not seem to be ableto develop well in the presence of the typical lactic acid nongas-forming bacteria. 

[Illustration: FIG. 16. Cheese made from "gassy" milk. ]

~"Sweet curdling" and digesting fermentations. ~ Not infrequently milk, instead of undergoing spontaneous souring, curdles in a weakly acid orneutral condition, in which state it is said to have undergone "sweetcurdling. " The coagulation of the milk is caused by the action of enzymsof a rennet type that are formed by the growth of various species ofbacteria. Later the whey separates more or less perfectly from the curd, producing a "wheyed off" condition. Generally the coagulum in thesecases is soft and somewhat slimy. The curd usually diminishes in bulk, due to the gradual digestion or peptonization of the casein byproteid-dissolving enzyms (tryptic type) that are also produced by thebacteria causing the change. 

A large number of bacteria possess the property of affecting milk inthis way. So far as known they are able to liquefy gelatin (also apeptonizing process) and form spores. The Tyrothrix type of bacteria (sonamed by Duclaux on account of the supposed relation to cheese ripening)belongs to this class. The hay and potato forms are also digesters. Organisms of this type are generally associated with filth and manure, and find their way into the milk from the accumulations on the coat ofthe animal. 

Conn[55] has separated the rennet enzym from bacterial cultures in arelatively pure condition, while Fermi[56] has isolated the digestiveferment from several species. 

Duclaux[57] has given to this digesting enzym the name _casease_ orcheese ferment. These isolated ferments when added to fresh milk possessthe power of causing the characteristic curdling and subsequentdigestion quite independent of cell development. The quantity of fermentproduced by different species differs materially in some cases. In thesedigestive fermentations, the chemical transformations are profound, thecomplex proteid molecule being broken down into albumoses, peptones, amido-acids (tyrosin and leucin) and ammonia as well as fatty acids. 

Not infrequently these fermentations gain the ascendency over the normalsouring change, but under ordinary conditions they are held in abeyance, although this type of bacteria is always present to some extent in milk. When the lactic acid bacteria are destroyed, as in boiled, sterilizedor pasteurized milk, these rennet-producing, digesting species develop. 

~Butyric acid fermentations. ~ The formation of butyric acid in milk whichmay be recognized by the "rancid butter" odor is not infrequently seenin old, sour milk, and for a long time was thought to be a continuationof the lactic fermentation, but it is now believed that these organismsfind more favorable conditions for growth, not so much on account of thelactic acid formed as in the absence of dissolved oxygen in the milkwhich is consumed by the sour-milk organisms. 

Most of the butyric class of bacteria are spore-bearing, and hence theyare frequently present in boiled or sterilized milk. The by-productsformed in this series of changes are quite numerous. In most cases, butyric acid is prominent, but in addition to this, other organic acids, as lactic, succinic, and acetic, are produced, likewise differentalcohols. Concerning the chemical origin of butyric acid there is yetsome doubt. Duclaux[58] affirms that the fat, sugar and casein are alldecomposed by various forms. In some cases, the reaction of the milk isalkaline, with other species it may be neutral or acid. This type offermentation has not received the study it deserves. 

In milk these organisms are not of great importance, as thisfermentation does not readily gain the ascendency over the lacticbacteria. 

~Ropy or slimy milk. ~ The viscosity of milk is often markedly increasedover that which it normally possesses. The intensity of this abnormalcondition may vary much; in some cases the milk becoming viscous orslimy; in others stringing out into long threads, several feet inlength, as in Fig. 17. Two sets of conditions are responsible for theseropy or slimy milks. The most common is where the milk is clotted orstringy when drawn, as in some forms of garget. This is generally due tothe presence of viscid pus, and is often accompanied by a bloodydischarge, such a condition representing an inflamed state of the udder. Ropiness of this character is not usually communicable from one lot ofmilk to another. 

[Illustration: FIG. 17. Ropy milk. ]

The communicable form of ropy milk only appears after the milk has beendrawn from the udder for a day or so, and is caused by the developmentof various species of bacteria which find their way into the milk afterit is drawn. These defects are liable to occur at any season of theyear. Their presence in a dairy is a source of much trouble, as theunsightly appearance of the milk precludes its use as food, althoughthere is no evidence that these ropy fermentations are dangerous tohealth. 

There are undoubtedly a number of different species of bacteria that arecapable of producing these viscid changes, [59] but it is quite probablethat they are not of equal importance in infecting milk under naturalconditions. 

In the majority of cases studied in this country, [60] the causalorganism seems to be _B. Lactis viscosus_, a form first found by Adametzin surface waters. [61] This organism possesses the property ofdeveloping at low temperatures (45°-50° F. ), and consequently it isoften able in winter to supplant the lactic-acid forms. Ward has foundthis germ repeatedly in water tanks where milk cans are cooled; andunder these conditions it is easy to see how infection of the milk mightoccur. Marshall[62] reports an outbreak which he traced to an externalinfection of the udder; in another case, the slime-forming organism wasabundant in the barn dust. A defect of this character is oftenperpetuated in a dairy for some time, and may therefore becomeexceedingly troublesome. In one instance in the writer's experience, amilk dealer lost over $150 a month for several months from ropy cream. Failure to properly sterilize cans, and particularly strainer cloths, isfrequently responsible for a continuance of trouble of this sort. 

The slimy substance formed in milk comes from various constituents ofthe milk, and the chemical character of the slime produced also varieswith different germs. In some cases the slimy material is merely theswollen outer cell membrane of the bacteria themselves as in the case of_B. Lactis viscosus_; in others it is due to the decomposition of theproteids, but often the chief decomposition product appears to come froma viscous fermentation of the milk-sugar. 

An interesting case of a fermentation of this class being utilized indairying is seen in the use of "lange wei" (long or stringy whey) whichis employed as a starter in Holland to control the gassy fermentationsin Edam cheese. This slimy change is due to the growth of_Streptococcus Hollandicus_. [63]

~Alcoholic fermentations. ~ Although glucose or cane-sugar solutions areextremely prone to undergo alcoholic fermentation, milk sugar does notreadily undergo this change. Where such changes are produced it is dueto yeasts. Several outbreaks attributable to such a cause have beenreported. [64] Russell and Hastings[65] have found these milk-sugarsplitting yeasts particularly abundant in regions where Swiss cheese ismade, a condition made possible by the use of whey-soaked rennets inmaking such cheese. 

Kephir and Koumiss are liquors much used in the Orient which are madefrom milk that has undergone alcoholic fermentation. Koumiss wasoriginally made from mare's milk but is now often made from cows' milkby adding cane sugar and yeast. In addition to the CO_{2} developed, alcohol, lactic acid, and casein-dissolving ferments are formed. Kephiris made by adding to milk Kephir grains, which are a mass of yeast andbacterial cells. The yeasts produce alcohol and CO_{2} while thebacteria change the casein of milk, rendering it more digestible. Thesebeverages are frequently recommended to persons who seem to be unable todigest raw milk readily. The exact nature of the changes produced arenot yet well understood. [66]

~Bitter milk. ~ The presence of bitter substances in milk may be ascribedto a variety of causes. A number of plants, such as lupines, ragweed andchicory, possess the property of affecting milk when the same areconsumed by animals. At certain stages in lactation, a bitter saltytaste is occasionally to be noted that is peculiar to individualanimals. 

A considerable number of cases of bitter milk have, however, been tracedto bacterial origin. For a number of years the bitter fermentation ofmilk was thought to be associated with the butyric fermentation, butWeigmann[67] showed that the two conditions were not dependent upon eachother. He found that the organism which produced the bitter taste actedupon the casein. 

Conn[68] observed a coccus form in bitter cream that was able to imparta bitter flavor to milk. Sometimes a bitter condition does not developin the milk, but may appear later in the milk products, as in the caseof a micrococcus which Freudenreich[69] found in cheese. 

Harrison[70] has traced a common bitter condition in Canadian milk to amilk-sugar splitting yeast, _Torula amara_ which not only grows rapidlyin milk but produces an undesirable bitterness in cheddar cheese. 

Cream ripened at low temperatures not infrequently develops a bitterflavor, showing that the optimum temperature for this type offermentation is below the typical lactic acid change. 

Milk that has been heated often develops a bitter condition. Theexplanation of this is that the bacteria producing the bitter substancesusually possess endospores, and that while the boiling or sterilizing ofmilk easily kills the lactic acid germs, these forms on account of theirgreater resisting powers are not destroyed by the heat. 

~Soapy milk:~ A soapy flavor in milk was traced by Weigmann and Zirn[71]to a specific bacillus, _B. Lactis saponacei_, that they found gainedaccess to the milk in one case from the bedding and in another instancefrom hay. A similar outbreak has been reported in this country, [72] dueto a germ acting on the casein and albumen. 

~Red milk. ~ The most common trouble of this nature in milk is due topresence of blood, which is most frequently caused by some wound in theudder. The ingestion of certain plants as sedges and scouring rushes isalso said to cause a bloody condition; madders impart a reddish tingedue to coloring matter absorbed. Defects of this class can be readilydistinguished from those due to germ growth because they are apparent attime of milking. Where blood is actually present, the corpuscles settleout in a short time if left undisturbed. 

There are a number of chromogenic or color-producing bacteria that areable to grow in milk, but their action is so slow that generally theyare not of much consequence. Moreover their development is usuallyconfined to the surface of the milk as it stands in a vessel. The mostimportant is the well-known _B. Prodigiosus_. Another form found attimes in milk possessing low acidity[73] is _B. Lactis erythrogenes_. This species only develops the red color in the dark. In the light, itforms a yellow pigment. Various other organisms have been reported atdifferent times. [74]

~Blue milk. ~ Blue milk has been known for many years, its communicablenature being established as long ago as 1838. It appears on the surfaceof milk first as isolated particles of bluish or grey color, whichlater become confluent, the blue color increasing in intensity as theacidity increases. The causal organism, _B. Cyanogenes_, is veryresistant toward drying, [75] thus accounting for its persistence. InMecklenberg an outbreak of this sort once continued for several years. It has frequently been observed in Europe in the past, but is not now sooften reported. Occasional outbreaks have been reported in this country. 

~Other kinds of colored milk. ~ Two or three chromogenic forms producingstill other colors have occasionally been found in milk. Adametz[76]discovered in a sample of cooked milk a peculiar form (_Bacillussynxanthus_) that produced a citron-yellow appearance which precipitatedand finally rendered soluble the casein. Adametz, Conn, and List havedescribed other species that confer tints of yellow on milk. Some ofthese are bright lemon, others orange, and some amber in color. 

Still other color-producing bacteria, such as those that produce violetor green changes in the milk, have been observed. In fact, almost any ofthe chromogenic bacteria are able to produce their color changes in milkas it is such an excellent food medium. Under ordinary conditions, thesedo not gain access to milk in sufficient numbers so that they modify theappearance of it except in occasional instances. 

~Treatment of abnormal fermentations. ~ If the taint is recognized as ofbacterial origin (see p. 57) and is found in the mixed milk of the herd, it is necessary to ascertain, first, whether it is a general trouble, orrestricted to one or more animals. This can sometimes be done byseparating the milk of the different cows and noting whether anyabnormal condition develops in the respective samples. 

~Fermentation tests. ~ The most satisfactory way to detect the presence ofthe taints more often present is to make a fermentation test of one kindor another. These tests are most frequently used at the factory, toenable the maker to detect the presence of milk that is likely to proveunfit for use, especially in cheese making. They are based upon theprinciple that if milk is held at a moderately high temperature, thebacteria will develop rapidly. A number of different methods have beendevised for this purpose. In Walther's lacto-fermentator samples of milkare simply allowed to stand in bottles or glass jars until they sour. They are examined at intervals of several hours. If the curdled milk ishomogeneous and has a pure acid smell, the milk is regarded as allright. If it floats in a turbid serum, is full of gas or ragged holes, it is abnormal. As generally carried out, no attempt is made to havethese vessels sterile. Gerber's test is a similar test that has beenextensively employed in Switzerland. Sometimes a few drops of rennet areadded to the milk so as to curdle the same, and thus permit of the moreready detection of the gas that is evolved. 

~Wisconsin curd test. ~ The method of testing milk described below wasdevised at the Wisconsin Experiment Station in 1895 by Babcock, Russelland Decker. [77] It was used first in connection with experimental workon the influence of gas-generating bacteria in cheese making, but itsapplicability to the detection of all taints in milk produced bybacteria makes it a valuable test for abnormal fermentations in general. 

In the curd test a small pat of curd is made in a glass jar from eachsample of milk. These tests may be made in any receptacle that has beencleaned in boiling water, and to keep the temperature more nearlyuniform these jars should be immersed in warm water, as in a wash tub orsome other receptacle. When the milk is about 95° F. , about ten drops ofrennet extract are added to each sample and mixed thoroughly with themilk. The jars should then remain undisturbed until the milk iscompletely curdled; then the curd is cut into small pieces with a caseknife and stirred to expel the whey. The whey should be poured off atfrequent intervals until the curd mats. If the sample be kept at bloodheat (98° F. ) for six to eight hours, it will be ready to examine. 

[Illustration: FIG. 18. Improved bottles for making curd test. _A_, testbottle complete; _B_, bottle showing construction of cover; _S_, sieveto hold back the curd when bottle is inverted; _C_, outer cover with _(DH)_ drain holes to permit of removal of whey. ]

More convenient types of this test than the improvised apparatus justalluded to have been devised by different dairy manufacturers. Generally, they consist of a special bottle having a full-sized top, thus permitting the easy removal of the curd. The one shown in Fig. 18is provided with a sieve of such construction that the bottles willdrain thoroughly if inclined in an inverted position. 

~Interpretation of results of test. ~ The curd from a good milk has a firm, solid texture, and should contain at most only a few small pin holes. Itmay have some large, irregular, "mechanical" holes where the curdparticles have failed to cement, as is seen in Fig. 19. If gas-producingbacteria are very prevalent in the milk, the conditions under which thetest is made cause such a rapid growth of the same that the evidence ofthe abnormal fermentation may be readily seen in the spongy texture ofthe curd (Fig. 20). If the undesirable organisms are not very abundantand the conditions not especially suited to their growth, the "pinholes" will be less frequent. 

[Illustration: FIG. 19. Curd from a good milk. The large irregular holesare mechanical. ]

Sometimes the curds show no evidence of gas, but their abnormalcondition can be recognized by the "mushy" texture and the presence of"off" flavors that are rendered more apparent by keeping them in closedbottles. This condition is abnormal and is apt to produce quite asserious results as if gas was formed. 

~Overcoming taints by use of starters. ~ Another method of combattingabnormal fermentations that is often fruitful, is that which rests uponthe inability of one kind of bacteria to grow in the same medium incompetition with certain other species. 

Some of the undesirable taints in factories can be controlled in largepart by the introduction of starters made from certain organisms thatare able to obtain the ascendency over the taint-producing germ. Such amethod is commonly followed when a lactic ferment, either a commercialpure culture, or a home-made starter, is added to milk to overcome theeffect of gas-generating bacteria. 

[Illustration: FIG. 20. Curd from a badly tainted milk. Large raggedholes are mechanical; numerous small holes due to gas. This curd was a"floater. "]

A similar illustration is seen in the case of the "lange wei" (slimywhey), that is used in the manufacture of Edam cheese to control thecharacter of the fermentation of the milk. 

This same method is sometimes applied in dealing with certain abnormalfermentations that are apt to occur on the farm. It is particularlyuseful with those tainted milks known as "sweet curdling. " The fermentorganisms concerned in this change are unable to develop in thepresence of lactic acid bacteria, so the addition of a clean sour milkas a starter restores the normal conditions by giving the ordinary milkbacteria the ascendency. 

~Chemical disinfection. ~ In exceptional instances it may be necessary toemploy chemical disinfectants to restore the normal conditions. Ofcourse with such diseases as tuberculosis, very stringent measures arerequired, as they are such a direct menace to human life, but with theseabnormal or taint-producing fermentations, care and cleanliness, welldirected, will usually overcome the trouble. 

If it becomes necessary to employ chemical substances as disinfectingagents, their use should always be preceded by a thorough cleansing withhot water so that the germicide may come in direct contact with thesurface to be disinfected. 

It must be borne in mind that many chemicals act as deodorants, _i. E. _, destroy the offensive odor, without destroying the cause of the trouble. 

_Sulfur_ is often recommended as a disinfecting agent, but its useshould be carefully controlled, otherwise the vapors have but littlegermicidal power. The common practice of burning a small quantity in aroom or any closed space for a few moments has little or no effect upongerm life. The effect of sulfur vapor (SO_{2}) alone upon germ life isrelatively slight, but if this gas is produced in the presence ofmoisture, sulfurous acid (H_{2}SO_{3}) is formed, which is much moreefficient. To use this agent effectively, it must be burned in largequantities in a moist atmosphere (three lbs. To every 1, 000 cubic feetof space), for at least twelve hours. After this operation, the spaceshould be thoroughly aired. 

_Formalin_, a watery solution of a gas known as formaldehyde, is a newdisinfectant that recent experience has demonstrated to be very useful. It may be used as a gas where rooms are to be disinfected, or applied asa liquid where desired. It is much more powerful in its action thansulfur, and it has a great advantage over mercury and other strongdisinfectants, as it is not so poisonous to man as it is to the lowerforms of life. 

_Bleaching powder or chloride of lime_ is often recommended where achemical can be advantageously used. This substance is a gooddisinfectant as well as a deodorant, and if applied as a wash, in theproportion of four to six ounces of the powder to one gallon of water, it will destroy most forms of life. In many cases this agent isinapplicable on account of its odor. 

_Corrosive sublimate_ (HgCl_{2}) for most purposes is a gooddisinfectant, but it is such an intense poison that its use is dangerousin places that are at all accessible to stock. 

For the disinfection of walls in stables and barns, common thin _whitewash_ Ca(OH)_{2} is admirably adapted if made from freshly-burned quicklime. It possesses strong germicidal powers, increases the amount oflight in the barn, is a good absorbent of odors, and is exceedinglycheap. 

Carbolic acid, creosote, and such products, while excellentdisinfectants, cannot well be used on account of their odor, especiallyin factories. 

For gutters, drains, and waste pipes in factories, _vitriol salts_(sulfates of copper, iron and zinc) are sometimes used. These aredeodorants as well as disinfectants, and are not so objectionable to useon account of their odor. 

These suggestions as to the use of chemicals, however, only apply toextreme cases and should not be brought into requisition until athorough application of hot water, soap, a little soda, and thescrubbing brush have failed to do their work. 

FOOTNOTES:

[51] Günther and Thierfelder, Arch. F. Hyg. , 25:164, 1895; Leichmann, Cent. F. Bakt. , 2:281, 1896; Esten, 9 Rept. Storrs Expt. Stat. , p. 44, 1896; Dinwiddie, Bull. 45, Ark. Expt. Stat. , May, 1897; Kozai, Zeit. F. Hyg. , 38:386, 1901; Weigmann, Hyg. Milk Congress, Hamburg, 1903, p. 375. 

[52] McDonnell, Inaug. Diss. , Kiel. 1899, p. 39. 

[53] Kayser, Cent. F. Bakt. II. Abt. 1:436. 

[54] Treadwell, Science, 1894, 17:178. 

[55] Conn, 5 Rept. Storrs Expt. Stat. , 1892, p. 396. 

[56] Fermi, Arch. F. Hyg. , 1892, 14:1. 

[57] Duclaux, Le Lait, p. 121. 

[58] Duclaux, Principes de Laiterie, p. 67. 

[59] Guillebeau (Milch Zeit. , 1892, p. 808) has studied over a dozendifferent forms that possess this property. 

[60] Ward, Bull. 165, Cornell Expt. Stat. , Mch. , 1899; also Bull. 195, Ibid. , Nov. , 1901. 

[61] Adametz, Landw. Jahr. , 1891, p. 185. 

[62] Marshall, Mich. Expt. Stat. , Bull. 140. 

[63] Milch Zeit. , 1899, p. 982. 

[64] Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn, Zeit. F. Hyg. , 46: 286, 1904. 

[65] Bull. 128, Wis. Expt. Stat. , Sept. 1905. 

[66] Freudenreich, Landw. Jahr. D. Schweiz, 1896, 10; 1. 

[67] Weigmann, Milch Zeit. , 1890, p. 881. 

[68] Conn, 3 Rept. Storrs Expt. Stat. , 1890, p. 158. 

[69] Freudenreich, Fühl. Landw. Ztg. 43: 361. 

[70] Harrison, Bull. 120 Ont. Agr'l. Coll. , May, 1902. 

[71] Milch Zeit. 22:569. 

[72] Marshall, Bull. 146, Mich. Expt. Stat. , p. 16. 

[73] Grotenfelt, Milch Zeit. , 1889, p. 263. 

[74] Menge, Cent. F. Bakt. , 6:596; Keferstein, Cent. F. Bakt. , 21:177. 

[75] Heim, Arb. A. D. Kais. Gesundheitsamte, 5:578. 

[76] Adametz, Milch Zeit. , 1890, p. 225. 

[77] 12 Rept. Wis. Expt. Stat. , 1895, p. 148; also Bull. 67, Ibid. , June, 1898. 

CHAPTER V. 

RELATION OF DISEASE-BACTERIA TO MILK. 

Practical experience with epidemic disease has abundantly demonstratedthe fact that milk not infrequently serves as a vehicle for thedissemination of contagion. Attention has been prominently called tothis relation by Ernest Hart, [78] who in 1880 compiled statisticalevidence showing the numerous outbreaks of various contagious diseasesthat had been associated with milk infection up to that time. Sincethen, further compilations have been made by Freeman, [79] and also byBusey and Kober, [80] who have collected the data with reference tooutbreaks from 1880 to 1899. 

These statistics indicate the relative importance of milk as a factor inthe dissemination of disease. 

The danger from this source is much intensified for the reason thatmilk, generally speaking, is consumed in a raw state; and also because aconsiderable number of disease-producing bacteria are able, not merelyto exist, but actually thrive and grow in milk, even though the normalmilk bacteria are also present. Moreover the recognition of the presenceof such pathogenic forms is complicated by the fact that often they donot alter the appearance of the milk sufficiently so that theirpresence can be detected by a physical examination. These facts whichhave been experimentally determined, coupled with the numerous clinicalcases on record, make a strong case against milk serving as an agent inthe dissemination of disease. 

~Origin of pathogenic bacteria in milk. ~ Disease-producing bacteria may begrouped with reference to their relation toward milk into two classes, depending upon the manner in which infection occurs:

Class I. Disease-producing bacteria capable of being transmitteddirectly from a diseased animal to man through the medium of infectedmilk. 

Class II. Bacteria pathogenic for man but not for cattle which arecapable of thriving in milk after it is drawn from the animal. 

In the first group the disease produced by the specific organism must becommon to both cattle and man. The organism must live a parasitic lifein the animal, developing in the udder, and so infect the milk supply. It may, of course, happen that diseases toward which domestic animalsalone are susceptible may be spread from one animal to another in thisway without affecting human beings. 

In the second group, the bacterial species lives a saprophyticexistence, growing in milk, if it happens to find its way therein. Insuch cases milk indirectly serves as an agent in the dissemination ofdisease, by giving conditions favorable to the growth of the diseasegerm. 

By far the most important of diseases that may be transmitted directlyfrom animal to man through a diseased milk supply is tuberculosis, butin addition to this, foot and mouth disease (aphthous fever inchildren), anthrax and acute enteric troubles have also been traced to asimilar source of infection. 

The most important specific diseases that have been disseminated throughsubsequent pollution of the milk are typhoid fever, diphtheria, scarletfever and cholera, but, of course, the possibility exists that anydisease germ capable of living and thriving in milk may be spread inthis way. In addition to these diseases that are caused by theintroduction of specific organisms (the causal organism of scarlet feverhas not yet been definitely determined), there are a large number ofmore or less illy-defined troubles of an intestinal character that occurespecially in infants and young children that are undoubtedlyattributable to the activity of microörganisms that gain access to milkduring and subsequent to the milking, and which produce changes in milkbefore or after its ingestion that result in the formation of toxicproducts. 

DISEASES TRANSMISSIBLE FROM ANIMAL TO MAN THROUGH DISEASED MILK. 

~Tuberculosis. ~ In view of the wide-spread distribution of this disease inboth the human and the bovine race, the relation of the same to milksupplies is a question of great importance. It is now generally admittedthat the different types of tubercular disease found in different kindsof animals and man are attributable to the development of the sameorganism, _Bacillus tuberculosis_, although there are varieties of thisorganism found in different species of animals that are sufficientlydistinct to permit of recognition. 

The question of prime importance is, whether the bovine type istransmissible to the human or not. Artificial inoculation of cattle withtuberculous human sputum as well as pure cultures of this variety showthat the human type is able to make but slight headway in cattle. Thiswould indicate that the danger of cattle acquiring the infection fromman would in all probability be very slight, but these experiments offerno answer as to the possibility of transmission from the bovine to thehuman. Manifestly it is impossible to solve this problem by directexperiment upon man except by artificial inoculation, but comparativeexperiments upon animals throw some light on the question. 

Theo. Smith[81] and others[82] have made parallel experiments withanimals such as guinea pigs, rabbits and pigeons, inoculated with bothbovine and human cultures of this organism. The results obtained in thecase of all animals tested show that the virulence of the two types wasmuch different, but that the bovine cultures were much more severe. While of course this does not prove that transmission from bovine tohuman is possible, still the importance of the fact must not beoverlooked. 

In a number of cases record of accidental infection from cattle to manhas been noted. [83] These have occurred with persons engaged in makingpost-mortem examinations on tuberculous animals, and the tubercularnature of the wound was proven in some cases by excision andinoculation. 

In addition to data of this sort that is practically experimental incharacter, there are also strong clinical reasons for considering thatinfection of human beings may occur through the medium of milk. Naturally such infection should produce intestinal tuberculosis, and itis noteworthy that this phase of the disease is quite common inchildren especially between the ages of two and five. [84] It isdifficult to determine, though, whether primary infection occurredthrough the intestine, for, usually, other organs also become involved. In a considerable number of cases in which tubercular infection by themost common channel, inhalation, seems to be excluded, the evidence isstrong that the disease was contracted through the medium of the milk, but it is always very difficult to exclude the possibility of pulmonaryinfection. 

Tuberculosis as a bovine disease has increased rapidly during recentdecades throughout many portions of the world. This has been most markedin dairy regions. Its extremely insidious nature does not permit of anearly recognition by physical means, and it was not until theintroduction of the tuberculin test[85] in 1892, as a diagnostic aidthat accurate knowledge of its distribution was possible. The quitegeneral introduction of this test in many regions has revealed analarmingly large percentage of animals as affected. In Denmark in 1894over forty per cent were diagnosed as tubercular. In some parts ofGermany almost as bad a condition has been revealed. Slaughter-housestatistics also show that the disease has increased rapidly since 1890. In this country the disease on the average is much less than in Europeand is also very irregularly distributed. In herds where it gained afoothold some years ago, often the majority of animals are frequentlyinfected; many herds, in fact the great majority, are wholly free fromall taint. The disease has undoubtedly been most frequently introducedthrough the purchase of apparently healthy but incipiently affectedanimals. Consequently in the older dairy regions where stock has beenimproved the most by breeding, more of the disease exists than among thewestern and southern cattle. 

[Illustration FIG. 21: Front view of a tuberculous udder, showing extentof swelling in single quarter. ]

~Infectiousness of milk of reacting animals. ~ Where the disease appears inthe udder the milk almost invariably contains the tubercle organism. Under such conditions the appearance of the milk is not materiallyaltered at first, but as the disease progresses the percentage of fatgenerally diminishes, and at times in the more advanced stages where thephysical condition of the udder is changed (Fig. 21), the milk maybecome "watery"; but the percentage of animals showing such udderlesions is not large, usually not more than a few per cent. (4 per cent. According to Ostertag. )

On the other hand, in the earlier phases of the disease, where itspresence has been recognized solely by the aid of the tuberculin test, before there are any recognizable physical symptoms in any part of theanimal, the milk is generally unaffected. Between these extremes, however, is found a large proportion of cases, concerning which sodefinite data are not available. The results of investigators on thispoint are conflicting and further information is much desired. Some haveasserted so long as the udder itself shows no lesions that no tuberclebacilli would be present, [86] but the findings of a considerable numberof investigators[87] indicate that even when the udder is apparently notdiseased the milk may contain the specific organism as revealed byinoculation experiments upon animals. In some cases, however, it hasbeen demonstrated by post-mortem examination that discoverable udderlesions existed that were not recognizable before autopsy was made. Inthe experimental evidence collected, a varying percentage of reactinganimals were found that gave positive results; and this number wasgenerally sufficient to indicate that the danger of using milk fromreacting animals was considerable, even though apparently no diseasecould be found in the udder. 

The infectiousness of milk can also be proven by the frequentcontraction of the disease in other animals, such as calves and pigswhich may be fed on the skim milk. The very rapid increase of thedisease among the swine of Germany and Denmark, [88] and the frequentlyreported cases of intestinal infection of young stock also attest thepresence of the organism in milk. 

The tubercle bacillus is so markedly parasitic in its habits, that, under ordinary conditions, it is incapable of growing at normal airtemperatures. There is, therefore, no danger of the germ developing inmilk after it is drawn from the animal, unless the same is kept atpractically blood heat. 

Even though the milk of some reacting animals may not contain thedangerous organism at the time of making the test, it is quiteimpossible to foretell how long it will remain free. As the diseasebecomes more generalized, or if tuberculous lesions should develop inthe udder, the milk may pass from a healthy to an infectious state. 

This fact makes it advisable to exclude from milk supplies intended forhuman use, all milk of animals that respond to the tuberculin test; orat least to treat it in a manner so as to render it safe. Whether it isnecessary to do this or not if the milk is made into butter or cheese isa somewhat different question. Exclusion or treatment is rendered moreimperative in milk supplies, because the danger is greater with childrenwith whom milk is often a prominent constituent of their diet, and alsofor the reason that the child is more susceptible to intestinalinfection than the adult. 

The danger of infection is much lessened in butter or cheese, becausethe processes of manufacture tend to diminish the number of organismsoriginally present in the milk, and inasmuch as no growth can ordinarilytake place in these products the danger is minimized. Moreover, the factthat these foods are consumed by the individual in smaller amounts thanis generally the case where milk is used, and also to a greater extentby adults, lessens still further the danger of infection. 

Notwithstanding this, numerous observers[89] especially in Germany havesucceeded in finding the tubercle bacillus in market butter, but thisfact is not so surprising when it is remembered that a very largefraction of their cattle show the presence of the disease as indicatedby the tuberculin test, a condition that does not obtain in any largesection in this country. 

The observations on the presence of the tubercle bacillus in butter havebeen questioned somewhat of late[2] by the determination of the factthat butter may contain an organism that possesses the property of beingstained in the same way as the tubercle organism. Differentiationbetween the two forms is rendered more difficult by the fact that thistubercle-like organism is also capable of producing in animals lesionsthat stimulate those of tuberculosis, although a careful examinationreveals definite differences. Petri[90] has recently determined thatboth the true tubercle and the acid-resisting butter organism may bereadily found in market butter. 

In the various milk products it has been experimentally determined thatthe true tubercle bacillus is able to retain its vitality in butter fora number of months and in cheese for nearly a year. 

~Treatment of milk from tuberculosis cows. ~ While it has been shown thatit is practically impossible to foretell whether the milk of anyreacting animal actually contains tubercle bacilli or not, still theinterests of public health demand that no milk from such stock be usedfor human food until it has been rendered safe by some satisfactorytreatment. 

_1. Heating. _ By far the best treatment that can be given such milk isto heat it. The temperature at which this should be done depends uponthe thermal death point of the tubercle bacillus, a question concerningwhich there has been considerable difference of opinion until veryrecently. According to the work of some of the earlier investigators, the tubercle bacillus in its vegetative stage is endowed with powers ofresistance greater than those possessed by any other pathogenicorganism. This work has not been substantiated by the most recentinvestigations on this subject. In determining the thermal death pointof this organism, as of any other, not only must the temperature beconsidered, but the period of exposure as well, and where that exposureis made in milk, another factor must be considered, viz. , the presenceof conditions permitting of the formation of a "scalded layer, " for asSmith[91] first pointed out, the resistance of the tubercle organismtoward heat is greatly increased under these conditions. If tuberculousmilk is heated in a closed receptacle where this scalded membrane cannotbe produced, the tubercle bacillus is killed at 140° F. In 15 to 20minutes. These results which were first determined by Smith, underlaboratory conditions, and confirmed by Russell and Hastings, [92] wheretuberculous milk was heated in commercial pasteurizers, have also beenverified by Hesse. [93] A great practical advantage which accrues fromthe treatment of milk at 140° F. Is that the natural creaming ispractically unaffected. Of course, where a higher temperature isemployed, the period of exposure may be materially lessened. If milk ismomentarily heated to 176° F. , it is certainly sufficient to destroy thetubercle bacillus. This is the plan practiced in Denmark where all skimmilk and whey must be heated to this temperature before it can be takenback to the farm, a plan which is designed to prevent the disseminationof tuberculosis and foot and mouth disease by means of the mixedcreamery by-products. This course renders it possible to utilize withperfect safety, for milk supplies, the milk of herds reacting to thetuberculin test, and as butter of the best quality can be made fromcream or milk heated to even high temperatures, [94] it thus becomespossible to prevent with slight expense what would otherwise entail alarge loss. 

_2. Dilution. _ Another method that has been suggested for the treatmentof this suspected milk is dilution with a relatively large volume ofperfectly healthy milk. It is a well known fact that to produceinfection, it requires the simultaneous introduction of a number oforganisms, and in the case of tuberculosis, especially that produced byingestion, this number is thought to be considerable. Gebhardt[95] foundthat the milk of tuberculous cows, which was virulent when injected byitself into animals, was innocuous when diluted with 40 to 100 times itsvolume of healthy milk. This fact is hardly to be relied upon inpractice, unless the proportion of reacting to healthy cows ispositively known. 

It has also been claimed in the centrifugal separation of cream frommilk[96] that by far the larger number of tubercle bacilli were thrownout with the separator slime. Moore[97] has shown that the tuberclebacilli in an artificially infected milk might be reduced in this way, so as to be no longer microscopically demonstrable, yet the purificationwas not complete enough to prevent the infection of animals inoculatedwith the milk. 

Another way to exclude all possibility of tubercular infection in milksupplies is to reject all milk from reacting animals. This method isoften followed where pasteurization or sterilization is not desired. Indairies where the keeping quality is dependent upon the exclusion ofbacteria by stringent conditions as to milking and handling ("sanitary"or "hygienic" milk), the tuberculin test is frequently used as a basisto insure healthy milk. 

~Foot and mouth disease. ~ The wide-spread extension of this diseasethroughout Europe in recent years has given abundant opportunity to showthat while it is distinctively an animal malady, it is alsotransmissible to man, although the disease is rarely fatal. The causalorganism has not been determined with certainty, but it has been shownthat the milk of affected animals possesses infectious properties[98]although appearing unchanged in earlier phases of the disease. 

Hertwig showed the direct transmissibility of the disease to man byexperiments made on himself and others. By ingesting milk from anaffected animal, he was able to produce the symptoms of the disease, themucous membrane of the mouth being covered with the small vesicles thatcharacterize the malady. It has also been shown that the virus of thedisease may be conveyed in butter. [99] This disease is practicallyunknown in this country, although widely spread in Europe. 

There are a number of other bovine diseases such as anthrax, [100]lockjaw, [101] and hydrophobia[102] in which it has been shown that thevirus of the disease is at times to be found in the milk supply, butoften the milk becomes visibly affected, so that the danger of using thesame is greatly minimized. 

There are also a number of inflammatory udder troubles known as gargetor mammitis. In most of these, the physical appearance of the milk is sochanged, and often pus is present to such a degree as to give a verydisagreeable appearance to the milk. Pus-forming bacteria (staphylococciand streptococci) are to be found associated with such troubles. Anumber of cases of gastric and intestinal catarrh have been reported ascaused by such milks. [103]

DISEASES TRANSMISSIBLE TO MAN THROUGH INFECTION OF MILK AFTERWITHDRAWAL. 

Milk is so well adapted to the development of bacteria in general, thatit is not surprising to find it a suitable medium for the growth of manypathogenic species even at ordinary temperatures. Not infrequently, disease-producing bacteria are able to grow in raw milk in competitionwith the normal milk bacteria, so that even a slight contamination maysuffice to produce infection. 

The diseases that are most frequently disseminated in this way aretyphoid fever, diphtheria, scarlet fever and cholera, together with thevarious illy-defined intestinal troubles of a toxic character that occurin children, especially under the name of cholera infantum, summercomplaint, etc. 

Diseases of this class are not derived directly from animals becausecattle are not susceptible to the same. 

~Modes of infection. ~ In a variety of ways, however, the milk may besubject to contaminating influences after it is drawn from the animal, and so give opportunity for the development of disease-producingbacteria. The more important methods of infection are as follows:

_1. Infection directly from a pre-existing case of disease on premises. _Quite frequently a person in the early stage of a diseased condition maycontinue at his usual vocation as helper in the barn or dairy, and sogive opportunity for direct infection to occur. In the so-called casesof "walking typhoid, " this danger is emphasized. It is noteworthy intyphoid fever that the bacilli frequently persist in the urine and indiphtheria they often remain in the throat until after convalescence. Insome cases infection has been traced to storage of the milk in rooms inthe house where it became polluted directly by the emanations of thepatient. [104] Among the dwellings of the lower classes where a singleroom has to be used in common this source of infection has been mostfrequently observed. 

_2. Infection through the medium of another person. _ Not infrequentlyanother individual may serve in the capacity of nurse or attendant to asick person, and also assist in the handling of the milk, either inmilking the animals or caring for the milk after it has been drawn. Busey and Kober report twenty-one outbreaks of typhoid fever in whichdairy employees also acted in the capacity of nurses. 

_3. Pollution of milk utensils. _ The most frequent method of infectionof cans, pails, etc. , is in cleaning them with water that may bepolluted with disease organisms. Often wells may be contaminated withdiseased matter of intestinal origin, as in typhoid fever, and the useof water at normal temperatures, or even in a lukewarm condition, giveconditions permitting of infection. Intentional adulteration of milkwith water inadvertently taken from polluted sources has caused quite anumber of typhoid outbreaks. [105] Sedgwick and Chapin[106] found in theSpringfield, Mass. , epidemic of typhoid that the milk cans were placedin a well to cool the milk, and it was subsequently shown that the wellwas polluted with typhoid fecal matter. 

_4. Pollution of udder_ of animal _by wading in infected water_, or bywashing same with contaminated water. This method of infection wouldonly be likely to occur in case of typhoid. An outbreak at theUniversity of Virginia in 1893[107] was ascribed to the latter cause. 

_5. Pollution of creamery by-products, skim-milk, etc. _ Where the milksupply of one patron becomes infected with pathogenic bacteria, it ispossible that disease may be disseminated through the medium of thecreamery, the infective agent remaining in the skim milk afterseparation and so polluting the mixed supply. This condition is morelikely to prevail with typhoid because of the greater tolerance of thisorganism for acids such as would be found in raw milk. The outbreaks atBrandon, [108] England, in 1893, Castle Island, [109] Ireland, andMarlboro, [110] Mass. , in 1894, were traced to such an origin. 

While most outbreaks of disease associated with a polluted milk supplyoriginate in the use of the milk itself, yet infected milk may serve tocause disease even when used in other ways. Several outbreaks of typhoidfever have been traced to the use of ice cream where there were strongreasons for believing that the milk used in the manufacture of theproduct was polluted. [111] Hankin[112] details a case of an Indianconfection made largely from milk that caused a typhoid outbreak in aBritish regiment. 

Although the evidence that milk may not infrequently serve as an agentin spreading disease is conclusive enough to satisfactorily prove theproposition, yet it should be borne in mind that the organism of anyspecific disease in question has rarely ever been found. The reasons forthis are quite the same as those that govern the situation in the caseof polluted waters, except that the difficulties of the problem are muchgreater in the case of milk than with water. The inability to readilyseparate the typhoid germ, for instance, from the colon bacillus, anorganism frequently found in milk, presents technical difficulties noteasily overcome. The most potent reason of failure to find diseasebacteria is the fact that infection in any case must occur sometimeprevious to the appearance of the outbreak. Not only is there the usualperiod of incubation, but it rarely happens that an outbreak isinvestigated until a number of cases have occurred. In this interim theoriginal cause of infection may have ceased to be operative. 

~Typhoid fever. ~ With reference to the diseases likely to to bedisseminated through the medium of milk, infected after being drawn fromthe animal, typhoid fever is the most important. The reason for this isdue (1) to the wide spread distribution of the disease; (2) to the factthat the typhoid bacillus is one that is capable of withstandingconsiderable amounts of acid, and consequently finds even in raw milkcontaining the normal lactic acid bacteria conditions favorable for itsgrowth. [113] Ability to grow under these conditions can be shown notonly experimentally, but there is abundant clinical evidence that even aslight infection often causes extensive outbreaks, as in the Stamford, Conn. , outbreak in 1895 where 386 cases developed in a few weeks, 97 percent. Of which occurred on the route of one milk-man. In this case themilk cans were thoroughly and properly cleaned, but were rinsed out with_cold_ water from a shallow well that was found to be polluted. 

The most common mode of pollution of milk with typhoid organisms iswhere the milk utensils are infected in one way or another. [114] Secondin importance is the carrying of infection by persons serving in thedual capacity of nurse and dairy attendant. 

~Cholera. ~ This germ does not find milk so favorable a nutrient medium asthe typhoid organism, because it is much more sensitive toward theaction of acids. Kitasato[115] found, however, that it could live inraw milk from one to four days, depending upon the amount of acidpresent. In boiled or sterilized milk it grows more freely, as theacid-producing forms are thereby eliminated. In butter it dies out in afew days (4 to 5). 

On account of the above relation not a large number of cholera outbreakshave been traced to milk, but Simpson[116] records a very striking casein India where a number of sailors, upon reaching port, secured aquantity of milk. Of the crew which consumed this, every one was takenill, and four out of ten died, while those who did not partake escapedwithout any disease. It was later shown that the milk was adulteratedwith water taken from an open pool in a cholera infected district. 

~Diphtheria. ~ Milk occasionally, though not often, serves as a medium forthe dissemination of diphtheria. Swithinbank and Newman[117] cites fourcases in which the causal organism has been isolated from milk. It hasbeen observed that growth occurs more rapidly in raw than in sterilizedmilk. [118]

Infection in this disease is more frequently attributable to directinfection from patient on account of the long persistence of this germin the throat, or indirectly through the medium of an attendant. 

~Scarlet fever. ~ Although it is more difficult to study the relation ofthis disease to contaminated milk supplies, because the causal germ ofscarlet fever is not yet known, yet the origin of a considerable numberof epidemics has been traced to polluted milk supplies. Milk doubtlessis infected most frequently from persons in the earlier stages of thedisease when the infectivity of the disease is greater. 

~Diarrhoeal diseases. ~ Milk not infrequently acquires the property ofproducing diseases of the digestive tract by reason of the developmentof various bacteria that form more or less poisonous by-products. Thesetroubles occur most frequently during the summer months, especially withinfants and children, as in cholera infantum and summer complaint. Thehigher mortality of bottle-fed infants[119] in comparison with thosethat are nursed directly is explicable on the theory that cows' milk isthe carrier of the infection, because in many cases it is not consumeduntil there has been ample time for the development of organisms in it. Where milk is pasteurized or boiled it is found that the mortality amongchildren is greatly reduced. As a cause of sickness and death thesediseases exceed in importance all other specific diseases previouslyreferred to. These troubles have generally been explained as produced bybacteria of the putrefactive class which find their way into the milkthrough the introduction of filth and dirt at time of milking. [120]Flügge[121] has demonstrated that certain peptonizing species possesstoxic properties for animals. Recent experimental inquiry[122] hasdemonstrated that the dysentery bacillus (Shiga) probably bears a causalrelation to some of these summer complaints. 

~Ptomaine poisoning. ~ Many cases of poisoning from food products are alsoreported with adults. These are due to the formation of various toxicproducts, generally ptomaines, that are produced as a result ofinfection of foods by different bacteria. One of these substances, _tyrotoxicon_, was isolated by Vaughan[123] from cheese and variousother products of milk, and found to possess the property of producingsymptoms of poisoning similar to those that are noted in such cases. Heattributes the production of this toxic effect to the decomposition ofthe elements in the milk induced by putrefactive forms of bacteria thatdevelop where milk is improperly kept. [124] Often outbreaks of thischaracter[125] assume the proportions of an epidemic, where a largenumber of persons use the tainted food. 

FOOTNOTES:

[78] Hart, Trans. Int. Med. Cong. , London, 1881, 4:491-544. 

[79] Freeman, Med. Rec. , March 28, 1896. 

[80] Busey and Kober, Rept. Health Off. Of Dist. Of Col. , Washington, D. C. , 1895, p. 299. These authors present in this report an elaboratearticle on morbific and infectious milk, giving a very completebibliography of 180 numbers. They append to Hart's list (which ispublished in full) additional outbreaks which have occurred since, together with full data as to extent of epidemic, circumstancesgoverning the outbreak, as well as name of original reporter andreference. 

[81] Smith, Theo. , Journ. Of Expt. Med. , 1898, 3:451. 

[82] Dinwiddie, Bull. 57, Ark. Expt. Stat. , June, 1899; Ravenel, Univ. Of Penn. Med. Bull. , Sept. 1901. 

[83] Ravenel, Journ. Of Comp. Med. & Vet. Arch. , Dec. 1897; Hartzell, Journ. Amer. Med. Ass'n, April 16, 1898. 

[84] Stille, Brit. Med. Journ. , Aug. 19, 1899. 

[85] This test is made by injecting into the animal a small quantity oftuberculin, which is a sterilized glycerin extract of cultures of thetubercle bacillus. In a tuberculous animal, even in the very earliestphases of the disease, tuberculin causes a temporary fever that lastsfor a few hours. By taking the temperature a number of times before andafter injection it is possible to readily recognize any febrilecondition. A positive diagnosis is made where the temperature afterinoculation is at least 2. 0° F. Above the average normal, and where thereaction fever is continued for a period of some hours. 

[86] Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculosesanimales, 1895. 

[87] C. O. Jensen, Milch Kunde und Milch hygiene, p. 69. 

[88] Ostertag, Milch Zeit. , 22:672. 

[89] Obermüller, Hyg. Rund. , 1897, p. 712; Petri, Arb. A. D. Kais. Ges. Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund. , 1898, p. 217. 

[90] Rabinowitsch, Zeit. F. Hyg. , 1897, 26: 90. 

[91] Th. Smith. Journ. Of Expt. Med. , 1899, 4:217. 

[92] Russell and Hastings, 18 Rept. Wis. Expt. Stat. , 1901. 

[93] Hesse, Zeit. F. Hyg. , 1900, 34:346. 

[94] Practically all of the finest butter made in Denmark is made fromcream that has been pasteurized at temperatures varying from 160°-185°F. 

[95] Gebhardt, Virch. Arch. , 1890, 119:12. 

[96] Scheurlen, Arb. A. D. K. Ges. Amte, 1891, 7:269; Bang, Milch Zeit. , 1893, p. 672. 

[97] Moore, Year Book of U. S. Dept. Agr. , 1895, p. 432. 

[98] Weigel and Noack, Jahres. D. Ges. Med. , 1890, p. 642; Weissenberg, Allg. Med. Cent. Zeit. , 1890, p. 1; Baum, Arch. F. Thierheilkunde, 1892, 18:16. 

[99] Schneider, Münch, med. Wochenschr. , 1893, No. 27; Fröhner, Zeit f. Fleisch u. Milchhygiene, 1891, p. 55. 

[100] Feser, Deutsche Zeit. F. Thiermed. , 1880, 6:166. 

[101] Nocard, Bull. Gén. , 1885, p. 54. 

[102] Deutsche Viertelsjahr. F. Offentl. Gesundheitspflege, 1890, 20:444. 

[103] Zeit. F. Fleisch und Milch hygiene, 11:114. 

[104] E. Roth, Deutsche Vierteljahresschr. F. Offentl. Gesundheitspfl. , 1890, 22:238

[105] S. W. North, London Practitioner, 1889, 43:393. 

[106] Sedgwick and Chapin, Boston Med. & Surg. Journ. , 1893, 129:485. 

[107] Dabney, Phila. Med. News, 1893, 63:630. 

[108] Welphy, London Lancet, 1894, 2:1085. 

[109] Brit. Med. Journ. , 1894, 1:815. 

[110] Mass. Bd. Health Rept. , 1894, p. 765. 

[111] Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med. Journ. , 1894, 2:829. 

[112] Hankin, Brit. Med. Journ. , 1894, 2:613. 

[113] Heim (Arb. A. D. Kais. Gesundheitsamte, 1889, 5:303) finds itcapable of living from 20-30 days in milk. 

[114] Schüder (Zeit. F. Hyg. , 1902, 38:34) examined the statistics of638 typhoid epidemics. He found 71 per cent. Due to infected drinkingwater, 17 per cent. To infected milk, and 3. 5 per cent. Caused by otherforms of food. 

[115] Kitasato. Arb. A. D. Kais. Gesundheitsamte, 1:470. 

[116] Simpson, London Practitioner, 1887, 39:144. 

[117] Swithinbank and Newman, Bacteriology of Milk, p. 341. 

[118] Schottelius and Ellerhorst. Milch Zeit. , 1897, pp. 40 and 73. 

[119] Baginsky, Hyg. Rund. , 1895, p. 176. 

[120] Gaffky, Deutsch. Med. Wochen. , 18:14. 

[121] Flügge. Zeit. , f. Hyg. , 17:272, 1894. 

[122] Duval and Bassett, Studies from the Rockefeller Inst. For Med. Research, 2:7, 1904. 

[123] Zeit. F. Physiol. Chemie, 10:146; 9 Intern. Hyg. Cong. (London), 1891, p. 118. 

[124] Vaughan and Perkins, Arch. F. Hyg. , 27:308. 

[125] Newton and Wallace (Phila. Med. News, 1887, 50:570) report threeoutbreaks at Long Branch, N. J. , two of which occurred in summer hotels. 

CHAPTER VI. 

BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OFPRESERVATION. 

To the milk dealer or distributor, bacteria are more or less of adetriment. None of the organisms that find their way into milk, nor theby-products formed by their growth, improve the quality of milksupplies. It is therefore especially desirable from the milk-dealer'spoint of view that these changes should be held in abeyance as much aspossible. Then too, the possibility that milk may serve as a medium forthe dissemination of disease-breeding bacteria makes it advisable toprotect this food supply from all possible infection from suspicioussources. 

In considering, therefore, the relation of bacteria to general milksupplies, the _economic_ and the _hygienic_ standpoints must be takeninto consideration. Ordinarily much more emphasis is laid upon the firstrequirement. If the supply presents no abnormal feature as to taste, odor and appearance, unfortunately but little attention is paid to thepossibility of infection by disease germs. The methods of control whichare applicable to general milk supplies are based on the followingfoundations: (1) the exclusion of all bacterial life, as far aspracticable, at the time the milk is drawn, and the subsequent storageof the same at temperatures unfavorable for the growth of the organismsthat do gain access; (2) the removal of the bacteria, wholly or in part, after they have once gained access. 

Until within comparatively recent years, practically no attention wasgiven to the character of milk supplies, except possibly as to thepercentage of butter fat, and sometimes the milk solids which itcontained. So long as the product could be placed in the hands of theconsumer in such shape as not to be rejected by him as unfit for food, no further attention was likely to be given to its character. Atpresent, however, much more emphasis is being given to the quality ofmilk, especially as to its germ content; and the milk dealer isbeginning to recognize the necessity of a greater degree of control. This control must not merely concern the handling of the product afterit reaches him, but should go back to the milk producer on the farm. Here especially, it is necessary to inculcate those methods ofcleanliness which will prevent in large measure the wholesale infectionthat ordinarily occurs. 

The two watch words which are of the utmost importance to the milkdealer are _cleanliness_ and _cold_. If the milk is properly drawn fromthe animal in a clean manner and is immediately and thoroughly chilled, the dealer has little to fear as to his product. Whenever seriousdifficulties do arise, attributable to bacterial changes, it is becausenegligence has been permitted in one or both directions. The influenceof cleanliness in diminishing the bacterial life in milk and that of lowtemperatures in repressing the growth of those forms which inevitablygain access has been fully dealt with in preceding chapters. It is ofcourse not practicable to take all of these precautions to whichreference has been made in the securing of large supplies of market milkfor city use, but great improvement over existing conditions could besecured if the public would demand a better supervision of thisimportant food article. Boards of health in our larger cities areawakening to the importance of this question and are becomingincreasingly active in the matter of better regulations and theenforcement of the same. 

New York City Board of Health has taken an advanced position inrequiring that all milk sold in the city shall be chilled down to 45° F. Immediately after milking and shall be transported to the city inrefrigerator cars. 

Reference has already been made to the application of the acid test(page 52) in the inspection of city milk supplies, and it is the opinionof the writer that the curd test (see page 76) could also be used withadvantage in determining the sanitary character of milk. This testreveals the presence of bacteria usually associated with dirt andpermits of the recognition of milks that have been carelessly handled. From personal knowledge of examinations made of the milk supplies in anumber of Wisconsin cities it appears that this test could be utilizedwith evident advantage. 

~"Sanitary" or "certified" milk supplies. ~ In a number of the largercities, the attempt has been made to improve the quality of the milksupplies by the installation of dairies in which is produced anespecially high grade of milk. Frequently the inspection of the dairy aswell as the examination of the milk at stated intervals is under thecontrol of milk commissions or medical societies and as it is customaryto distribute the certificate of the examining board with the product, such milks are frequently known as "certified. " In such dairies thetuberculin test is used at regular intervals, and the herd inspectedfrequently by competent veterinarians. The methods of controlinaugurated as to clean milking and subsequent handling are such as toinsure the diminution of the bacteria to the lowest possible point. Thebacterial limit set by the Pediatric Society of Philadelphia is 10, 000organisms per cc. Often it is possible to improve very materially onthis standard and not infrequently is the supply produced where itcontains only a few thousand organisms per cc. Where such a degree ofcare is exercised, naturally a considerably higher price must be paidfor the product, [126] and it should be remembered that the developmentof such a system is only possible in relatively large centers where thedealer can cater to a selected high-class trade. Moreover, it shouldalso be borne in mind that such a method of control is only feasible indairies that are under individual control. The impossibility ofexercising adequate control with reference to the milking process andthe care which should be given the milk immediately thereafter, when thesame is produced on different farms under various auspices is evident. 

PRESERVATION OF MILK SUPPLIES. 

While much can be done to improve the quality of milk supplies byexcluding a large proportion of the bacteria which normally gain accessto the milk, and preventing the rapid growth of those that do find theirway therein, yet for general municipal purposes, any practical method ofpreservation[127] that is applicable on a commercial scale must restlargely upon the destruction of bacteria that are present in the milk. 

The two possible methods by which bacteria can be destroyed after theyhave once gained access is (1) by the use of chemical preservatives; (2)by the aid of physical methods. 

~Chemical preservatives. ~ Numerous attempts have been made to find somechemical substance that could be added to milk which would preserve itwithout interfering with its nutritive properties, but as a general rulea substance that is toxic enough to destroy or inhibit the growth ofbacterial life exerts a prejudicial effect on the tissues of the body. The use of chemicals, such as carbolic acid, mercury salts and mineralacids, that are able to entirely destroy all life, is of courseexcluded, except when milk is preserved for analytical purposes; but anumber of milder substances are more or less extensively employed, although the statutes of practically all states forbid their use. 

The substances so used may be grouped in two classes:

1. Those that unite chemically with certain by-products of bacterialgrowth to form inert substances. Thus bicarbonate of soda neutralizesthe acid in souring milk, although it does not destroy the lactic acidbacteria. 

2. Those that act directly upon the bacteria in milk, restraining orinhibiting their development. The substances most frequently utilizedare salicylic acid, formaldehyde and boracic acid. These are nearlyalways sold to the milk handler, under some proprietary name, at pricesgreatly in excess of what the crude chemicals could be bought for in theopen market. Formaldehyde has been widely advertised of late, but itsuse is fraught with the greatest danger, for it practically rendersinsoluble all albuminous matter and its toxic effect is greatlyincreased in larger doses. 

These substances are generally used by milk handlers who know nothing oftheir poisonous action, and although it may be possible for adults towithstand their use in dilute form, without serious results, yet theiraddition to general milk supplies that may be used by children islittle short of criminal. The sale of these preparations for use in milkfinds its only outlet with those dairymen who are anxious to escape theexactions that must be met by all who attempt to handle milk in the bestpossible manner. Farrington has suggested a simple means for thedetection of preservalin (boracic acid). [128] When this substance isadded to fresh milk, it increases the acidity of milk without affectingits taste. As normal milk tastes sour when it contains about 0. 3 percent lactic acid, a milk that tests as much or more than this withouttasting sour has been probably treated with this antiseptic agent. 

~Physical methods of preservation. ~ Methods based upon the application ofphysical forces are less likely to injure the nutritive value of milk, and are consequently more effective, if of any value whatever. A numberof methods have been tried more or less thoroughly in an experimentalway that have not yet been reduced to a practical basis, as electricity, use of a vacuum, and increased pressure. [129] Condensation has long beenused with great success, but in this process the nature of the milk ismaterially changed. The keeping quality in condensed milk often dependsupon the action of another principle, viz. , the inhibition of bacterialgrowth by reason of the concentration of the medium. This condition isreached either by adding sugar and so increasing the soluble solids, orby driving off the water by evaporation, preferably in a vacuum pan. Temperature changes are, however, of the most value in preserving milk, for by a variation in temperature all bacterial growth can be brought toa standstill, and under proper conditions thoroughly destroyed. 

~Use of low temperatures. ~ The effect of chilling or rapid cooling on thekeeping quality of milk is well known. When the temperature of milk islowered to the neighborhood of 45° F. , the development of bacterial lifeis so slow as to materially increase the period that milk remains sweet. Within recent years, attempts have been made to preserve milk so that itcould be shipped long distances by freezing the product, which in theform of milk-ice could be held for an indefinite period withoutchange. [130] A modification of this process known as Casse's system hasbeen in use more or less extensively in Copenhagen and in several placesin Germany. This consists of adding a small block of milk-ice (frozenmilk) to large cans of milk (one part to about fifty of milk) which mayor may not be pasteurized. [131] This reduces the temperature so that themilk remains sweet considerably longer. Such a process might permit ofthe shipment of milk for long distances with safety but as a matter offact, the system has not met with especial favor. 

[Illustration: FIG. 22. Microscopic appearance of normal milk showingthe fat-globules aggregated in clusters. ]

~Use of high temperatures. ~ Heat has long been used as a preserving agent. Milk has been scalded or cooked to keep it from time immemorial. Heatmay be used at different temperatures, and when so applied exerts avarying effect, depending upon temperature employed. All methods ofpreservation by heat rest, however, upon the application of the heatunder the following conditions:

1. A temperature above the maximum growing-point (105°-115° F. ) andbelow the thermal death-point (130°-140° F. ) will prevent furthergrowth, and consequently fermentative action. 

2. A temperature above the thermal death-point destroys bacteria, andthereby stops all changes. This temperature varies, however, with thecondition of the bacteria, and for spores is much higher than forvegetative forms. 

Attempts have been made to employ the first principle in shipping milkby rail, viz. , prolonged heating above growing temperature, but whenmilk is so heated, its physical appearance is changed. [132] The methodsof heating most satisfactorily used are known as sterilization andpasteurization, in which a degree of temperature is used approximatingthe boiling and scalding points respectively. 

[Illustration: FIG. 23. Microscopic appearance of milk heated above 140°F. , showing the homogeneous distribution of fat-globules. The physicalchange noted in comparison with Fig. 22 causes the diminishedconsistency of pasteurized cream. ]

~Effect of heat on milk. ~ When milk is subjected to the action of heat, anumber of changes in its physical and chemical properties are to benoted. 

_1. Diminished "body. "_ When milk, but more especially cream, is heatedto 140° F. Or above, it becomes thinner in consistency or "body, " acondition which is due to a change in the grouping of the fat globules. In normal milk, the butter fat for the most part is massed inmicroscopic clots as (Fig. 22). When exposed to 140° F. Or above for tenminutes these fat-globule clots break down, and the globules becomehomogeneously distributed (Fig. 23). A _momentary_ exposure to heat ashigh as 158°-160° may be made without serious effect on the cream lime;but above this the cream rises so poorly and slowly that it gives theimpression of thinner milk. 

_2. Cooked Taste. _ If milk is heated for some minutes to 160° F. , itacquires a cooked taste that becomes more pronounced as the temperatureis further raised. Milk so heated develops on its surface a pellicle or"skin. " The cause of this change in taste is not well known. Usually ithas been explained as being produced by changes in the nitrogenouselements in the milk, particularly in the albumen. Thoerner[133] haspointed out the coincidence that exists between the appearance of acooked taste and the loss of certain gases that are expelled by heating. He finds that the milk heated in closed vessels from which the gascannot escape has a much less pronounced cooked flavor than if heated inan open vessel. The so-called "skin" on the surface of heated milk isnot formed when the milk is heated in a tightly-closed receptacle. Bysome[134] it is asserted that this layer is composed of albumen, butthere is evidence to show that it is modified casein due to the rapidevaporation of the milk serum at the surface of the milk. 

_3. Digestibility. _ Considerable difference of opinion has existed inthe minds of medical men as to the relative digestibility of raw andheated milks. A considerable amount of experimental work has been doneby making artificial digestion experiments with enzyms, also digestionexperiments with animals, and in a few cases with children. The resultsobtained by different investigators are quite contradictory, althoughthe preponderance of evidence seems to be in favor of the view thatheating does impair the digestibility of milk, especially if thetemperature attains the sterilizing point. [135] It has been observedthat there is a noteworthy increase in amount of rickets, [136] scurvyand marasmus in children where highly-heated milks are employed. Theseobjections do not obtain with reference to milk heated to moderatetemperatures, as in pasteurization, although even this lower temperaturelessens slightly its digestibility. The successful use of pasteurizedmilks in children's hospitals is evidence of its usefulness. 

_4. Fermentative changes. _ The normal souring change in milk is due tothe predominance of the lactic acid bacteria, but as these organisms asa class do not possess spores, they are readily killed when heated abovethe thermal death-point of the developing cell. The destruction of thelactic forms leaves the spore-bearing types possessors of the field, andconsequently the fermentative changes in heated milk are not those thatusually occur, but are characterized by the curdling of the milk fromthe action of rennet enzyms. 

_5. Action of rennet. _ Heating milk causes the soluble lime salts to beprecipitated, and as the curdling of milk by rennet (in cheese-making)is dependent upon the presence of these salts, their absence in heatedmilks greatly retards the action of rennet. This renders it difficult toutilize heated milks in cheese-making unless the soluble lime salts arerestored, which can be done by adding solutions of calcium chlorid. 

~Sterilization. ~ As ordinarily used in dairying, sterilization means theapplication of heat at temperatures approximating, if not exceeding, 212° F. It does not necessarily imply that milk so treated is sterile, i. E. , germ-free; for, on account of the resistance of spores, it ispractically impossible to destroy entirely _all_ these hardy forms. Ifmilk is heated at temperatures above the boiling point, as is done wheresteam pressure is utilized, it can be rendered practically germ-free. Such methods are employed where it is designed to keep milk sweet for along period of time. The treatment of milk by sterilization has not metwith any general favor in this country, although it has been more widelyintroduced abroad. In most cases the process is carried out after themilk is bottled; and considerable ingenuity has been exercised in theconstruction of devices which will permit of the closure of the bottlesafter the sterilizing process has been completed. Milks heated to sohigh a temperature have a more or less pronounced boiled or cookedtaste, a condition that does not meet with general favor in thiscountry. The apparatus suitable for this purpose must, of necessity, beso constructed as to withstand steam pressure, and consequently isconsiderably more expensive than that required for the simplerpasteurizing process. 

~Pasteurization. ~ In this method the degree of heat used ranges from 140°to 185° F. And the application is made for only a limited length oftime. The process was first extensively used by Pasteur (from whom itderives its name) in combating various maladies of beer and wine. Itsimportance as a means of increasing the keeping quality of milk was notgenerally recognized until a few years ago; but the method is nowgrowing rapidly in favor as a means of preserving milk for commercialpurposes. The method does not destroy all germ-life in milk; it affectsonly those organisms that are in a growing, vegetative condition; but ifthe milk is quickly cooled, it enhances the keeping quality verymaterially. It is unfortunate that this same term is used in connectionwith the heating of cream as a preparatory step to the use of purecultures in cream-ripening in butter-making. The objects to beaccomplished vary materially and the details of the two processes arealso quite different. 

While pasteurizing can be performed on a small scale by the individual, the process can also be adapted to the commercial treatment of largequantities of milk. The apparatus necessary for this purpose is notnearly so expensive as that used in sterilizing, a factor of importancewhen other advantages are considered. In this country pasteurization hasmade considerable headway, not only in supplying a milk that is designedto serve as children's food, but even for general purposes. 

~Requirements essential in pasteurization. ~ While considerable latitudewith reference to pasteurizing limits is permitted, yet there arecertain conditions which should be observed, and these, in a sense, fixthe limits that should be employed. These may be designated as (1) the_physical_, and (2) the _biological_ requirements. 

~Physical requirements. ~ _1. Avoidance of scalded or cooked taste. _ TheEnglish and American people are so averse to a scalded or cooked flavorin milk that it is practically impossible for a highly heated product tobe sold in competition with ordinary raw milk. In pasteurization then, care must be taken not to exceed the temperature at which a permanentlycooked flavor is developed. As previously observed, this point varieswith the period of exposure. A momentary exposure to a temperature ofabout 170° F. May be made without any material alteration, but if theheat is maintained for a few minutes (ten minutes or over), atemperature of 158° to 160° F. Is about the maximum that can be employedwith safety. 

_2. Normal creaming of the milk. _ It is especially desirable that asharp and definite cream line be evident on the milk soon afterpasteurization. If this fails to appear, the natural inference of theconsumer is that the milk is skimmed. If the milk be heated to atemperature sufficiently high to cause the fat-globule clusters todisintegrate (see Figs. 22 and 23), the globules do not rise to thesurface as readily as before and the cream line remains indistinct. Where the exposure is made for a considerable period of time (10 minutesor more), the maximum temperature which can be used without producingthis change is about 140° F. ; if the exposure is made for a very brieftime, a minute or less, the milk may be heated to 158°-160 F. ° withoutinjuring the creaming property. 

_3. No diminution in cream "body. "_ Coincident with this change whichtakes place in the creaming of the milk is the change in body orconsistency which is noted where cream is pasteurized at too high atemperature. For the same reason as given under (2) cream heated abovethese temperatures is reduced in apparent thickness and appears tocontain less butter-fat. Of course the pasteurizing process does notchange the fat content, but its "body" is apparently so affected. Thus a25 per cent. Cream may seem to be no thicker or heavier than an 18 percent. Raw cream. This real reduction in consistency naturally affectsthe readiness with which the cream can be whipped. 

~Biological requirements. ~ _1. Enhanced keeping quality. _ In commercialpractice the essential biological requirement is expressed in theenhanced keeping quality of the pasteurized milk. This expresses in apractical way the reduction in germ life accomplished by thepasteurizing process. The improvement in keeping quality depends uponthe temperature and time of exposure, but fully as much also on the wayin which the pasteurized product is handled after heating. The lowesttemperature which can be used with success to kill the active, vegetative bacteria is about 140° F. , at which point it requires aboutten minutes exposure. If this period is curtailed the temperature mustbe raised accordingly. An exposure to a temperature of 175° F. For aminute has approximately the same effect as the lower degree of heat forthe longer time. 

The following bacteriological studies as to the effect which a variationin temperature exerts on bacterial life in milk are of importance asindicating the foundation for the selection of the proper limits. In thefollowing table the exposures were made for a uniform period (20minutes):

_The bacterial content of milk heated at different temperatures. _

 Number of bacteria per cc. In milk. 45° C. 50° C. 55° C. 60° C. 65° C. 70° C. Unheated 113° F. 122° F. 131° F. 140° F. 149° F. 158° F. Series I. 2, 895, 000 ---- 1, 260, 000 798, 000 32, 000 5, 770 3, 900Series II. 750, 000 665, 000 262, 400 201, 000 950 700 705Series III. 1, 350, 000 1, 100, 000 260, 000 215, 000 575 610 650Series IV. 1, 750, 000 ---- 87, 360 ---- 4, 000 3, 500 3, 600

It appears from these results that the most marked decrease intemperature occurs at 140° F. (60° C. ). It should also be observed thatan increase in heat above this temperature did not materially diminishthe number of organisms present, indicating that those forms remainingwere in a spore or resistant condition. It was noted, however, that thedeveloping colonies grew more slowly in the plates made from the highlyheated milk, showing that their vitality was injured to a greater extenteven though not killed. 

_2. Destruction of disease bacteria. _ While milk should be pasteurizedso as to destroy all active, multiplying bacteria, it is particularlyimportant to destroy any organisms of a disease nature that might findtheir way into the same. Fortunately most of the bacteria capable ofthriving in milk before or after it is drawn from the animal are notable to form spores and hence succumb to proper pasteurization. Such isthe case with the diphtheria, cholera and typhoid organisms. 

The organism that is invested with most interest in this connection isthe tubercle bacillus. On account of its more or less frequentoccurrence in milk and its reputed high powers of resistance, it maywell be taken as a standard in pasteurizing. 

~Thermal death limits of tubercle bacillus. ~ Concerning the exacttemperature at which this germ is destroyed there is considerabledifference of opinion. Part of this arises from the inherent difficultyin determining exactly when the organism is killed (due to its failureto grow readily on artificial media), and part from the lack of uniformconditions of exposure. The standards that previously have been mostgenerally accepted are those of De Man, [137] who found that thirtyminutes exposure at 149° F. , fifteen minutes at 155° F. , or ten minutesat 167° F. , sufficed to destroy this germ. 

More recently it has been demonstrated, [138] and these resultsconfirmed, [139] that if tuberculous milk is heated in closed receptacleswhere the surface pellicle does not form, the vitality of this diseasegerm is destroyed at 140° F. In 10-15 minutes, while an exposure at 160°F. Requires only about one minute. [140] If the conditions of heating aresuch that the surface of the milk is exposed to the air, the resistanceof bacteria is greatly increased. When heated in open vessels Smithfound that the tubercle organism was not killed in some cases where theexposure was made for at least an hour. Russell and Hastings[141] haveshown an instance where the thermal death-point of a micrococcusisolated from pasteurized milk was increased 12. 5° F. , by heating itunder conditions that permitted of the formation of the scalded layer. It is therefore apparent that apparatus used for pasteurization shouldbe constructed so as to avoid this defect. 

~Methods of treatment. ~ Two different systems of pasteurization have grownup in the treatment of milk. One of these has been developed from thehygienic or sanitary aspect of the problem and is used more particularlyin the treatment of cream and relatively small milk supplies. The othersystem has been developed primarily from the commercial point of viewwhere a large amount of milk must be treated in the minimum time. In thefirst method the milk is heated for a longer period of time, aboutfifteen minutes at a relatively low temperature from 140°-155° F. ; inthe other, the milk is exposed to the source of heat only while it ispassing rapidly through the apparatus. Naturally, the exposure undersuch conditions must be made at a considerably higher temperature, usually in the neighborhood of 160° F. 

The types of apparatus used in these respective processes naturallyvaries. Where the heating is prolonged, the apparatus employed is builton the principle of a _tank_ or _reservoir_ in which a given volume ofmilk may be held at any given temperature for any given period of time. 

When the heat is applied for a much shorter period of time, the milk ispassed in a continuous stream through the machine. Naturally thecapacity of a continuous-flow apparatus is much greater than a machinethat operates on the intermittent principle; hence, for large supplies, as in city distribution, this system has a great advantage. The questionas to relative efficiency is however one which should be given mostcareful consideration. 

~Pasteurizing apparatus. ~ The problems to be solved in the pasteurizationof milk and cream designed for direct consumption are so materiallydifferent from where the process is used in butter-making that the typeof machinery for each purpose is quite different. The equipmentnecessary for the first purpose may be divided into two general classes:

1. Apparatus of limited capacity designed for family use. 

2. Apparatus of sufficient capacity to pasteurize on a commercial scale. 

~Domestic pasteurizers. ~ In pasteurizing milk for individual use, it isnot desirable to treat at one time more than will be consumed in oneday; hence an apparatus holding a few bottles will suffice. In this casethe treatment can best be performed in the bottle itself, therebylessening the danger of infection. Several different types ofpasteurizers are on the market; but special apparatus is by no meansnecessary for the purpose. The process can be efficiently performed byany one with the addition of an ordinary dairy thermometer to the commonutensils found in the kitchen. Fig. 24 indicates a simple contrivancethat can be readily arranged for this purpose. 

The following suggestions indicate the different steps of the process:

1. Use only fresh milk. 

2. Place milk in clean bottles or fruit cans, filling to a uniformlevel, closing bottles tightly with a cork or cover. If pint and quartcans are used at the same time, an inverted bowl will equalize thelevel. Set these in a flat-bottomed tin pail and fill with warm water tosame level as milk. An inverted pie tin punched with holes will serve asa stand on which to place the bottles during the heating process. 

3. Heat water in pail until the temperature of same reaches 155° to 160°F. ; then remove from source of direct heat, cover with a cloth or tincover, and allow the whole to stand for half an hour. In the preparationof milk for children, it is not advisable to use the low-temperaturetreatment (140° F. ) that is recommended for commercial city delivery. 

[Illustration: FIG. 24. A home-made pasteurizer. ]

4. Remove bottles of milk and cool them as rapidly as possible withoutdanger to bottles and store in a refrigerator. 

~Commercial pasteurizers. ~ The two methods of pasteurization practicedcommercially for the preservation of milk and cream have been developedbecause of the two types of machinery now in use. Apparatus constructedon the reservoir or tank principle permits of the retention of the milkfor any desired period of time. Therefore, a lower temperature can beemployed in the treatment. In those machines where the milk flowsthrough the heater in a more or less continuous stream, the period ofexposure is necessarily curtailed, thereby necessitating a highertemperature. 

~Reservoir pasteurizers. ~ The simplest type of apparatus suitable forpasteurizing on this principle is where the milk is placed in shotguncans and immersed in water heated by steam. Ordinary tanks surroundedwith water spaces can also be used successfully. The Boyd cream ripeningvat has also been tried. In this the milk is heated by a swinging coilimmersed in the vat through which hot water circulates. 

In 1894 the writer[142] constructed a tank pasteurizer which consistedof a long, narrow vat surrounded by a steam-heated water chamber. Boththe milk and the water chambers were provided with mechanical agitatorshaving a to-and-fro movement. 

[Illustration: FIG. 25. Pott's pasteurizer. ]

Another machine which has been quite generally introduced is the Potts'rotating pasteurizer. This apparatus has a central milk chamber that issurrounded with an outer shell containing hot water. The whole machinerevolves on a horizontal axis, and the cream or milk is thus thoroughlyagitated during the heating process. 

~Continuous-flow pasteurizers. ~ The demand for greater capacity than canbe secured in the reservoir machines has led to the perfection ofseveral kinds of apparatus where the milk is heated momentarily as itflows through the apparatus. Most of these were primarily introduced forthe treatment of cream for butter-making purposes, but they arefrequently employed for the treatment of milk on a large scale in citymilk trade. Many of them are of European origin although of late yearsseveral have been devised in this country. 

The general principle of construction is much the same in most of them. The milk is spread out in a thin sheet, and is treated by passing itover a surface, heated either with steam directly or preferably with hotwater. 

Where steam is used directly, it is impossible to prevent the "scaldingon" of the milk proteids to the heated surface. 

In some of these machines (Thiel, Kuehne, Lawrence, De Laval, andHochmuth), a ribbed surface is employed over which the milk flows, whilethe opposite surface is heated with hot water or steam. Monrad, Lefeldtand Lentsch employ a centrifugal apparatus in which a thin layer of milkis heated in a revolving drum. 

In some types of apparatus, as in the Miller machine, an Americanpasteurizer, the milk is forced in a thin sheet between two heatedsurfaces, thereby facilitating the heating process. In the Farringtonmachine heated discs rotate in a reservoir through which the milk flowsin a continuous stream. 

One of the most economical types of apparatus is the regenerator type (aGerman machine), in which the milk passes over the heating surface in athin stream and then is carried back over the incoming cold milk sothat the heated liquid is partially cooled by the inflowing fresh milk. In machines of this class it requires very much less steam to heat upthe milk than in those in which the cold milk is heated wholly by thehot water. 

A number of machines have been constructed on the principle of areservoir which is fed by a constantly flowing stream. In some kinds ofapparatus of this type no attempt is made to prevent the mixing of therecently introduced milk with that which has been partially heated. Thepattern for this reservoir type is Fjord's heater, in which the milk isstirred by a stirrer. This apparatus was originally designed as a heaterfor milk before separation, but it has since been materially modified sothat it is better adapted to the purposes of pasteurization. Reid wasthe first to introduce this type of machine into America. 

~Objections to continuous flow pasteurizers. ~ In all continuous flowpasteurizers certain defects are more or less evident. While theyfulfill the important requirement of large capacity, an absoluteessential where large volumes of milk are being handled, it does not ofnecessity follow that they conform to all the hygienic and physicalrequirements that should be kept in mind. The greatest difficulty is theshortened period of exposure. The period which the milk is actuallyheated is often not more than a minute or so. Another serious defect isthe inability to heat _all_ of the milk for a uniform period of time. Atbest, the milk is exposed for an extremely short time, but even thenportions pass through the machine much more quickly than do theremainder. Those portions in contact with the walls of the apparatus areretarded by friction and are materially delayed in their passage, whilethe particles in the center of the stream, however thin, flow throughin the least possible time. 

The following simple method enables the factory operator to test theperiod of exposure in the machine: Start the machine full of water, andafter the same has become heated to the proper temperature, change theinflow to full-cream milk, continuing at the same rate. Note the exacttime of change and also when first evidence of milkiness begins toappear at outflow. If samples are taken from first appearance of milkycondition and thereafter at different intervals for several minutes, itis possible, by determining the amount of butter-fat in the same, tocalculate with exactness how long it takes for the milk to entirelyreplace the water. 

Tests made by the writer[143] on the Miller pasteurizer showed, when fedat the rate of 1, 700 pounds per hour, the minimum period of exposure tobe 15 seconds, and the maximum about 60-70 seconds, while abouttwo-thirds of the milk passed the machine in 40-50 seconds. Thismanifest variation in the rate of flow of the milk through the machineis undoubtedly the reason why the results of this type of treatment aresubject to so much variation. Naturally, even a fatal temperature tobacterial life can be reduced to a point where actual destruction ofeven vegetating cells does not occur. 

~Bacterial efficiency of reservoir pasteurizers. ~ The bacterial content ofpasteurized milk and cream will depend somewhat on the number oforganisms originally present in the same. Naturally, if mixed milkbrought to a creamery is pasteurized, the number of organisms remainingafter treatment would be greater than if the raw material was fresh andproduced on a single farm. 

An examination of milk and cream pasteurized on a commercial scale inthe Russell vat at the Wisconsin Dairy school showed that over 99. 8 percent of the bacterial life in raw milk or cream was destroyed by theheat employed, i. E. , 155° F. For twenty minutes duration. [144] Innearly one-half of the samples of milk, the germ content in thepasteurized sample fell below 1, 000 bacteria per cc. , and the average oftwenty-five samples contained 6, 140 bacteria per cc. In cream the germcontent was higher, averaging about 25, 000 bacteria per cc. This milkwas taken from the general creamery supply, which was high in organisms, containing on an average 3, 675, 000 bacteria per cc. De Schweinitz[145]has reported the germ content of a supply furnished in Washington whichwas treated at 158° to 160° F. For fifteen minutes. This supply camefrom a single source. Figures reported were from 48-hour-old agarplates. Undoubtedly these would have been higher if a longer period ofincubation had been maintained. The average of 82 samples, taken for theperiod of one year, showed 325 bacteria per cc. 

[Illustration: FIG. 26. Effect of pasteurizing on germ content of milk. Black square represents bacteria of raw milk; small white square, thoseremaining after pasteurization. ]

~Bacterial efficiency of continuous-flow pasteurizers. ~ A quantitativedetermination of the bacteria found in milk and cream when treated inmachinery of this class almost always shows a degree of variation inresults that is not to be noted in the discontinuous apparatus. 

[Illustration: FIG. 27. Reid's Continuous Pasteurizer. ]

Harding and Rogers[146] have tested the efficiency of one of the Danishtype of continuous pasteurizers. These experiments were made at 158°, 176° and 185° F. They found the efficiency of the machine not whollysatisfactory at the lower temperatures. At 158° F. The average offourteen tests gave 15, 300 bacteria per cc. , with a maximum to minimumrange from 62, 790 to 120. Twenty-five examinations at 176° F. Showed anaverage of only 117, with a range from 300 to 20. The results at 185° F. Showed practically the same results as noted at 176° F. Considerabletrouble was experienced with the "scalding on" of the milk to the wallsof the machine when milk of high acidity was used. 

Jensen[147] details the results of 139 tests in 1899, made by theCopenhagen Health Commission. In 66 samples from one hundred thousand toone million organisms per cc. Were found, and in 22 cases from one tofive millions. Nineteen tests showed less than 10, 000 per cc. 

In a series of tests conducted by the writer[148] on a Millerpasteurizer in commercial operation, an average of 21 tests showed12, 350 bacteria remaining in the milk when the milk was pasteurized from156°-164° F. The raw milk in these tests ran from 115, 000 to about onemillion organisms per cc. 

A recently devised machine of this type (Pasteur) has been tested byLehmann, who found that it was necessary to heat the milk as high as176° to 185° F. , in order to secure satisfactory results on thebacterial content of the cream. 

The writer tested Reid's pasteurizer at 155° to 165° F. With thefollowing results: in some cases as many as 40 per cent. Of the bacteriasurvived, which number in some cases exceeded 2, 000, 000 bacteria percc. 

~Pasteurizing details. ~ While the pasteurizing process is exceedinglysimple, yet, in order to secure the best results, certain conditionsmust be rigidly observed in the treatment before and after the heatingprocess. 

It is important to select the best possible milk for pasteurizing, forif the milk has not been milked under clean conditions, it is likely tobe rich in the spore-bearing bacteria. Old milk, or milk that has notbeen kept at a low temperature, is much richer in germ-life thanperfectly fresh or thoroughly chilled milk. 

The true standard for selecting milk for pasteurization should be todetermine the actual number of bacterial _spores_ that are able toresist the heating process, but this method is impracticable undercommercial conditions. 

The following method, while only approximate in its results, will befound helpful: Assuming that the age or treatment of the milk bears acertain relation to the presence of spores, and that the acid increasesin a general way with an increase in age or temperature, the amount ofacid present may be taken as an approximate index of the suitability ofthe milk for pasteurizing purposes. Biological tests were carried out inthe author's laboratory[149] on milks having a high and low acidcontent, and it was shown that the milk with the least acid was, as arule, the freest from spore-bearing bacteria. 

This acid determination can be made at the weigh-can by employing theFarrington alkaline tablet which is used in cream-ripening. Where milkis pasteurized under general creamery conditions, none should be usedcontaining more than 0. 2 per cent acidity. If only perfectly fresh milkis used, the amount of acid will generally be about 0. 15 per cent withphenolphthalein as indicator. 

[Illustration: FIG. 28. Diagram showing temperature changes inpasteurizing, and the relation of same to bacterial growth. 

Shaded zone represents limits of bacterial growth, 50°-109° F. (10°-43°C. ), the intensity of shading indicating rapidity of development. Thesolid black line shows temperature of milk during the process. Thenecessity for rapid cooling is evident as the milk falls in temperatureto that of growing zone. ]

Emphasis has already been laid on the selection of a proper limit ofpasteurizing (p. 114). It should be kept constantly in mind that thethermal death-point of any organism depends not alone on the temperatureused, but on the period of exposure. With the lower limits given, 140°F. , it is necessary to expose the milk for not less than fifteenminutes. If a higher heat is employed (and the cooked flavordisregarded) the period of exposure may be curtailed. 

~Chilling the milk. ~ It is very essential in pasteurizing that the heatedmilk be immediately chilled in order to prevent the germination of theresistant spores, for if germination once occurs, growth can go on atrelatively low temperatures. 

The following experiments by Marshall[150] are of interest as showingthe influence of refrigeration on germination of spores:

Cultures of organisms that had been isolated from pasteurized milk wereinoculated into bouillon. One set was left to grow at room temperature, another was pasteurized and allowed to stand at same temperature, whileanother heated set was kept in a refrigerator. The unheated cultures atroom temperature showed evidence of growth in thirty trials in anaverage of 26 hours; 29 heated cultures at room temperature alldeveloped in an average of 50 hours, while the heated cultures kept inrefrigerator showed no growth in 45 days with but four exceptions. 

Practically all of the rapid-process machines are provided withespecially constructed cooling devices. In some of them, as in theMiller and Farrington, the cooling is effected by passing the milkthrough two separate coolers that are constructed in the same generalway as the heater. With the first cooler, cold running water isemployed, the temperature often being lowered in this way to 58° or 60°F. Further lessening of the temperature is secured by an additional icewater or brine cooler which brings the temperature down to 40°-50° F. 

In the economical use of ice the ice itself should be applied as closelyas possibly to the milk to be cooled, for the larger part of thechilling value of ice comes from the melting of the same. To convert apound of ice at 32° F. Into a pound of water at the same temperature, ifwe disregard radiation, would require as much heat as would suffice toraise 142 pounds of water one degree F. , or one pound of water 142° F. The absorptive capacity of milk for heat (specific heat) is not quitethe same as it is with water, being . 847 for milk in comparison with 1. 0for water. [151] Hot milk would therefore require somewhat less ice tocool it than would be required by any equal volume of water at the sametemperature. 

~Bottling the product. ~ If the milk has been properly pasteurized, itshould, of course, be dispensed in sterilized bottles. Glass bottleswith plain pulp caps are best, and these should be thoroughly sterilizedin steam before using. The bottling can best be done in a commercialbottling machine. Care must be taken to thoroughly clean this apparatusafter use each day. Rubber valves in these machines suffer deteriorationrapidly. 

[Illustration: FIG. 29. Relative consistency of pasteurized cream before(A) and after (B) treatment with viscogen as shown by rate of flow downinclined glass plate. ]

~Restoration of "body" of pasteurized cream. ~ The action of heat causesthe tiny groupings of fat globules in normal milk (Fig. 22) to break up, and with this change, which occurs in the neighborhood of 140° F. , wherethe milk is heated for about 15 minutes and at about 160-165° F. Whererapidly heated in a continuous stream, the consistency of the liquid isdiminished, notwithstanding the fact that the fat-content remainsunchanged. Babcock and the writer[152] devised the following "cure" forthis apparent defect. If a strong solution of cane sugar is added tofreshly slacked lime and the mixture allowed to stand, a clear fluid canbe decanted off. The addition of this alkaline liquid, which is called"viscogen, " to pasteurized cream in proportions of about one part ofsugar-lime solution to 100 to 150 of cream, restores the consistency ofthe cream, as it causes the fat globules to cluster together in smallgroups. 

The relative viscosity of creams can easily be determined by thefollowing method (Fig. 29):

Take a perfectly clean piece of glass (plate or picture glass ispreferable, as it is less liable to be wavy). Drop on one edge two orthree drops of cream at intervals of an inch or so. Then incline pieceof glass at such an angle as to cause the cream to flow down surface ofglass. The cream, having the heavier body or viscosity, will move moreslowly. If several samples of each cream are taken, then the aggregatelengths of the different cream paths may be taken, thereby eliminatingslight differences due to condition of glass. 

FOOTNOTES:

[126] From 10 to 16 cents per quart is usually paid for such milks. 

[127] Much improvement in quality could be made by more careful controlof milk during shipment, especially as to refrigeration; also as to thecare taken on the farms. The use of the ordinary milking machine (seepage 37), would go far to reduce the germ content of milk. 

[128] Farrington, Journ. Amer. Chem. Soc. , Sept. , 1896. 

[129] Hite, Bull. 58, West Va. Expt. Stat. , 1899. 

[130] Milch Zeit. , 1895, No. 9. 

[131] Ibid. , 1897, No. 33. 

[132] Bernstein, Milch Zeit. , 1894, pp. 184, 200. 

[133] Thoerner, Chem. Zeit. , 18:845. 

[134] Snyder, Chemistry of Dairying, p. 59. 

[135] Doane and Price (Bull. 77, Md. Expt. Stat. , Aug. 1901) give quitea full resumé of the work on this subject in connection with ratherextensive experiments made by them on feeding animals with raw, pasteurized and sterilized milks. 

[136] Rickets is a disease in which the bones lack sufficient mineralmatter to give them proper firmness. Marasmus is a condition in whichthe ingested food seems to fail to nourish the body and gradual wastingaway occurs. 

[137] De Man, Arch. F. Hyg. , 1893, 18:133. 

[138] Th. Smith, Journ. Of Expt. Med. , 1899, 4:217. 

[139] Russell and Hastings, 17 Rept. Wis. Expt. Stat. , 1900, p. 147. 

[140] Russell and Hastings, 21 Rept. Ibid. , 1904. 

[141] Russell and Hastings, 18 Rept. Ibid. , 1901. 

[142] Russell, Bull. 44, Wis. Expt. Stat. 

[143] Russell, 22 Wis. Expt. Stat. Rept. , 1905, p. 232. 

[144] Russell, 12 Wis. Expt. Stat. Rept. , 1895, p. 160. 

[145] De Schweinitz, Nat. Med. Rev. , 1899, No. 11. 

[146] Harding and Rogers. Bull. 182, N. Y. (Geneva) Expt. Stat. , Dec. , 1899. 

[147] Jensen, Milchkunde und Milch Hygiene, p. 132. 

[148] 22 Wis. Expt. Stat. Rept. , 1905, p. 236. 

[149] Shockley, Thesis, Univ. Of Wis. , 1896. 

[150] Marshall, Mich. Expt. Stat. , Bull. 147, p. 47. 

[151] Fleischmann, Landw. Versuchts Stat. , 17:251. 

[152] Babcock and Russell, Bull. 54, Wis. Expt. Stat. , Aug. 1896. 

CHAPTER VII. 

BACTERIA AND BUTTER-MAKING. 

In making butter from the butter fat in milk, it is necessary toconcentrate the fat globules into cream, preliminary to the churningprocess. The cream may be raised by the gravity process or separatedfrom the milk by centrifugal action. In either case the bacteria thatare normally present in the milk differentiate themselves in varyingnumbers in the cream and the skim-milk. The cream always contains percc. A great many more than the skim-milk, the reason for this being thatthe bacteria are caught and held in the masses of fat globules, which, on account of their lighter specific gravity, move toward the surface ofthe milk or toward the interior of the separator bowl. This filteringaction of the fat globules is similar to what happens in muddy waterupon standing. As the suspended particles fall to the bottom they carrywith them a large number of the organisms that are in the liquid. 

~Various creaming methods. ~ The creaming method has an important bearingon the kind as well as the number of the bacteria that are to be foundin the cream. The difference in species is largely determined by thedifference in ripening temperature, while the varying number is governedmore by the age of the milk. 

_1. Primitive gravity methods. _ In the old shallow-pan process, thetemperature of the milk is relatively high, as the milk is allowed tocool naturally. This comparatively high temperature favors especiallythe development of those forms whose optimum growing-point is near theair temperature. By this method the cream layer is exposed to the airfor a longer time than with any other, and consequently thecontamination from this source is greater. Usually cream obtained by theshallow-pan process will contain a larger number of species and alsohave a higher acid content. 

_2. Modern gravity methods. _ In the Cooley process, or any of the moderngravity methods where cold water or ice is used to lower thetemperature, the conditions do not favor the growth of a large varietyof species. The number of bacteria in the cream will depend largely uponthe manner in which the milk is handled previous to setting. If care isused in milking, and the milk is kept so as to exclude outsidecontamination, the cream will be freer from bacteria than ifcarelessness prevails in handling the milk. Only those forms willdevelop in abundance that are able to grow at the low temperature atwhich the milk is set. Cream raised by this method is less frequentlyinfected with undesirable forms than that which is creamed at a highertemperature. 

_3. Centrifugal method. _ Separator cream should contain less germ-lifethan that which is secured in the old way. It should contain only thoseforms that have found their way into the milk during and subsequent tothe milking, for the cream is ordinarily separated so soon that there isbut little opportunity of infection, if care is taken in the handling. As a consequence, the number of species found therein is smaller. 

Where milk is separated, it is always prudent to cool the cream so as tocheck growth, as the milk is generally heated before separating in orderto skim efficiently. 

Although cream is numerically much richer in bacteria than milk, yetthe changes due to bacterial action are slower; hence milk sours morerapidly than cream. For this same reason, cream will sour sooner when itremains on the milk than it will if it is separated as soon as possible. This fact indicates the necessity of early creaming, so as to increasethe keeping quality of the product, and is another argument in favor ofthe separator process. 

~Ripening of cream. ~ If cream is allowed to remain at ordinarytemperatures, it undergoes a series of fermentation changes that areexceedingly complex in character, the result of which is to produce inbutter made from the same the characteristic flavor and aroma that areso well known in this article. We are so accustomed to the developmentof these flavors in butter that they are not generally recognized asbeing intimately associated with bacterial activity unless compared withbutter made from perfectly fresh cream. Sweet-cream butter lacks thearomatic principle that is prominent in the ripened product, and whilethe flavor is delicate, it is relatively unpronounced. 

In the primitive method of butter-making, where the butter was made onthe farm, the ripening of cream became a necessity in order thatsufficient material might be accumulated to make a churning. Theripening change occurred spontaneously without the exercise of anyespecial control. With the development of the creamery system came thenecessity of exercising a control of this process, and therefore themodern butter-maker must understand the principles which are involved inthis series of complex changes that largely give to his product itscommercial value. 

In these ripening changes three different factors are to be taken intoconsideration: the development of acid, flavor and aroma. Much confusionin the past has arisen from a failure to discriminate between thesequalities. While all three are produced simultaneously in ordinaryripening, it does not necessarily follow that they are produced by thesame cause. If the ripening changes are allowed to go too far, undesirable rather than beneficial decomposition products are produced. These greatly impair the value of butter, so that it becomes necessaryto know just to what extent this process should be carried. 

In cream ripening there is a very marked bacterial growth, the extent ofwhich is determined mainly by the temperature of the cream. Conn andEsten[153] find that the number of organisms may vary widely inunripened cream, but that the germ content of the ripened product ismore uniform. When cream is ready for the churn, it often contains500, 000, 000 organisms per cc. , and frequently even a higher number. Thisrepresents a germ content that has no parallel in any natural material. 

The larger proportion of bacteria in cream as it is found in thecreamery belong to the acid-producing class, but in the process ofripening, these forms seem to thrive still better, so that when it isready for churning the germ content of the cream is practically made upof this type. 

~Effect on churning. ~ In fresh cream the fat globules which are suspendedin the milk serum are surrounded by a film of albuminous material whichprevents them from coalescing readily. During the ripening changes, thisenveloping substance is modified, probably by partial solution, so thatthe globules cohere when agitated, as in churning. The result is thatripened cream churns more easily, and as it is possible to cause alarger number of the smaller fat-globules to cohere to the buttergranules, the yield is slightly larger--a point of considerableeconomic importance where large quantities of butter are made. 

~Development of acid. ~ The result of this enormous bacterialmultiplication is that acid is produced in cream, lactic being theprincipal acid so formed. 

Other organic acids are undoubtedly formed as well as certain aromaticproducts. While the production of acid as a result of fermentativeactivity is usually accompanied with a development of flavor, the flavoris not directly produced by the formation of acid. If cream is treatedin proper proportions with a commercial acid, as hydrochloric, [154] itassumes the same churning properties as found in normally ripened cream, but is devoid of the desired aromatic qualities. Lactic acid[155] hasalso been used in a similar way but with no better results. 

The amount of acidity that should be developed under natural conditionsso as to secure the optimum quality as to flavor and aroma is the mostimportant question in cream ripening. Concerning this there have beentwo somewhat divergent views as to what is best in practice, someholding that better results were obtained with cream ripened to a highdegree of acidity than where a less amount was developed. [156] Thepresent tendency seems to be to develop somewhat more than formerly, asit is thought that this secures more of the "high, quick" flavor wantedin the market. On the average, cream is ripened to about 0. 5 to 0. 65 percent. Acidity, a higher percentage than this giving a strong-flavoredbutter. In the determination of acidity, the most convenient method isto employ the Farrington alkaline tablet, which permits of an accurateand rapid estimation of the acidity in the ripening cream. The amount ofacidity to be produced must of necessity be governed by the amount ofbutter-fat present, for the formation of acid is confined to the serumof the cream; consequently, a rich cream would show less acid bytitration than a thinner cream, and still contain really as much acid asthe other. The importance of this factor is evident in gathered-creamfactories. 

The rate of ripening is dependent upon the conditions that affect therate of growth of bacterial life, such as time and temperature, numberof organisms in cream and also the per cent of butter fat in the cream. Some years ago it was customary to ripen cream at about 50° to 60° F. , but more recently better results have been obtained, it is claimed, where the ripening temperature is increased and the period of ripeninglessened. As high a temperature as 70° to 75° F. Has been recommended. It should be said that this variation in practice may have a validscientific foundation, for the temperature of the ripening cream isundoubtedly the most potent factor in determining what kind of bacteriawill develop most luxuriantly. It is well known that those forms thatare capable of producing bitter flavors are able to thrive better at alower temperature than some of the desirable ripening species. 

The importance of this factor would be lessened where a pure culture wasused in pasteurized cream, because here practically the selectedorganism alone controls the field. 

It is frequently asserted that better results are obtained by stirringthe cream and so exposing it to the air as much as possible. Experimentsmade at the Ontario Agricultural College, however, show practically nodifference in the quality of the butter made by these two methods. Thegreat majority of the bacteria in the cream belong to the facultativeclass, and are able to grow under conditions where they are not indirect contact with the air. 

~Flavor and aroma. ~ The basis for the peculiar flavor or taste whichripened cream-butter possesses is due, in large part, to the formationof certain decomposition products formed by various bacteria. Aroma is aquality often confounded with flavor, but this is produced by volatileproducts only, which appeal to the sense of smell rather than taste. Generally a good flavor is accompanied by a desirable aroma, but theorigin of the two qualities is not necessarily dependent on the sameorganisms. The quality of flavor and aroma in butter is, of course, alsoaffected by other conditions, as, for instance, the presence or absenceof salt, as well as the inherent qualities of the milk, that arecontrolled, to some extent at least, by the character of the feed whichis consumed by the animal. The exact source of these desirable butevanescent qualities in butter is not yet satisfactorily determined. According to Storch, [157] flavors are produced by the decomposition ofthe milk sugar and the absorption of the volatile flavors by the butterfat. Conn[158] holds that the nitrogenous elements in cream serve asfood for bacteria, and in the decomposition of which the desiredaromatic substance is produced. The change is unquestionably a complexone, and cannot be explained as a single fermentation. 

There is no longer much doubt but that both acid-forming andcasein-digesting species can take part in the production of properflavors as well as desirable aromas. The researches of Conn, [159] whohas studied this question most exhaustively, indicate that both of thesetypes of decomposition participate in the production of flavor andaroma. He has shown that both flavor and aroma production areindependent of acid; that many good flavor-producing forms belong tothat class which renders milk alkaline, or do not change the reaction atall. Some of these species liquefied gelatin and would therefore belongto the casein-dissolving class. Those species that produced bad flavorsare also included in both fermentative types. Conn has found a number oforganisms that are favorable flavor-producers; in fact they were muchmore numerous than desirable aroma-yielding species. None of thefavorable aroma forms according to his investigations were lactic-acidspecies, --a view which is also shared by Weigmann. [160]

McDonnell[161] has found that the production of aroma in certain casesvaries at different temperatures, the most pronounced being evolved nearthe optimum growing temperature, which, as a general rule, is too highfor cream ripening. 

The majority of bacteria in ripening cream do not seem to exert anymarked influence in butter. A considerable number of species arepositively beneficial, inasmuch as they produce a good flavor or aroma. A more limited number are concerned in the production of undesirableripening changes. This condition being true, it may seem strange thatbutter is as good as it is, because so frequently the requisite care isnot given to the development of proper ripening. In all probability thechief reason why this is so is that those bacteria that find milk andcream pre-eminently suited to their development, e. G. The lactic-acidclass, are either neutral or beneficial in their effect on butter. 

~Use of starters. ~ Experience has amply demonstrated that it is possibleto control the nature of the fermentative changes that occur in ripeningcream to such an extent as to materially improve the quality of thebutter. This is frequently done by the addition of a "starter. " Whilestarters have been employed for many years for the purpose mentioned, itis only recently that their nature has been understood. A starter may beselected from widely divergent sources, but in all cases it is sure tocontain a large number of bacteria, and the presumption is that they areof such a nature as to produce desirable fermentative changes in thecream. 

In the selection of these so-called natural starters, it follows thatthey must be chosen under such conditions as experience has shown togive favorable results. For this purpose, whole milk from a singleanimal is often used where the same is observed to sour with theproduction of no gas or other undesirable taint. A skim-milk starterfrom a mixed supply is recommended by many. Butter milk is frequentlyemployed, but in the opinion of butter experts is not as suitable as theothers mentioned. 

It not infrequently happens that the practical operator may be misled inselecting a starter that is not desirable, or by continuing its useafter it has become contaminated. 

In 1890[162] a new system of cream ripening was introduced in Denmark byStorch that possesses the merit of being a truly scientific and at thesame time practical method. This consisted in the use of pure culturesof specific organisms that were selected on account of their ability toproduce a desirable ripening change in cream. The introduction of theseso-called culture starters has become universal in Denmark, and in partsof Germany. Their use is also rapidly extending in this country, Australia and New Zealand. 

~Principles of pure-culture cream-ripening. ~ In the proper use of purecultures for ripening cream, it is necessary first to eliminate as faras possible the bacteria already present in cream before the culturestarter is added. This result is accomplished by heating the cream to atemperature sufficiently high to destroy the vegetating organisms. Theaddition of a properly selected starter will then give the chosenorganism such an impetus as will generally enable it to gain theascendency over any other bacteria and so control the character of theripening. The principle employed is quite like that practiced in raisinggrain. The farmer prepares his soil by plowing, in this way killing theweeds. Then he sows his selected grain, which is merely a pure culture, and by the rapid growth of this, other forms are held in check. 

The attempt has been made to use these culture starters in raw sweetcream, but it can scarcely be expected that the most beneficial resultswill be attained in this way. This method has been justified on thebasis of the following experiments. Where cream is pasteurized and nostarter is added, the spore-bearing forms frequently produce undesirableflavors. These can almost always be controlled if a culture starter isadded, the obnoxious form being repressed by the presence of the addedstarter. This condition is interpreted as indicating that the additionof a starter to cream which already contains developing bacteria willprevent those originally present in the cream from growing. [163] Thisrepressive action of one species on another is a well-knownbacteriological fact, but it must be remembered that such an explanationis only applicable in those cases where the culture organism is betterable to develop than those forms that already exist in the cream. 

If the culture organism is added to raw milk or cream which alreadycontains a flora that is well suited to develop in this medium, it isquite doubtful whether it would gain the supremacy in the ripeningcream. The above method of adding a culture to raw cream renderscream-ripening details less burdensome, but at the same time Danishexperience, which is entitled to most credence on this question, isopposed to this method. 

~Reputed advantages of culture starters. ~ _1. Flavor and aroma. _ Naturallythe flavor produced by pure-culture ferments depends upon the characterof the organism used. Those which are most extensively used are able toproduce a perfectly clean but mild flavor, and a delicate but notpronounced aroma. The "high, quick" flavor and aroma that is so muchdesired in the American market is not readily obtained by the use ofcultures. It is quite problematical whether the use of any singlespecies will give any more marked aroma than normally occurs in naturalripening. 

_2. Uniformity of product. _ Culture starters produce a more uniformproduct because the type of fermentation is under more complete control, and herein is the greatest advantage to be derived from their use. Eventhe best butter-maker at times will fail to secure uniform results ifhis starter is not perfectly satisfactory. 

_3. Keeping quality of product. _ Butter made from pasteurized cream towhich a pure-culture starter has been added will keep much better thanthe ordinary product, because the diversity of the bacterial flora isless and the milk is therefore not so likely to contain those organismsthat produce an "off" condition. 

_4. Elimination of taints. _ Many defective conditions in butter areattributable to the growth of undesirable bacteria in the cream thatresult in the formation of "off" flavors and taints. If cream ispasteurized, thereby destroying these organisms, then ripened with pureferments, it is generally possible to eliminate the abnormalconditions. [164] Taints may also be present in cream due to directabsorption from the cow or through exposure to foul odors. [165] Troublesof this sort may thus be carried over to the butter. This isparticularly true in regions where leeks and wild onions abound, as insome of the Atlantic States. The heating of the cream tends to expelthese volatile taints, so that a fairly good article of butter can bemade from what would otherwise be a relatively worthless product. 

~Characteristics desired in culture starters. ~ Certain conditions as thefollowing are desirable in starters made from pure cultures:

1. Vigorous growth in milk at ordinary ripening temperatures. 

2. Ability to form acid so as to facilitate churning and increase theyield of butter. 

3. Able to produce a clean flavor and desirable aroma. 

4. Impart a good keeping quality to butter. 

5. Not easily modified in its flavor-producing qualities by artificialcultivation. 

These different conditions are difficult to attain, for the reason thatsome of them seem to be in part incompatible. Weigmann[166] found that agood aroma was generally an evanescent property, and therefore opposedto good keeping quality. Conn has shown that the functions ofacid-formation, flavor and aroma production are not necessarily related, and therefore the chances of finding a single organism that possessesall the desirable attributes are not very good. 

In all probability no one germ possesses all of these desirablequalities, but natural ripening is the resultant of the action ofseveral forms. [167] This idea has led to the attempt at mixing selectedorganisms that have been chosen on account of certain favorablecharacteristics which they might possess. The difficulty of maintainingsuch a composite culture in its correct proportions when it ispropagated in the creamery is seemingly well nigh insuperable, as oneorganism is very apt to develop more or less rapidly than the other. 

A very satisfactory way in which these cultures are marketed is to mixthe bacterial growth with some sterile, inert, dry substance. This isthe method used in most of the Danish cultures. In this country, some ofthe more prominent cultures employed are marketed in a liquid form. 

~Culture vs. Home-made starters. ~ One great advantage which has accruedfrom the use of culture or commercial starters has been that inemphasizing the need of closer control of the ripening process, greaterattention has been paid to the carrying out of the details. In thehands of the better operators, the differences in flavor of butter madewith a culture or a natural starter are not marked, [168] but in thehands of those who fail to make a good product under ordinaryconditions, an improvement is often secured where a commercial cultureis used. 

~Pasteurization as applied to butter-making. ~ This process, as applied tobutter making, is often confounded with the treatment of milk and creamfor direct consumption. It is unfortunate that the same term is used inconnection with the two methods, for they have but little in commonexcept in the use of heat to destroy the germ life of the milk. Inpasteurizing cream for butter-making, it is not necessary to observe thestringent precautions that are to be noted in the preservation of milk;for the addition of a rapidly developing starter controls at once thefermentative changes that subsequently occur. Then again, the physicalrequirement as to the production of a cooked taste is not so stringentin butter-making. While a cooked taste is imparted to milk or even creamat about 158° F. , it is possible to make butter that shows no permanentcooked taste from cream that has been raised as high as 185° or even195° F. This is due to the fact that the fat does not readily take upthose substances that give to scalded milk its peculiar flavor. 

Unless care is taken in the manipulation of the heated cream, the grainor body of the butter may be injured. This tendency can be overcome ifthe ripened cream is chilled to 48° F. For about two hours beforechurning. It is also essential that the heated cream should be quicklyand thoroughly chilled after being pasteurized. 

The Danes, who were the first to employ pasteurization in butter-making, used, in the beginning, a temperature ranging from 158° to 167° F. , butowing to the prevalence of such diseases as tuberculosis andfoot-and-mouth disease, it became necessary to treat all of the skimmilk that was returned from the creameries. For this purpose the skimmilk is heated to a temperature of 176° F. , it having been more recentlydetermined that this degree of heat is sufficient to destroy the seedsof disease. With the use of this higher temperature the capacity of thepasteurizing apparatus is considerably reduced, but the highertemperature is rendered necessary by the prevailing conditions as todisease. 

When the system was first introduced in Denmark, two methods ofprocedure were followed: the whole milk was heated to a sufficientlyhigh temperature to thoroughly pasteurize it before it was separated, orit was separated first, and the cream pasteurized afterwards. In thelatter case, it is necessary to heat the skim milk after separation todestroy the disease organisms, but this can be quickly done by the useof steam directly. Much more care must be used in heating the cream inorder to prevent injury to the grain of the butter. In spite of theextra trouble of heating the cream and skim milk separately, this methodhas practically supplanted the single heating. With the continual spreadof tuberculosis in America the heating of skim milk separately isbeginning to be introduced. [169]

~Use of starters in pasteurized and unpasteurized cream. ~ In order tosecure the beneficial results presumably attributable to the use of astarter, natural as well as a pure culture, it should be employed incream in which the bacteria have first been killed out bypasteurization. This is certainly the most logical and scientific methodand is the way in which the process has been developed in Denmark. 

Here in this country, the use of pure cultures has been quite rapidlyextended, but the system of heating the cream has been used in only aslight measure. The increased labor and expense incurred in pasteurizingthe cream has naturally militated somewhat against the wide-spread useof the process, but doubtless the main factor has been the inability tosecure as high a flavor where the cream was heated as in the unheatedproduct. As the demands of the market change from a high, quick flavorto one that is somewhat milder but of better keeping quality, doubtlesspasteurization of the cream will become more and more popular. That sucha change is gradually occurring is already evident, although as yet onlya small proportion of butter made in this country is now made in thisway. Where the cream is unheated, a considerable number of species willbe found, and even the addition of a pure culture, if that culture is ofthe lactic acid-producing species, will to some extent control the typeof fermentation that occurs. Such would not be the case with a culturecomposed of the casein-digesting type of bacteria. Only those formscould thus be used which are especially well suited to development inraw cream. For this reason the pure culture ferments that are generallyemployed in creamery practice are organisms of the lactic acid type, able to grow rapidly in cream and produce a pure cream flavor in thebutter. 

~Purity of commercial starters. ~ Naturally the butter maker is forced torely on the laboratory for his commercial starter, and the question willoften arise as to the purity and vigor of the various ferments employed. As there is no way for the factory operator to ascertain the actualcondition of the starter, except by using the same, the greatest careshould be taken by the manufacturer to insure the absolute purity of theseed used. 

A bacteriological examination of the various cultures which have beenplaced on the market not infrequently reveals an impure condition. Inseveral cases the writer has found a not inconsiderable number ofliquefying bacteria mixed with the selected organism. Molds notinfrequently are found in cultures put up in the dry form. Doubtless theeffect of these accidental contaminations is considerably less in thecase of a starter composed of a distinctively lactic acid-producingorganism than with a form which is less capable of thriving vigorouslyin milk, and it should be said that these impurities can frequently beeliminated by continued propagation. 

The virility and vigor of the starter is also a fluctuating factor, dependent in part at least, upon the conditions under which the organismis grown. In some cases the germ is cultivated in solutions in whichacid cannot be formed in abundance. Where the conditions permit of theformation of acid, as would be the case if sugar was present with alactic acid-producing species, the vitality of the culture is oftenimpaired by the action of the gradually accumulating acid. Somemanufacturers attempt to minimize this deleterious condition by addingcarbonate of lime which unites with the acid that is formed. 

~Propagation of starters for cream-ripening. ~ The preparation andpropagation of a starter for cream-ripening is a process involvingconsiderable bacteriological knowledge, whether the starter is ofdomestic origin or prepared from a pure-culture ferment. In any event, it is necessary that the starter should be handled in a way so as toprevent the introduction of foreign bacteria as far as possible. Itshould be remembered at all times that the starter is a live thing andmust be handled throughout its entire history in a way so as to retainits vitality and vigor unimpaired. The following points should be takeninto consideration in growing the starter and transferring it from dayto day:

1. If a commercial starter is used, see that it is fresh and that theseal has not been broken. If the culture is too old, the larger part ofthe organisms may have died out before it is transferred, in which casethe effect of its addition to the sterilized milk would be of littlevalue. 

When the commercial ferment is received, it should be stored in therefrigerator pending its use so as to retard as much as possible thechanges that naturally go on in the culture liquid. Be careful that thebottle is not exposed to the influence of direct sunlight for in atransparent medium the organisms may be readily killed by thedisinfecting action of the sun's rays. 

2. If a home-made starter is employed, use the greatest possible care inselecting the milk that is to be used as a basis for the starter. 

3. For the propagation and perpetuation of the starter from day to day, it is necessary that the same should be grown in milk that is asgerm-free as it is possible to secure it. For this purpose sterilizesome fresh skim-milk in a covered can that has previously been wellsteamed. This can be done easily by setting cans containing skim-milk ina vat filled with water and heating the same to 180° F. Or above forone-half hour or more. Steam should not be introduced directly. Thisprocess destroys all but a few of the most resistant spore-bearingorganisms. This will give a cooked flavor to the milk, but will notaffect the cream to which the starter is added. Dairy supply houses arenow introducing the use of starter cans that are specially made for thispurpose. 

4. After the heated milk is cooled down to about 70° or 80° F. , it canbe inoculated with the desired culture. Sometimes it is desirable to"build up" the starter by propagating it first in a smaller volume ofmilk, and then after this has developed, adding it to a larger amount. 

This method is of particular value where a large amount of starter isneeded for the cream-ripening. 

5. After the milk has been inoculated, it should be kept at atemperature that is suitable for the rapid development of the containedbacteria, 65°-75° F. , which temperature should be kept as uniform aspossible. 

This can best be done by setting the covered can in a vat filled withwarm water. The starter cans are often arranged so that temperature canbe controlled by circulating water. 

6. The starter should not be too thoroughly curdled when it is neededfor use, but should be well soured and only partially curdled for it isdifficult to break up thoroughly the curd particles if the starter iscompletely curdled. If these curd masses are added to ripening cream, white specks may appear in the butter. 

7. The vigor of the starter is in all probability stronger when the milkis on the point of curdling than it is after the curd has been formedsome time. The continued formation of lactic acid kills many of thebacteria and thus weakens the fermentative action. It is thereforehighly important that the acidity of the starter should be closelywatched. 

8. Do not refrigerate the starter when it has reached the proper stageof development, as this retards the bacterial growth in the same manneras cold weather checks the growth of grain. It is preferable to dilutethe starter, if it cannot be used when ready, with sufficient freshlysterilized sweet milk to hold the acidity at the proper point and thuskeep the bacteria in the starter in a condition which will favorvigorous growth. 

9. The starter should be propagated from day to day by adding a smallquantity to a new lot of freshly prepared milk. For this purpose twopropagating cans should be provided so that one starter may be in usewhile the other is being prepared. 

~How long should a starter be propagated?~ No hard-and-fast rule can begiven for this, for it depends largely upon how carefully the starter ishandled during its propagation. If the starter is grown in sterilizedmilk kept in steamed vessels and is handled with sterile dippers, it ispossible to maintain it in a state of relative purity for a considerableperiod of time; if, however, no especial care is given, it will soonbecome infected by the air, and the retention of its purity will dependmore upon the ability of the contained organism to choke out foreigngrowths than upon any other factor. Experience seems to indicate thatpure-culture starters "run out" sooner than domestic starters. While itis possible, by bacteriological methods, to determine with accuracy theactual condition of a starter as to its germ content, still such methodsare inapplicable in creamery practice. Here the maker must rely largelyupon the general appearance of the starter as determined by taste andsmell. The supply houses that deal in cultures of this class generallyexpect to supply a new culture at least every month. 

~Bacteria in butter. ~ As ripened cream is necessarily rich in bacteria, itfollows that butter will also contain germ life in varying amounts, butas butter-fat is not well adapted for bacterial food, the number ofgerms in butter is usually less than in ripened cream. 

Sweet-cream butter is naturally poorer in germ life than that made fromripened cream. Grotenfelt reports in sweet-cream butter, the so-called"Paris butter, " only a few bacteria while in acid cream butter the germcontent runs from scores to hundreds of thousands. 

~Effect of bacteria in wash water. ~ An important factor in contaminationmay be the wash water that is used. Much carelessness often prevailsregarding the location and drainage of the creamery well, and if samebecomes polluted with organic matter, bacterial growth goes on apace. Melick[170] has made some interesting studies on using pasteurized andsterilized well waters for washing. He found a direct relation to existbetween the bacterial content of the wash water and the keeping qualityof the butter. Some creameries have tried filtered water but underordinary conditions a filter, unless it is tended to with greatregularity, becomes a source of infection rather than otherwise. 

~Changes in germ content. ~ The bacteria that are incorporated with thebutter as it first "comes" undergo a slight increase for the first fewdays. The duration of this period of increase is dependent largely uponthe condition of the butter. If the buttermilk is well worked out of thebutter, the increase is slight and lasts for a few days only, while thepresence of so nutritious a medium as buttermilk affords conditions muchmore favorable for the continued growth of the organisms. 

While there may be many varieties in butter when it is fresh, they arevery soon reduced in kind as well as number. The lactic acid group oforganisms disappear quite rapidly; the spore-bearing species remainingfor a somewhat longer time. Butter examined after it is several monthsold is often found to be almost free from germs. 

In the manufacture of butter there is much that is dependent upon themechanical processes of churning, washing, salting and working theproduct. These processes do not involve any bacteriological principlesother than those that are incident to cleanliness. The cream, if ripenedproperly, will contain such enormous numbers of favorable forms that theaccess of the few organisms that are derived from the churn, the air, orthe water in washing will have little effect, unless the conditions areabnormal. 

BACTERIAL DEFECTS IN BUTTER. 

~Rancid change in butter. ~ Fresh butter has a peculiar aroma that is verydesirable and one that enhances the market price, if it can be retained;but this delicate flavor is more or less evanescent, soon disappearing, even in the best makes. While a good butter loses with age some of thepeculiar aroma that it possesses when first made, yet a gilt-edgedproduct should retain its good keeping qualities for some length oftime. All butters, however, sooner or later undergo a change thatrenders them worthless for table use. This change is usually a ranciditythat is observed in all stale products of this class. The cause of thisrancid condition in butter was at first attributed to the formation ofbutyric acid, but it is now recognized that other changes also enterin. [171] Light and especially air also exert a marked effect on theflavor of butter. Where butter is kept in small packages it is much moreprone to develop off flavors than when packed in large tubs. From thecarefully executed experiments of Jensen it appears that some of themolds as well as certain species of bacteria are able to incite thesechanges. These organisms are common in the air and water and ittherefore readily follows that inoculation occurs. 

Practically, rancidity is held in check by storing butter at lowtemperatures where germ growth is quite suspended. 

~Lack of flavor. ~ Often this may be due to improper handling of the creamin not allowing it to ripen far enough, but sometimes it is impossibleto produce a high flavor. The lack of flavor in this case is due to theabsence of the proper flavor-producing organisms. This condition canusually be overcome by the addition of a proper starter. 

~Putrid butter. ~ This specific butter trouble has been observed inDenmark, where it has been studied by Jensen. [172] Butter affected by itrapidly acquires a peculiar putrid odor that ruins it for table use. Sometimes, this flavor may be developed in the cream previous tochurning. 

Jensen found the trouble to be due to several different putrefactivebacteria. One form which he called _Bacillus foetidus lactis_, a closeally of the common feces bacillus, produced this rotten odor and tastein milk in a very short time. Fortunately, this organism was easilykilled by a comparatively low heat, so that pasteurization of the creamand use of a culture starter quickly eliminated the trouble, where itwas tried. 

~Turnip-flavored butter. ~ Butter sometimes acquires a peculiar flavorrecalling the order of turnips, rutabagas, and other root crops. Oftenthis trouble is due to feeding, there being in several of these crops, aromatic substances that pass directly into the milk, but in someinstances the trouble arises from bacteria that are able to producedecomposition products, [173] the odor and taste of which stronglyrecalls these vegetables. 

~"Cowy" butter. ~ Frequently there is to be noted in milk a peculiar odorthat resembles that of the cow stable. Usually this defect in milk hasbeen ascribed to the absorption of impure gases by the milk as it cools, although the gases and odors naturally present in fresh milk have thispeculiar property that is demonstrable by certain methods of aeration. Occasionally it is transmitted to butter, and recently Pammel[174] hasisolated from butter a bacillus that produced in milk the same peculiarodor so commonly present in stables. 

~Lardy and tallowy butter. ~ The presence of this unpleasant taste inbutter may be due to a variety of causes. In some instances, improperfood seems to be the source of the trouble; then again, butter exposedto direct sunlight bleaches in color and develops a lardy flavor. [175]In addition to these, cases have been found in which the defect has beentraced to the action of bacteria. Storch[176] has described alactic-acid form in a sample of tallowy butter that was able to producethis disagreeable odor. 

~Oily butter. ~ Jensen has isolated one of the causes of the dreaded oilybutter that is reported quite frequently in Denmark. The specificorganism that he found belongs to the sour-milk bacteria. In twenty-fourhours it curdles milk, the curd being solid like that of ordinary sourmilk. There is produced, however, in addition to this, an unpleasantodor and taste resembling that of machine oil, a peculiarity that istransmitted directly to butter made from affected cream. 

~Bitter butter. ~ Now and then butter develops a bitter taste that may bedue to a variety of different bacterial forms. In most cases, the bitterflavor in the butter is derived primarily from the bacteria present inthe cream or milk. Several of the fermentations of this character inmilk are also to be found in butter. In addition to these defectsproduced by a biological cause, bitter flavors in butter are sometimesproduced by the milk being impregnated with volatile, bitter substancesderived from weeds. 

~Moldy butter. ~ This defect is perhaps the most serious because mostcommon. It is produced by the development of a number of differentvarieties of molds. The trouble appears most frequently in packed butteron the outside of the mass of butter in contact with the tub. Moldspores are so widely disseminated that if proper conditions are givenfor their germination, they are almost sure to develop. In some casesthe mold is due to the growth of the ordinary bread mold, _Penicilliumglaucum_; in other cases a black mold develops, due often to_Cladosporium butyri_. Not infrequently trouble of this character isassociated with the use of parchment wrappers. The difficulty can easilybe held in check by soaking the parchment linings and the tubs in astrong brine, or paraffining the inside of the tub. 

~Fishy butter. ~ Considerable trouble has been experienced in Australianbutter exported to Europe in which a fishy flavor developed. It wasnoted that the production of this defect seemed to be dependent upon thestorage temperature at which the butter was kept. When the butter wasrefrigerated at 15° F. No further difficulty was experienced. It isclaimed that the cause of this condition is due to the formation oftrimethylamine (herring brine odor) due to the growth of the mold fungus_Oidium lactis_, developing in combination with the lactic-acidbacteria. 

A fishy taste is sometimes noted in canned butter. Rogers[177] hasdetermined that this flavor is caused by yeasts (_Torula_) which producefat-splitting enzyms capable of producing this undesirable change. 

FOOTNOTES:

[153] Conn and Esten, Cent. F. Bakt. , II Abt. , 1901, 7:746. 

[154] Tiemann, Milch Zeit. , 23:701. 

[155] Milch Zeit. , 1889, p. 7; 1894, p. 624; 1895, p. 383. 

[156] Dean, Ont. Agr. Coll. , 1897, p. 66. 

[157] Storch, Nogle, Unders. Over Floed. Syrning, 1890. 

[158] Conn, 6 Storrs Expt. Stat. , 1893, p. 66. 

[159] Conn, 9 Storrs Expt. Stat. , 1896, p. 17. 

[160] Weigmann, Milch Zeit. , 1891, p. 793

[161] McDonnell, ü. Milchsäure Bakterien (Diss. Kiel, 1899), p. 43. 

[162] Storch, Milch Zeit. , 1890, p. 304. 

[163] Conn, 9 Storrs Expt. Stat. , 1896, p. 25. 

[164] Milch Zeit. , 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p. 163. 

[165] McKay, Bull. 32, Iowa Expt. Stat. , p. 47

[166] Weigmann, Landw. Woch. F. Schl. Hol. , No. 2, 1890. 

[167] Weigmann, Cent. F. Bakt. , II Abt. , 3:497, 1897. 

[168] At the National Creamery Buttermakers' Association for 1901, 193out of 240 exhibitors used starters. Of those that employed starters, nearly one-half used commercial cultures. There was practically nodifference in the average score of the two classes of starters, butthose using starters ranked nearly two points higher in flavor thanthose that did not. 

[169] Russell, Bull. 143, Wis. Expt. Stat. , Feb. 1907. 

[170] Melick, Bull. 138, Kansas Expt. Stat. , June 1906. 

[171] Reinmann, Cent. F. Bakt. , 1900, 6:131; Jensen, Landw. Jahr. D. Schweiz, 1901. 

[172] Jensen, Cent. F. Bakt. , 1891, 11:409. 

[173] Jensen, Milch Zeit. , 1892, 6, Nos. 5 and 6. 

[174] Pammel, Bull. 21, Iowa Expt. Stat. , p. 803. 

[175] Fischer, Hyg. Rund. , 5:573. 

[176] Storch, 18 Rept. Danish Agric. Expt. Stat. , 1890. 

[177] Rogers Bull. 57, B. A. I. U. S. Dept Agric. , 1904. 

CHAPTER VIII. 

BACTERIA IN CHEESE. 

The art of cheese-making, like all other phases of dairying, has beendeveloped mainly as a result of empirical methods. Within the lastdecade or so, the subject has received more attention from thescientific point of view and the underlying causes determined to someextent. Since the subject has been investigated from the bacteriologicalpoint of view, much light has been thrown on the cause of many changesthat were heretofore inexplicable. Our knowledge, as yet, is quitemeager, but enough has already been determined to indicate that thewhole industry is largely based on the phenomena of ferment action, andthat the application of bacteriological principles and ideas is sure toyield more than ordinary results, in explaining, in a rational way, thereasons underlying many of the processes to be observed in thisindustry. 

The problem of good milk is a vital one in any phase of dairy activity, but it is pre-eminently so in cheese-making, for the ability to make afirst-class product depends to a large extent on the quality of the rawmaterial. Cheese contains so large a proportion of nitrogenousconstituents that it is admirably suited, as a food medium, to thedevelopment of bacteria; much better, in fact, than butter. 

INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES. 

In the manufacture of cheddar cheese bacteria exert a marked influencein the initial stages of the process. To produce the proper texture thatcharacterizes cheddar cheese, it is necessary to develop a certainamount of acid which acts upon the casein. This acidity is measured bythe development of the lactic-acid bacteria that normally abound in themilk; or, as the cheese-maker expresses it, the milk is "ripened" to theproper point. The action of the rennet, which is added to precipitatethe casein of the milk, is markedly affected by the amount of acidpresent, as well as the temperature. Hence it is desirable to have astandard amount of acidity as well as a standard temperature forcoagulation, so as to unify conditions. It frequently happens that themilk is abnormal with reference to its bacterial content, on account ofthe absence of the proper lactic bacteria, or the presence of formscapable of producing fermentative changes of an undesirable character. In such cases the maker attempts to overcome the effect of the unwelcomebacteria by adding a "starter;" or he must vary his method ofmanufacture to some extent to meet these new conditions. 

~Use of starters. ~ A starter may be employed to hasten the ripening ofmilk that is extremely sweet, so as to curtail the time necessary to getthe cheese to press; or it may be used to overcome the effect ofabnormal conditions. 

The starter that is employed is generally one of domestic origin, and isusually taken from skim milk that has been allowed to ferment and sourunder carefully controlled conditions. Of course much depends upon thequality of the starter, and in a natural starter there is always thepossibility that it may not be perfectly pure. 

Within recent years the attempt has been made to control the effect ofthe starter more thoroughly by using pure cultures of some desirablelactic-acid form. [178] This has rendered the making of cheese not onlymore uniform, but has aided in repressing abnormal fermentationsparticularly those that are characterized by the production of gas. 

Recently, pure cultures of Adametz's _B. Nobilis_, a digesting organismthat is claimed to be the cause of the breaking down of the casein andalso of the peculiar aroma of Emmenthaler cheese, has been placed on themarket under the name _Tyrogen_. It is claimed that the use of thisstarter, which is added directly to the milk and also rubbed on thesurface of the cheese, results in the improvement of the curds, assistsin the development of the proper holes, imparts a favorable aroma andhastens ripening. [179]

Campbell[180] states that the discoloration of cheese in England, whichis due to the formation of white spots that are produced by thebleaching of the coloring matter in the cheese, may be overcome by theuse of lactic-acid starters. 

The use of stringy or slimy whey has been advocated in Holland for someyears as a means of overcoming the tendency toward gas formation in Edamcheese which is made from practically sweet milk. This fermentation, theessential feature of which is produced by a culture of _StreptococcusHollandicus_, [181] develops acid in a marked degree, thereby inhibitingthe production of gas. 

The use of masses of moldy bread in directing the fermentation ofRoquefort cheese is another illustration of the empirical development ofstarters, although in this instance it is added after the curds havebeen prepared for the press. 

~Pasteurizing milk for cheese-making. ~ If it were possible to use properlypasteurized milk in cheese-making, then practically all abnormalconditions could be controlled by the use of properly selected starters. Numerous attempts have been made to perfect this system with referenceto cheddar cheese, but so far they have been attended with imperfectsuccess. The reason for this is that in pasteurizing milk, the solublelime salts are precipitated by the action of heat, and under theseconditions rennet extract does not curdle the casein in a normal manner. This condition can be restored, in part at least, by the addition ofsoluble lime salts, such as calcium chlorid; but in our experience, desirable results were not obtained where heated milks to which thiscalcium solution had been added were made into cheddar cheese. Considerable experience has been gained in the use of heated milks inthe manufacture of certain types of foreign cheese. Klein[182] findsthat Brick cheese can be successfully made even where the milk is heatedas high as 185° F. An increased weight is secured by the addition of thecoagulated albumin and also increased moisture. 

~Bacteria in rennet. ~ In the use of natural rennets, such as arefrequently employed in the making of Swiss cheese, considerable numbersof bacteria are added to the milk. Although these rennets are preservedin salt, alcohol or boric acid, they are never free from bacteria. Adametz[183] found ten different species and from 640, 000 to 900, 000bacteria per cc. In natural rennets. Freudenreich has shown that rennetextract solutions can be used in Swiss cheese-making quite as well asnatural rennets; but to secure the best results, a small quantity ofpure lactic ferment must be added to simulate the conditions thatprevail when natural rennets are soaked in whey, which, it must beremembered, is a fluid rich in bacterial life. 

Where rennet extract or tablets are used, as is generally the case incheddar making, the number of bacteria added is so infinitesimal as tobe negligible. 

~Development of acid. ~ In the manufacture of cheddar cheese, thedevelopment of acid exerts an important influence on the character ofthe product. This is brought about by holding the curds at temperaturesfavorable to the growth of the bacteria in the same. Under theseconditions the lactic-acid organisms, which usually predominate, developvery rapidly, producing thereby considerable quantities of acid whichchange materially the texture of the curds. The lactic acid acts uponthe casein in solutions containing salt, causing it to dissolve to someextent, thus forming the initial compounds of digestion. [184] Thissolution of the casein is expressed physically by the "stringing" of thecurds on a hot iron. This causes the curds to mat, producing a close, solid body, free from mechanical holes. Still further, the developmentof this acid is necessary for the digestive activity of the pepsin inthe rennet extract. 

In some varieties of cheese, as the Swiss, acid is not developed and thecharacter of the cheese is much different from that of cheddar. In allsuch varieties, a great deal more trouble is experienced from theproduction of "gassy" curds, because the development of thegas-producing bacteria is held in check by the rapid growth of thelactic acid-producing species. 

~Bacteria in green cheese. ~ The conditions under which cheese is madepermit of the development of bacteria throughout the entire process. Thecooking or heating of curds to expel the excessive moisture is never sohigh as to be fatal to germ life; on the contrary, the acidity of thecurd and whey is continually increased by the development of bacteria inthe same. 

The body of green cheese fresh from the press is, to a considerableextent, dependent upon the acid produced in the curds. If the curds areput to press in a relatively sweet condition the texture is open andporous. The curd particles do not mat closely together and "mechanicalholes, " rough and irregular in outline, occur. Very often, at relativelyhigh temperatures, such cheese begin to "huff, " soon after being takenfrom the press, a condition due to the development of gas, produced bygas-generating bacteria acting on the sugar in the curd. This gas findsits way readily into these ragged holes, greatly distending them, as inFig. 30. 

[Illustration: FIG. 30. _L_, a sweet curd cheese direct from the press. "Mechanical" holes due to lack of acid development; _P_, same cheesefour days later, mechanical holes distended by development of gas. ]

~Physical changes in ripening cheese. ~ When a green cheese is taken fromthe press, the curd is tough, firm, but elastic. It has no value as afood product for immediate use, because it lacks a desirable flavor andis not readily digestible. It is nothing but precipitated casein andfat. In a short time, a deep-seated change occurs. Physically thischange is demonstrated in the modification that the curd undergoes. Gradually it breaks down and becomes plastic, the elastic, tough curdbeing changed into a softened mass. This change in texture of the cheeseis also accompanied by a marked change in flavor. The green cheese hasno distinctively cheese flavor, but in course of time, with the gradualchange of texture, the peculiar flavor incident to ripe cheese isdeveloped. 

The characteristic texture and flavor are susceptible of considerablemodification that is induced not only by variation in methods ofmanufacture, but by the conditions under which the cheese are cured. Theamount of moisture incorporated with the curd materially affects thephysical appearance of the cheese, and the rate of change in the same. The ripening temperature, likewise the moisture content of thesurrounding air, also exerts a marked influence on the physicalproperties of the cheese. To some extent the action of these forces ispurely physical, as in the gradual loss by drying, but in other respectsthey are associated with chemical transformations. 

~Chemical changes in ripening cheese. ~ Coincident with the physicalbreaking down of the curd comes a change in the chemical nature of thecasein. The hitherto insoluble casein is gradually transformed intosoluble nitrogenous substances (_caseone_ of Duclaux, or _caseogluten_of Weigmann). This chemical phenomenon is a breaking-down process thatis analogous to the peptonization of proteids, although in addition tothe peptones and albumoses characteristic of peptic digestion, amido-acids and ammonia are to be found. The quantity of these lowerproducts increases with the age of the cheese. 

The chemical reaction of cheese is normally acid to phenolphthalein, although there is generally no free acid, as shown by Congo red, thelactic acid being converted into salts as fast as formed. In very oldcheese, undergoing putrefactive changes, especially on the outside, analkaline reaction may be present, due to the formation of free ammonia. 

The changes that occur in a ripening cheese are for the most partconfined to the proteids. According to most investigators the fatremains practically unchanged, although the researches of Weigmann andBacke[185] show that fatty acids are formed from the fat. In the greencheese considerable milk-sugar is present, but, as a result of thefermentation that occurs, this is rapidly converted into acid products. 

~Bacterial flora of cheese. ~ It might naturally be expected that the greencheese, fresh from the press, would contain practically the same kind ofbacteria that are in the milk, but a study of cheese shows a peculiarchange in the character of the flora. In the first place, fresh cottagecheese, made by the coagulation of the casein through the action ofacid, has a more diversified flora than cheese made with rennet, for thereason, as given by Lafar, [186] that the fermentative process is fartheradvanced. 

When different varieties of cheese are made from milk in the samelocality, the germ content of even the ripened product has a markedsimilarity, as is illustrated by Adametz's work[187] on Emmenthaler orSwiss hard cheese, and Schweitzer Hauskäse, a soft variety. Of the ninespecies of bacilli and cocci found in mature Emmenthaler, eight of themwere also present in ripened Hauskäse. 

Different investigators have studied the bacterial flora of variouskinds of cheese, but as yet little comparative systematic work has beendone. Freudenreich[188] has determined the character and number ofbacteria in Emmenthaler cheese, and Russell[189] the same for cheddarcheese. The same general law has also been noted in Canadian[190] andEnglish[191] cheese. At first a marked decrease in numbers is usuallynoted, lasting for a day or two. This is followed by an enormousincrease, caused by the rapid growth of the lactic-acid type. Thedevelopment may reach scores of millions and often over a hundredmillion organisms per gram. Synchronous with this increase, thepeptonizing and gas-producing bacteria gradually disappear. This rapiddevelopment, which lasts only for a few weeks, is followed by a generaldecline. 

In the ripening of cheese a question arises as to whether the processgoes on throughout the entire mass of cheese, or whether it is moreactive at or near the surface. In the case of many of the soft cheese, such as Brie and limburger, bacterial and mold development isexceedingly active on the exterior, and the enzyms secreted by theseorganisms diffuse toward the interior. That such a condition occurs inthe hard type of cheese made with rennet is extremely improbable. Mostobservers agree that in this type of cheese the ripening progressesthroughout the entire mass, although Adametz opposes this view andconsiders that in Emmenthaler cheese the development of the specificaroma-producing organism occurs in the superficial layers. Jensen hasshown, however, that the greatest amount of soluble nitrogenous productsare to be found in the innermost part of the cheese, a condition that isnot reconcilable with the view that the most active ripening is on theexterior. [192]

The course of development of bacteria in cheddar cheese is materiallyinfluenced by the ripening temperature. In cheese ripened at relativelylow temperatures (50°-55° F. ), [193] a high germ content is maintainedfor a much longer period of time than at higher temperatures. Underthese conditions the lactic-acid type continues in the ascendancy asusual. In cheese cured at high temperatures (80°-86° F. ) the number oforganisms is greatly diminished, and they fail to persist in appreciablenumbers for as long a time as in cheese cured at temperatures morefrequently employed. 

~Influence of temperature on curing. ~ Temperature exerts a most potentinfluence on the quality of the cheese, as determined not only by therate of ripening but the nature of the process itself. Much of the poorquality of cheese is attributable to the effect of improper curingconditions. Probably in the initial stage of this industry cheese wereallowed to ripen without any sort of control, with the inevitable resultthat during the summer months the temperature generally fluctuated somuch as to impair seriously the quality. The effect of high temperatures(70° F. And above) is to produce a rapid curing, and, therefore, a shortlived cheese; also a sharp, strong flavor, and generally a more or lessopen texture. Unless the cheese is made from the best quality of milk, it is very apt to undergo abnormal fermentations, more especially thoseof a gassy character. 

[Illustration: FIG. 31. Influence of curing temperature on texture ofcheese. Upper row ripened eight months at 60° F. ; lower row at 40° F. ]

Where cheese is ripened at low temperatures, ranging from 50° F. Down tonearly the freezing temperatures, it is found that the quality isgreatly improved. [194] Such cheese are thoroughly broken down from aphysical point of view even though they may not show such a high percent of soluble nitrogenous products. They have an excellent texture, generally solid and firm, free from all tendency to openness; and, moreover, their flavor is clean and entirely devoid of the sharp, undesirable tang that so frequently appears in old cheese. The keepingquality of such cheese is much superior to the ordinary product. Theintroduction of this new system of cheese-curing promises much from apractical point of view, and undoubtedly a more complete study of thesubject from a scientific point of view will aid materially inunraveling some of the problems as to flavor production. 

~Theories of cheese curing. ~ Within the last few years considerable studyhas been given the subject of cheese curing or ripening, in order toexplain how this physical and chemical transformation is brought about. 

Much of the misconception that has arisen relative to the cause ofcheese ripening comes from a confusion of terms. In the ordinary use ofthe word, ripening or curing of cheese is intended to signify the sumtotal of all the changes that result in converting the green product asit comes from the press into the edible substance that is known as curedcheese. As previously shown, the most marked chemical transformationthat occurs is that which has to do with the peptonization or breakingdown of the casein. It is true that under ordinary conditions thisdecomposition process is also accompanied with the formation of certainflavor-producing substances, more or less aromatic in character; but itby no means follows that these two processes are necessarily due to thesame cause. The majority of investigators have failed to consider thesetwo questions of casein decomposition and flavor as independent, or atleast as not necessarily related. They are undoubtedly closely boundtogether, but it will be shown later that the problems are quitedifferent and possibly susceptible of more thorough understanding whenconsidered separately. 

In the earlier theories of cheese ripening it was thought to be purely achemical change, but, with the growth of bacteriological science, evidence was forthcoming that seemed to indicate that the activity oforganisms entered into the problem. Schaffer[195] showed that if milkwas boiled and made into cheese, the casein failed to break down. Adametz[196] added to green cheese various disinfectants, as creolin andthymol, and found that this practically stopped the curing process. Fromthese experiments he drew the conclusion that bacteria must be the causeof the change, because these organisms were killed; but when it isconsidered that such treatment would also destroy the activity of enzymsas well as vital ferments, it is evident that these experiments werequite indecisive. 

A determination of the nature of the by-products found in maturingcheese indicates that the general character of the ripening change is apeptonization or digestion of the casein. 

Until recently the most widely accepted views relating to the cause ofthis change have been those which ascribed the transformation to theactivity of micro-organisms, although concerning the nature of theseorganisms there has been no unanimity of opinion. The overwhelmingdevelopment of bacteria in all cheeses naturally gave support to thisview; and such experiments as detailed above strengthened the idea thatthe casein transformation could not occur where these ferment organismswere destroyed. 

The very nature of the changes produced in the casein signified that totake part in this process any organism must possess the property ofdissolving the proteid molecule, casein, and forming therefromby-products that are most generally found in other digestive orpeptonizing changes of this class. 

~Digestive bacterial theory. ~ The first theory propounded was that ofDuclaux, [197] who in 1887 advanced the idea that this change was due tothat type of bacteria which is able to liquefy gelatin, peptonize milk, and cause a hydrolytic change in proteids. To this widely-spread groupthat he found in cheese, he gave the generic name _Tyrothrix_ (cheesehairs). According to him, these organisms do not function directly asripening agents, but they secrete an enzym or unorganized ferment towhich he applies the name _casease_. This ferment acts upon the caseinof milk, converting it into a soluble product known as _caseone_. Theseorganisms are found in normal milk, and if they function as caseintransformers, one would naturally expect them to be present, at leastfrequently, if not predominating in the ripening cheese; but such is notthe case. In typical cheddar or Swiss cheese, they rapidly disappear (p. 168), although in the moister, softer varieties, they persist forconsiderable periods of time. According to Freudenreich, even wherethese organisms are added in large numbers to the curd, they soonperish, an observation that is not regarded as correct by the lateradherents to the digestive bacterial theory, as Adametz and Winkler. 

Duclaux's experiments were made with liquid media for isolationpurposes, and his work, therefore, cannot be regarded as satisfactory asthat carried out with more modern technical methods. Recently thistheory has been revived by Adametz, [198] who claims to have found inEmmenthaler cheese a digesting species, one of the Tyrothrix type, whichis capable of peptonizing the casein and at the same time producing thecharacteristic flavor of this class of cheese. This organism, called byhim _Bacillus nobilis_, the Edelpilz of Emmenthaler cheese, has beensubjected to comparative experiments, and in the cheese made with purecultures of this germ better results are claimed to have been secured. Sufficient experiments have not as yet been reported by otherinvestigators to warrant the acceptance of the claims made relative tothe effect of this organism. 

~Lactic-acid bacterial theory. ~ It has already been shown that thelactic-acid bacteria seems to find in the green cheese the optimumconditions of development; that they increase enormously in numbers fora short period, and then finally decline. This marked development, coincident with the breaking down of the casein, has led to the viewwhich has been so ably expounded by Freudenreich[199] that this type ofbacterial action is concerned in the ripening of cheese. This group ofbacteria is, under ordinary conditions, unable to liquefy gelatin, ordigest milk, or, in fact, to exert, under ordinary conditions, anyproteolytic or peptonizing properties. This has been the stumbling-blockto the acceptance of this hypothesis, as an explanation of the breakingdown of the casein. Freudenreich has recently carried on experimentswhich he believes solve the problem. By growing cultures of theseorganisms in milk, to which sterile, freshly precipitated chalk had beenadded, he was able to prolong the development of bacteria for aconsiderable period of time, and as a result finds that an appreciablepart of the casein is digested; but this action is so slow compared withwhat normally occurs in a cheese, that exception may well be taken tothis type of experiment alone. Weigmann[200] inclines to the view thatthe lactic-acid bacteria are not the true cause of the peptonizingprocess, but that their development prepares the soil, as it were, forthose forms that are more directly concerned in the peptonizing process. This they do by developing an acid substratum that renders possible themore luxuriant growth of the aroma-producing species. According toGorini, [201] certain of the Tyrothrix forms function at hightemperatures as lactic acid producing bacteria, while at lowertemperatures they act as peptonizers. On this basis he seeks toreconcile the discrepancies that appear in the experiments of otherinvestigators. 

~Digestive milk enzym theory. ~ In 1897 Babcock and the writer[202] showedthat milk underwent digestive changes spontaneously when bacterialactivity was suspended by the addition of such anaesthetics as ether, chloroform and benzol. The chemical nature of the by-products producedby this auto-digestion of milk resembles quite closely those found inripened cheese, except that ammonia is not produced as is the case inold cheese. The cause of the decomposition of the casein, they found tobe due to the action of a milk enzym which is inherent to the milkitself. This digestive ferment may be separated from fresh milk byconcentrating centrifuge slime extracts by the usual physiologicalreagents. This ferment, called by them _galactase_, on account of itsorigin in milk, is a proteolytic enzym of the tryptic type. Its activityis destroyed by strong chemicals such as formaldehyde, corrosivesublimate, also when heated to 175° F. Or above. When such extracts areadded to boiled milk, the digestive process is started anew, and theby-products produced are very similar to those noted in a normal cheese. 

Jensen[203] has also shown that the addition of pancreatic extracts tocheese accelerated the formation of soluble nitrogenous products. 

The action of galactase in milk and cheese has been confirmed byFreudenreich[204] and Jensen, [205] as well as by American investigators, and this enzym is now generally accepted as one of the factors concernedin the decomposition of the casein. Freudenreich believes it is able tochange casein into albumose and peptones, but that the lactic-acidbacteria are chiefly responsible for the further decomposition of thenitrogen to amid form. 

Failure before to recognize the presence of galactase in milk isattributable to the fact that all attempts to secure sterile milk hadbeen made by heating the same, in which case galactase was necessarilydestroyed. A brief exposure at 176° F. Is sufficient to destroy itsactivity, and even an exposure at lower temperatures weakens its actionconsiderably, especially if the reaction of the medium is acid. Thisundoubtedly explains the contradictory results obtained in the ripeningof cheese from pasteurized milk, such cheese occasionally breaking downin an abnormal manner. 

The results mentioned on page 172, in which cheese failed to ripen whentreated with disinfectants, --experiments which were supposed at thattime to be the foundation of the bacterial theory of caseindigestion--are now explicable on an entirely different basis. In thesecases the casein was not peptonized, because these strong disinfectantsdestroyed the activity of the enzyms as well as the bacteria. 

Another important factor in the breaking down of the casein is the_pepsin_ in the rennet extract. The digestive influence of this agentwas first demonstrated for cheddar cheese by Babcock, Russell andVivian, [206] and simultaneously, although independently, by Jensen[207]in Emmenthaler cheese. In this digestive action, only albumoses andhigher peptones are produced. The activity of pepsin does not becomemanifest until there is about 0. 3 per cent. Acid which is approximatelythe amount developed in the cheddar process. These two factorsundoubtedly account for by far the larger proportion of the changes inthe casein; and yet, the formation of ammonia in well ripened cheese isnot accounted for by these factors. This by-product is the main endproduct of proteid digestion by the liquefying bacteria but theirapparent infrequency in cheese makes it difficult to understand how theycan function prominently in the change, unless the small quantity ofdigestive enzyms excreted by them in their growth in milk is capable ofcontinuing its action until a cumulative effect is obtained. Althoughmuch light has been thrown on this question by the researches of thelast few years, the matter is far from being satisfactorily settled atthe present time and the subject needs much more critical work. Ifliquefying bacteria abound in the milk, doubtless they exert someaction, but the rôle of bacteria is doubtless much greater in theproduction of flavor than in the decomposition of the curd. 

~Conditions determining quality. ~ In determining the quality of cheese, several factors are to be taken into consideration. First and foremostis the flavor, which determines more than anything else the value of theproduct. This should be mild and pleasant, although with age theintensity of the same generally increases but at no time should it haveany bitter, sour, or otherwise undesirable taste or aroma. Textureregisters more accurately the physical nature of the ripening. Thecheese should not be curdy and harsh, but should yield quite readily topressure under the thumb, becoming on manipulation waxy and plasticinstead of crumbly or mealy. Body refers to the openness or closeness ofthe curd particles, a close, compact mass being most desirable. Thecolor of cheese should be even, not wavy, streaked or bleached. 

For a cheese to possess all of these characteristics in an optimumdegree is to be perfect in every respect--a condition that is rarelyreached. 

So many factors influence this condition that the problem of making aperfect cheese becomes exceedingly difficult. Not only must the qualityof the milk--the raw material to be used in the manufacture--beperfectly satisfactory, but the factory management while the curds arein the vat demands great skill and careful attention; and finally, thelong period of curing in which variation in temperature or moistureconditions may seriously affect the quality, --all of these stages, moreor less critical, must be successfully gone through, before the productreaches its highest state of development. 

It is of course true that many phases of this complex series ofprocesses have no direct relation to bacteria, yet it frequently happensthat the result attained is influenced at some preceding stage by theaction of bacteria in one way or another. Thus the influence of theacidity developed in the curds is felt throughout the whole life of thecheese, an over-development of lactic-acid bacteria producing a sourcondition that leaves its impress not only on flavor but texture. Aninsufficient development of acid fails to soften the curd-particles soas to permit of close matting, the consequence being that the body ofthe cheese remains loose and open, a condition favorable to thedevelopment of gas-generating organisms. 

~Production of flavor. ~ The importance of flavor as determining thequality of cheese makes it imperative that the nature of the substancesthat confer on cheese its peculiar aromatic qualities and taste bethoroughly understood. It is to be regretted that the results obtainedso far are not more satisfactory, for improvement in technique is hardlyto be expected until the reason for the process is thoroughlyunderstood. 

The view that is most generally accepted is that this most importantphase of cheese curing is dependent upon bacterial activity, but theorganisms that are concerned in this process have not as yet beensatisfactorily determined. In a number of cases, different species ofbacteria have been separated from milk and cheese that have the power ofproducing aromatic compounds that resemble, in some cases, the peculiarflavors and odors that characterize some of the foreign kinds of cheese;but an introduction of these into curd has not resulted in theproduction of the peculiar variety, even though the methods ofmanufacture and curing were closely followed. The similarity in germcontent in different varieties of cheese made in the same locality hasperhaps a bearing on this question of flavor as related to bacteria. Ofthe nine different species of bacteria found in Emmenthaler cheese byAdametz, eight of them were also present in ripened Hauskäse. Ifspecific flavors are solely the result of specific bacterial action, itmight naturally be expected that the character of the flora woulddiffer. 

Some suggestive experiments were made by Babcock and Russell on thequestion of flavor as related to bacterial growth, by changing thenature of the environment in cheese by washing the curds on the rackswith warm water. In this way the sugar and most of the ash were removed. Under such conditions the character of the bacterial flora wasmaterially modified. While the liquefying type of bacteria was verysparse in normal cheddar, they developed luxuriantly in the washedcheese. The flavor at the same time was markedly affected. The controlcheddar was of good quality, while that made from the washed curds wasdecidedly off, and in the course of ripening became vile. It may bethese two results are simply coincidences, but other data[208] bear outthe view that the flavor was to some extent related to the nature of thebacteria developing in the cheese. This was strengthened materially byadding different sugars to washed curds, in which case it was found thatthe flavor was much improved, while the more normal lactic-acid type ofbacteria again became predominant. 

~Ripening of moldy cheese. ~ In a number of foreign cheeses, the peculiarflavor obtained is in part due to the action of various fungi which growin the cheese, and there produce certain by-products that flavor thecheese. Among the most important of these are the Roquefort cheese ofFrance, Stilton of England, and Gorgonzola of Italy. 

Roquefort cheese is made from goat's or cow's milk, and in order tointroduce the desired mold, which is the ordinary bread-mold, _Penicillium glaucum_, carefully-prepared moldy bread-crumbs are addedto the curd. 

At ordinary temperatures this organism develops too rapidly, so that thecheese to ripen properly must be kept at a low temperature. The town ofRoquefort is situated in a limestone country, in a region full ofcaves, and it is in these natural caves that most of the ripening isdone. These caverns are always very moist and have a temperature rangingfrom 35° to 44° F. , so that the growth of the fungus is retardedconsiderably. The spread of the mold throughout the ripening mass isalso assisted in a mechanical way. The partially-matured cheese are runthrough a machine that pricks them full of small holes. These slendercanals allow the mold organism to penetrate the whole mass morethoroughly, the moldy straw matting upon which the ripening cheese areplaced helping to furnish an abundant seeding of the desired germ. 

When new factories are constructed it is of advantage to introduce thisnecessary germ in quantities, and the practice is sometimes followed ofrubbing the walls and cellars of the new location with material takenfrom the old established factory. In this custom, developed in purely anempirical manner, is to be seen a striking illustration of abacteriological process crudely carried out. 

In the Stilton cheese, one of the highly prized moldy cheeses ofEngland, the desired mold fungus is introduced into the green cheese byexchanging plugs taken with a cheese trier from a ripe Stilton. 

~Ripening of soft cheese. ~ The type of ripening which takes place in thesoft cheeses is materially different from that which occurs in the hardtype. The peptonizing action does not go on uniformly throughout thecheese, but is hastened by the development of molds and bacteria on theoutside that exert a solvent action on the casein. For this reason, softcheeses are usually made up in small sizes, so that this action may behastened. The organisms that take part in this process are those thatare able to form enzyms (similar in their action to trypsin, galactase, etc. ), and these soluble ferments gradually diffuse from the outsidethrough the cheese. 

Most of these peptonizing bacteria are hindered in their growth by thepresence of lactic acid, so that in many cases the appearance of thedigesting organisms on the surface is delayed until the acidity of themass is reduced to the proper point by the development of otherorganisms, principally molds, which prefer an acid substratum for theirgrowth. 

In Brie cheese a blue coating of mold develops on the surface. In thecourse of a few weeks, a white felting appears which later changes tored. This slimy coat below the mold layer is made up of diverse speciesof bacteria and fungi that are able to grow after the acid is reduced bythe blue mold. The organisms in the red slimy coat act upon the casein, producing an alkaline reaction that is unfavorable to the growth of theblue mold. Two sets of organisms are, therefore essential in theripening process, one preparing the soil for the ferment that laterproduces the requisite ripening changes. As ordinarily carried on, theprocess is an empirical one, and if the red coat does not develop asexpected, the maker resorts to all kinds of devices to bring out thedesired ferment. The appearance of the right form is dependent, however, upon the proper reaction of the cheese, and if this is not suitable, thewished-for growth will not appear. 

INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES. 

The reason why cheese is more subject to abnormal fermentation thanbutter is because its high nitrogen content favors the continueddevelopment of bacteria for some time after it is made. It must beborne in mind, in considering the more important of these changes, thatnot all defective conditions in cheese are attributable to the influenceof living organisms. Troubles frequently arise from errors inmanufacturing details, as too prolonged cooking of curds, too highheating, or the development of insufficient or too much acid. Thenagain, the production of undesirable flavors or impairment in texturemay arise from imperfect curing conditions. 

Our knowledge regarding the exact nature of these indefinite faults isas yet too inadequate to enable many of these undesirable conditions tobe traced to their proper source; but in many cases the taints observedin a factory are due to the abnormal development of certain bacteria, capable of evolving unpleasant or even putrid odors. Most of them areseeded in the milk before it comes to the factory and are due tocareless manipulation of the milk while it is still on the farm. Othersgain access to the milk in the factory, owing to unclean conditions ofone sort or another. Sometimes the cheese-maker is able to overcomethese taints by vigorous treatment, but often they pass on into thecheese, only to detract from the market value of the product. Mostfrequently these "off" flavors appear in cheese that are cured at toohigh temperatures, say above 65° F. 

~"Gassy" fermentations in cheese. ~ One of the worst and at the same timemost common troubles in cheese-making is where the cheese undergoes afermentation marked by the evolution of gas. The presence of gas isrecognized by the appearance either of spherical or lens-shaped holes ofvarious sizes in the green cheese; often they appear in the curd beforeit is put to press. Usually in this condition the curds look as if theyhad been punctured with a pin, and are known as "pin holey" curds. Wherethe gas holes are larger, they are known as "Swiss holes" from theirresemblance to the normal holes in the Swiss product. If the developmentof gas is abundant, these holes are restricted in size. Often theformation of gas may be so intense as to cause the curds to float on thesurface of the whey before they are removed. Such curds are known as"floaters" or "bloaters. "

If "gassy" curds are put to press, the abnormal fermentation maycontinue. The further production of gas causes the green cheese to"huff" or swell, until it may be considerably distorted as in Fig. 33. In such cases the texture of the cheese is greatly injured, and theflavor is generally impaired. 

[Illustration: FIG. 33. Cheese made from gassy milk. ]

Such abnormal changes may occur at any season of the year, but thetrouble is most common in summer, especially in the latter part. 

This defect is less likely to occur in cheese that is well cheddaredthan in sweet curd cheese. When acidity is produced, these gassyfermentations are checked, and in good cheddar the body is so close andfirm as not readily to permit of gaseous changes. 

In Swiss cheese, which is essentially a sweet curd cheese, thesefermentations are very troublesome. Where large holes are formed inabundance (blähen), the trouble reaches its maximum. If the gas holesare very numerous and therefore small it is called a "nissler. "Sometimes the normal "eyes" are even wanting when it is said to be"blind" or a "gläsler. "

[Illustration: FIG. 34. Block Swiss cheese showing "gassy"fermentation. ]

One method of procedure which is likely to cause trouble in Swissfactories is often produced by the use of sour, fermented whey in whichto soak the natural rennets. Freudenreich and Steinegger[209] have shownthat a much more uniform quality of cheese can be made with rennetextract if it is prepared with a starter made from a pure lacticferment. 

The cause of the difficulty has long been charged to various sources, such as a lack of aeration, improper feeding, retention of animal gases, etc. , but in all these cases it was nothing more than a surmise. Veryoften the milk does not betray any visible symptom of fermentation whenreceived, and the trouble is not to be recognized until the process ofcheese-making is well advanced. 

Studies from a biological standpoint have, however, thrown much light onthis troublesome problem; and it is now known that the formation of gas, either in the curd or after it has been put to press, is due entirely tothe breaking down of certain elements, such as the sugar of milk, due tothe influence of various living germs. This trouble is, then, a typefermentation, and is, therefore, much more widely distributed than itwould be if it was caused by a single specific organism. Thesegas-producing organisms are to be found, sparingly at least, in almostall milks, but are normally held in check by the ordinary lacticspecies. Among them are a large number of the bacteria, although yeastsand allied germs are often present and are likewise able to set upfermentative changes of this sort. In these cases the milk-sugar isdecomposed in such a way as to give off CO_{2} and H, and in some cases, alcohol. Russell and Hastings[210] found a lactose-splitting yeast in asevere outbreak of gassy cheese in a Swiss factory. In this case the gasdid not develop until the cheese were a few weeks old. In severe casesthe cheese actually cracked to pieces. 

According to Guillebeau, a close relation exists between those germsthat are able to produce an infectious inflammation (mastitis) in theudder of the cow and some forms capable of gas evolution. 

If pure cultures of these gas-producing bacteria are added to perfectlysweet milk, it is possible to artificially produce the conditions incheese that so frequently appear in practice. 

~Treatment of "pin-holey" curds. ~ When this type of fermentation appearsduring the manufacture of the cheese, the maker can control it in partwithin certain limits. These methods of treatment are, as a rule, purelymechanical, as when the curds are piled and turned, and subsequentlyground in a curd mill. After the gas has been forced out, the curds arethen put to press and the whole mats into a compact mass. 

Another method of treatment based upon bacteriological principles is theaddition of a starter to induce the formation of acid. Where acid isdeveloped as a result of the growth of the lactic-acid bacteria, thegas-producing species do not readily thrive. Another reason why acidaids in repressing the development of gas is that the curd particles arepartially softened or digested by the action of the acid. This causesthem to mat together more closely, and there is not left in the cheesethe irregular mechanical openings in which the developing gas may findlodgment. 

Another method that is also useful with these curds is to employ salt. This represses gaseous fermentations, and the use of more salt thanusual in making the cheese will very often restrain the production ofgas. Tendency to form gas in Edam cheese is controlled by the additionof a starter prepared from slimy whey (lange wei) which is caused by thedevelopment of an acid-forming organism. 

Some have recommended the custom of washing the curds to remove the wheyand the gas-producing bacteria contained therein. Care must be taken notto carry this too far, for the removal of the sugar permitstaint-producing organisms to thrive. [211]

The temperature at which the cheese is cured also materially affects thedevelopment of gas. At high curing temperatures, gas-producing organismsdevelop rapidly; therefore more trouble is experienced in summer than atother seasons. 

If milks which are prone to undergo "gassy" development are excludedfrom the general supply, it would be possible to eliminate the source ofthe entire trouble. To aid in the early recognition of such milks thatare not apparently affected when brought to the factory, fermentation orcurd tests (p. 76) are of great value. The use of this test in the handsof the factory operator often enables him to detect the exact source ofthe trouble, which may frequently be confined to the milk delivered by asingle patron. 

~"Fruity" or "sweet" flavor. ~ Not infrequently the product of a factorymay acquire during the process of ripening what is known as a "sweet" or"fruity" flavor. This flavor resembles the odor of fermented fruit orthe bouquet of certain kinds of wine. It has been noted in widelydifferent sections of the country and its presence bears no relation tothe other qualities of the cheese. The cause of this trouble hasrecently been traced[212] to the presence of various kinds of yeasts. Ordinarily yeasts are rarely present in good cheese, but in cheeseaffected with this trouble they abound. The addition of starters madefrom yeast cultures resulted in the production of the undesirablecondition. 

~Mottled cheese. ~ The color of cheese is sometimes cut to that extent thatthe cheese presents a wavy or mottled appearance. This condition is aptto appear if the ripening temperature is somewhat high, or largerquantities of rennet used than usual. The cause of the defect isobscure, but it has been demonstrated that the same is communicable if astarter is made by grating some of this mottled cheese into milk. Thebacteriology of the trouble has not yet been worked out, but the defectis undoubtedly due to an organism that is able to grow in the ripeningcheese. It has been claimed that the use of a pure lactic ferment as astarter enables one to overcome this defect. 

~Bitter cheese. ~ Bitter flavors are sometimes developed in cheeseespecially where the ripening process is carried on at a low temperaturein the presence of an excess of moisture for a considerable length oftime. 

Guillebeau[213] isolated several forms from Emmenthaler cheese which heconnected with udder inflammation that were able to produce a bittersubstance in cheese. 

Von Freudenreich[214] has described a new form _Micrococcus casei amari_(micrococcus of bitter cheese) that was found in a sample of bittercheese. This germ is closely related to Conn's micrococcus of bittermilk. It develops lactic acid rapidly, coagulating the milk andproducing an intensely bitter taste in the course of one to three days. When milk infected with this organism is made into cheese, there isformed in a few days a decomposition product that imparts a markedbitter flavor to the cheese. 

Harrison[215] has recently found a yeast that grows in the milk and alsoin the cheese which produces an undesirable bitter change. 

It is peculiar that some of the organisms that are able to producebitter products in milk do not retain this property when the milk isworked up into cheese. 

~Putrid or rotten cheese. ~ Sometimes cheese undergoes a putrefactivedecomposition in which the texture is profoundly modified and variousfoul smelling gases are evolved. These often begin on the exterior assmall circumscribed spots that slowly extend into the cheese, changingthe casein into a soft slimy mass. Then, again, the interior of thecheese undergoes this slimy decomposition. The soft varieties are moreprone toward this fermentation than the hard, although the firm cheesesare by no means exempt from the trouble. The "Verlaufen" or "running" oflimburger cheese is a fermentation allied to this. It is where theinside of the cheese breaks down into a soft semi-fluid mass. In severecases, the rind may even be ruptured, in which case the whole interiorof the cheese flows out as a thick slimy mass, having sometimes a putridodor. The conditions favoring this putrid decomposition are usuallyassociated with an excess of moisture, and an abnormally low ripeningtemperature. 

~Rusty spot. ~ This name is applied to the development of smallyellowish-red or orange spots that are formed sometimes throughout thewhole mass of cheddar cheese. A close inspection shows the coloredpoints to be located along the edges of the curd particles. According toHarding, [216] this trouble is most common in spring and fall. The causeof the difficulty has been traced by Connell[217] to the development ofa chromogenic bacterium, _Bacillus rudensis_. The organism can be mostreadily isolated on a potato surface rather than with the usualisolating media, agar or gelatin. 

~Other pigment changes. ~ Occasionally, with the hard type of cheese, butmore frequently with the softer foreign varieties, various abnormalconditions arise that are marked by the production of different pigmentsin or on the cheese. More frequently these are merely superficial andaffect only the outer layers of the cheese. Generally they areattributable to the development of certain chromogenic organisms(bacteria, molds and yeasts), although occasionally due to other causes, as in the case of a blue discoloration sometimes noted in foreign cheesemade in copper kettles. [218]

De Vries[219] has described a blue condition that is found in Edamcheese. It appears first as a small blue spot on the inside, increasingrapidly in size until the whole mass is affected. This defect he wasable to show was produced by a pigment-forming organism, _B. Cyaneo-fuscus_. By the use of slimy whey (lange wei) this abnormalchange was controlled. 

~Moldy cheese. ~ With many varieties of cheese, especially some of theforeign types, the presence of mold on the exterior is not regarded asdetrimental; in fact a limited development is much desired. In hardrennet cheese as cheddar or Swiss, the market demands a product freefrom mold, although it should be said that this condition is imposed bythe desire to secure a good-looking cheese rather than any injury inflavor that the mold causes. Mold spores are so widely distributed that, if proper temperature and moisture conditions prevail, these spores willalways develop. At temperatures in the neighborhood of 40° F. Andbelow, mold growth is exceedingly slow, and often fructification doesnot occur, the only evidence of the mold being the white, felt-likecovering that is made up of the vegetating filaments. The use ofparaffin has been suggested as a means of overcoming this growth, thecheese being dipped at an early stage into melted paraffin. Recentexperiments have shown that "off" flavors sometimes develop where cheeseare paraffined directly from the press. If paraffin is too hard, it hasa tendency to crack and separate from the rind, thus allowing molds todevelop beneath the paraffin coat, where the conditions are ideal as tomoisture, for evaporation is excluded and the air consequentlysaturated. The use of formalin (2% solution) has been suggested as awash for the outside of the cheese. This substance or sulfur is alsoapplied in a gaseous form. Double bandaging is also resorted to as ameans of making the cheese more presentable through the removal of theouter bandage. 

The nature of these molds has not been thoroughly studied as yet. Theordinary blue-green bread mold, _Penicillium glaucum_, is mostfrequently found, but there are numerous other forms that appear, especially at low temperatures. 

~Poisonous cheese. ~ Cases of acute poisoning arising from the ingestion ofcheese are reported from time to time. Vaughan has succeeded in showingthat this condition is due to the formation of a highly poisonousalkaloid which he has isolated, and which he calls _tyrotoxicon_. [220]This poisonous ptomaine has also been demonstrated in milk and othermilk products, and is undoubtedly due to the development of variousputrefactive bacteria that find their way into the milk. It seems quiteprobable that the development of these toxic organisms can also go onin the cheese after it is taken from the press. 

~Prevention or cheese defects. ~ The defective conditions previouslyreferred to can rarely be overcome in cheese so as to improve theaffected product, for they only become manifest in most cases during thelater stages of the curing process. The only remedy against future lossis to recognize the conditions that are apt to prevail during theoccurrence of an outbreak and see that the cheese are handled in such away as to prevent a recurrence of the difficulty. 

Many abnormal and undesirable results are incident to the manufacture ofthe product, such as "sour" or "mealy" cheese, conditions due to thedevelopment of too much acid in the milk or too high a "cook. " These areunder the direct control of the maker and for them he alone isresponsible. The development of taints due to the growth of unwelcomebacteria that have gained access to the milk while it is yet on the farmare generally beyond the control of the cheese maker, unless they are sopronounced as to appear during the handling of the curds. If this doesoccur he is sometimes able, through the intervention of a starter or byvarying some detail in making, to handle the milk in such a way as tominimize the trouble, but rarely is he able to eliminate it entirely. 

One of the most strenuous duties which the maker must perform at alltimes is to point out to his patrons the absolute necessity of theirhandling the milk in such a way as to prevent the introduction oforganisms of a baleful type. 

FOOTNOTES:

[178] Russell, 13 Rept. Wis. Expt. Stat. , 1896, p. 112; Campbell, Trans. High. & Agr. Soc. Scotland, 5 ser. , 1898, 10:181. 

[179] Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900. 

[180] Campbell, No. Brit. , Agric. , May 12, 1897. 

[181] Weigmann, Milch Zeit. , No. 50, 1889. 

[182] Klein, Milch Zeit. (Hildesheim), No. 17, 1900. 

[183] Adametz, Landw. Jahr. , 18:256. 

[184] Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat. , July 1902. 

[185] Milch Zeit. , 1898, No. 49. 

[186] Lafar, Technical Mycology, p. 216. 

[187] Adametz, Landw. Jahr. , 18:228. 

[188] Freudenreich, Landw. Jahr. D. Schweiz, 4:17; 5:16. 

[189] Russell, 13 Rept. Wis. Expt. Stat. , 1896, p. 95. 

[190] Harrison and Connell, Rev. Gen. Du Lait, Nos. 4, 5, 6, 7 and 8, 1903-04. 

[191] Lloyd, Bath and West of Eng. Soc. Rept. , 1892, 2:180. 

[192] Freudenreich, Landw. Jahr. D. Schweiz, 1900; Adametz, Oest. Molk. Zeit. , 1899, No. 7. 

[193] Russell, 14 Wis. Expt. Stat. , 1897, p. 203. Harrison and Connell, Rev. Gen. Du Lait Nos. 4, etc. , 1903-04. 

[194] Babcock and Russell, 18 Rept. Wis. Expt. Stat. , 1901. Dean, Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll. , June 1902. 

[195] Schaffer, Milch Zeit. , 1889, p. 146. 

[196] Adametz, Landw. Jahr. , 18:261. 

[197] Duclaux, Le Lait, p. 213. 

[198] Adametz, Oest. Molk. Zeit. , 1900, Nos. 16-18. 

[199] Freudenreich, Landw. Jahr. D. Schweiz, 1897, p. 85. 

[200] Weigmann, Cent. F. Bakt. , II Abt. , 1898, 4:593; also 1899, 5:630. 

[201] Gorini, Abs. In Expt. Stat. Rec. , 11:388. 

[202] Babcock and Russell, 14 Rept. Wis. Expt. Stat. , 1897, p. 161. 

[203] Jensen, Cent. F. Bakt. , II Abt. , 3:752. 

[204] Freudenreich, Cent. F. Bakt. , II Abt. , 1900, 6:332. 

[205] Jensen, Ibid. , 1900, 6:734. 

[206] 17 Rept. Wis. Expt. Stat. , 1900, p. 102. 

[207] Jensen, Landw. Jahr. D. Schweiz, 1900. 

[208] Babcock and Russell, 18 Rept. Wis. Expt. Stat. , 1901. 

[209] Cent. F. Bakt. 1899, p. 14. 

[210] Bull. 128, Wis. Expt. Stat. , Sept. 1905. 

[211] Babcock and Russell, 18 Rept. Wis. Expt. Stat. , 1901. 

[212] Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat. , Dec. , 1900. 

[213] Guillebeau, Landw. Jahr. , 1890, p. 27. 

[214] Freudenreich, Füehl. Landw. Ztg. , 43:361. 

[215] Harrison, Bull. 123 Ont. Agr'l. Coll. , May, 1902. 

[216] Bull. 183, N. Y. (Geneva) Expt. Stat. , Dec. 1900. 

[217] Connell, Bull. Canadian Dept. Of Agr. , 1897. 

[218] Schmöger, Milch Zeit. , 1883, p. 483. 

[219] De Vries, Milch Zeit. , 1888, pp. 861, 885. 

[220] Zeit. F. Physiol. Chemie, 10:146. 

INDEX. 

Acid, effect of, on churning, 137; in butter-making, 138. 

Acid test, 52. 

Aeration of milk, 59. 

Aerobic bacteria, 7. 

Alcoholic fermentation in milk, 72. 

Anaerobic bacteria, 7. 

Animal, influence of, on milk infection, 34. 

Animal odor, 56. 

Anthrax, 94. 

Antiseptics, 9, 88. 

Aroma, of butter, 140. 

Bacillus: definition of, 2. _acidi lactici_, 64; _cyaneo-fuscus_, 188; _cyanogenus_, 74; _foetidus lactis_, 157; _lactis aerogenes_, 65; _lactis erythrogenes_, 74; _lactis saponacei_, 67; _lactis viscosus_, 71; _nobilis_, 162, 174; _prodigiosus_, 74; _rudensis_, 188; _synxanthus_, 75; _tuberculosis_, 84. 

Bacteria: on hairs, 35; kinds in milk, 63; in barn air, 42; in milk pails, 27; in butter, 154; classification of, 4; in cheese, 160; culture of, 17; in cream, 128; discovery of, 1; external conditions affecting, 8; form of, 2; in butter, 142; in butter-making, 127; in centrifuge slime, 39; In fore milk, 28; in rennet, 163; In separator slime, 39; manure, 37; number of, in milk, 50. Distribution of: milk of American cities, 50; European cities, 50; in relation to cheese, 168. Of disease: anthrax, 94; cholera, 98; diphtheria, 99; lockjaw, 94; toxic, 100; tuberculosis, 84; typhoid fever, 98. Methods of study of: culture, 15; culture media, 13; isolation, 14. 

Bitter butter, 158; cheese, 189; milk, 72. 

Bloody milk, 74. 

Blue cheese, 191; milk, 74. 

Bovine tuberculosis, 84. 

Brie cheese, 182. 

Butter: bacteria in, 154; bitter, 158; "cowy, " 157; fishy, 159; lardy, 157; moldy, 158; mottled, 156; oily, 158; putrid, 156; rancid, 155; tallowy, 157; turnip flavor in, 157. Making: aroma, 140; flavor in, 140; pure culture, 143; in ripening of cream, 136. 

Butyric acid fermentation, 69. 

By-products of factory, methods of preserving, 25. 

Casease, 68. 

Caseone, 68. 

Centrifugal force, cleaning milk by, 38. 

Cheese: bacterial flora of, 168; bitter, 189; blue, 187; Brie, 182; Edam, 72, 162; Emmenthaler, 185; flavor of, 179; gassy fermentations in, 183; Gorgonzola, 180; molds on, 191; mottled, 189; "nissler, " 185; poisonous, 192; putrid, 190; ripening of moldy, 180; ripening of soft, 181; Roquefort, 180; rusty spot in, 188; Stilton, 180; Swiss, 185. Making and curing: chemical changes in curing, 166; influence of temperature on curing, 169; influence of rennet, 177; physical changes in curing, 165; prevention of defects, 193; starters in, 161; temperature in relation to bacterial influence, 169. Theories of curing: digestive, 173; galactase, 175, 177; lactic acid, 174. 

Chemical changes in cheese-ripening, 166. 

Chemical disinfectants in milk: bleaching powder, 81; corrosive sublimate, 81; formalin, 80; sulfur, 80; whitewash, 81; vitriol, 81. 

Chemical preservatives, 80. 

Children, milk for, 45. 

Cholera in milk, 98. 

Classification by separator, 38. 

Coccus, definition of, 2. 

Cold, influence on bacteria, 8, 48. 

Contamination of milk through disease germs, 95, 191. 

Covered milk pails, 41. 

Cream, bacterial changes in, 135; mechanical causes for bacteria in, 135; pasteurized, 113; restoration of consistency of pasteurized, 132. Ripening of, 136; advantage of pure cultures in, 144; by natural starters, 142; characteristics of pure cultures in, 145; objections to pure cultures in, 146; principles of pure cultures in, 143; propagation of pure cultures, 151; purity of commercial starters, 150; home-made starters in, 146. 

Creaming methods, 134. 

Curd test, 76. 

Dairy utensils a source of contamination, 21. 

Diarrhoeal diseases, 100. 

Digesting bacteria, 67. 

Digestibility of heated milk, 111. 

Diphtheria, 99. 

Dirt in milk, 34. 

Dirt, exclusion of, 36. 

Disease germs in milk, 95; effect of heat on, 91; origin of, 83. 

Disinfectants, 9: carbolic acid, 81; chloride of lime, 81; corrosive sublimate, 81; formalin, 80; sulfur, 80; vitriol salts, 81; whitewash, 79. 

Disinfectants in milk: alkaline salts, 106; boracic acid, 106; formalin, 106; preservaline, 107; salicylic acid, 106. 

Domestic pasteurizing apparatus, 119. 

Drugs, taints in milk due to, 56. 

Drying, effect of, 8. 

Edam cheese, 72, 162. 

Emmenthaler cheese, 185. 

Endospores, 3. 

Enzyms, 10. 

Factory by-products, 22; treatment of, 25. 

Farrington alkaline tablet, 52. 

Fecal bacteria, effect of, on butter, 35. 

Fermentation: In cheese: gassy, 183. In milk: alcoholic, 72; bitter, 72; blue, 74; butyric, 69; digesting, 67; gassy, 66; kephir, 72; koumiss, 72; lactic acid, 63; lange-wei, 72; red, 74; ropy, 69; slimy, 69; soapy, 73; souring, 63; sweet curdling, 67; treatment of, 75. Tests, 76; Gerber's, 76; Walther's, 76; Wisconsin curd, 76. 

Filtration of milk, 38. 

Fishy butter, 159. 

Flavor: of butter, 140; of cheese, 179. 

Foot and mouth disease, 93. 

Fore milk, 28. 

Formaldehyde, 80. 

Formalin, 80. 

Fruity flavor in cheese, 188. 

Galactase in cheese, 175. 

Gassy fermentations: in cheese, 183; in milk, 67; in Swiss cheese, 167. 

Gläsler, 185. 

Gorgonzola cheese, 180. 

Growth of bacteria, essential conditions for, 4; in milk, 46. 

Hair, bacteria on, 35. 

Heat, influence on bacterial growth, 8. 

Heated milk: characteristics of, 109; action toward rennet, 112; body, 110; digestibility, 111; fermentative changes, 111; flavor, 110; hydrogen peroxid test in, 23; Storch's test, 23. 

Hygienic milk, bacteria in, 45. 

Infection of milk: animal, 34; dairy utensils, 21; fore milk, 28; milker, 36. 

Isolation of bacteria, methods of, 14. 

Kephir, 72. 

Koumiss, 72. 

Lactic acid: fermentation in milk, 63; theory in cheese-curing, 174. 

Lange-wei, 72. 

Lardy butter, 157. 

Light, action on bacteria, 9. 

Manure, bacteria in, 33. 

Methods: of isolation, 14; culture, 15. 

_Micrococcus casei amari_, 189. 

Microscope, use of, 17. 

Milk: a bacterial food medium, 19; bacteria in, 48. Disease organisms in: anthrax, 94; cholera, 98; diphtheria, 99; foot and mouth disease, 93; poisonous, 101; ptomaines, 101; scarlet fever, 99; tuberculosis, 84; typhoid fever, 98. Contamination, 20: from air, 42; from animal odors, 55; dirt, 34; distinction between bacterial and non-bacterial, 57; fore milk, 28; infection in factory, 59; milker, 36; relative importance of various kinds, 43; utensils, 21. 

Milk fermentations: alcoholic, 72; bitter, 72; bloody, 74; blue, 74; butyric acid, 69; gassy, 66, 167; kephir, 72; koumiss, 72; lactic acid, 63; red, 72; ropy, 69; slimy, 69; soapy, 74; souring, 63; sweet curdling, 67; tests for, 76; treatment of, 75; yellow, 75. 

Milk, heated: action towards rennet, 112; digestibility, 111; flavor of, 110; fermentative changes in, 111; hydrogen peroxid test, 110. 

Milking machines, influence of, on germ content, 37. 

Milk preservation: chemical agents in, 106; condensation, 107; freezing, 108; heat, 108; pasteurization, 113; sterilization, 112. 

Milk-sugar as bacterial food, 19. 

Mold, in butter, 158; in cheese, 191. 

Mottled cheese, 189. 

"Nissler" cheese, 185. 

Odors, direct absorption of, in milk, 55. 

_Oidium lactis_, 159. 

Oily butter, 158. 

Pasteurization of milk; acid test in, 128; bacteriological study of, 124, 126, 149; for butter, 147; for cheese, 162; for direct use, 113; of skim milk, 25; details of, 128; temperature and time limit in, 118. 

Pasteurizing apparatus: continuous flow, 122; coolers, 131; Danish, 123; domestic, 119; Farrington, 122; intermittant flow, 121; Miller, 122; Potts, 121; regenerator, 122; Reid, 126; Russell, 121; testing rate of flow, 124. 

_Penicillium glaucum_, 159, 180, 190. 

Pepsin, 10. 

Physical changes in cheese-ripening, 165. 

Poisonous bacteria: in cheese, 192; in milk, 100, 101. 

Preservaline, 167. 

Preservation of milk: by exclusion, 103; chemical agents, 106; condensing, 107; filtration, 38; freezing, 108; pasteurization, 112; physical agents, 107; sterilization, 112. 

Ptomaine poisoning, 101. 

Pure cultures, 15. 

Pure culture starters: advantages of, 144; characteristics of, 145; home-made cultures compared with, 146; propagation of, 151. 

Putrid cheese, 190; butter, 156. 

Rancidity in butter, 155. 

Red milk, 74. 

Rennet: action in heated milk, 112; bacteria in, 163; influence of, on cheese-ripening, 177. 

Restoration of consistency in pasteurized cream, 132. 

Ripening of cheese: moldy cheese, 180; soft cheese, 181. Of cream, 136; artificial starters, 143; natural starters, 142; principles of pure culture starters in, 143. 

Ropy milk, 69. 

Roquefort cheese, 180. 

Rusty spot in cheese, 190. 

Rusty cans: effect of, on acidity, 53. 

Sanitary milk, 45, 104. 

Sanitary pails, 41. 

Scarlet fever in milk, 99. 

Separator slime: bacteria in, 39; tubercle bacillus in, 93. 

Scalded layer, resistance of bacteria in, 91. 

Skim-milk, a distributor of disease, 96. 

Slimy milk, 69. 

Soapy milk, 74. 

Soft cheese, ripening of, 186. 

Sources of contamination in milk: barn air, 42; dairy utensils, 21; dirt from animals, 34; factory cans, 25; fore-milk, 28; milker, 36. 

Souring of milk, 63. 

Spirillum, definition of, 2. 

Spores, 3. 

Starters: in cheese-making, 161; in butter-making, 142; propagation of, 151; pure cultures in cream-ripening, 143. 

Sterilization of milk, 112. 

_Streptococcus Hollandicus_, 72, 162. 

Stilton cheese, 181. 

Storch's test, 23. 

Sulfur as a disinfectant, 81. 

Sweet curdling milk, 68. 

Sweet flavor in cheese, 188. 

Swiss cheese, 177; gassy fermentations in, 24, 185. 

Taints, absorption of, 55. 

Taints, bacterial vs. Physical, 58. 

Taints in milk, absorption of, 55. 

Taints, use of starters in overcoming, 79. 

Taints in butter: putrid, 156; rancidity, 155; turnip flavor, 157. 

Tallowy butter, 157. 

Temperature: effect on bacterial development, 6, 48; effect of low, 108; effect of high, 108; and time limit in milk pasteurization, 113. 

Tests for milk: fermentation, 76; Storch's, 23; acid, 52. 

Theories in cheese-curing: digestive, 171; galactase, 175, 177; lactic acid, 174. 

Trypsin, 10. 

Tubercle bacillus: in milk, 88; in separator slime, 93; thermal death limits, 117. 

Tuberculin test, 86. 

Tuberculosis, bovine, 84. 

Turnip flavor in butter, 157. 

Typhoid fever, 98. 

Tyrogen, 162. 

Tyrotoxicon, 101, 190. 

Udder: artificial introduction of bacteria into, 32; milk germ-free in, 19; infection of, 28; washing, 89; tuberculosis in, 87. 

Viscogen, 132. 

Water: as a source of infection, 61. 

Whey, pollution of vats, 23; method of preserving, 25; treatment of, in vats, 25. 

Whitewash, 81. 

Wisconsin curd test, 76. 

Yeasts: alcoholic ferments in milk, 73; fruity flavor in cheese, 186; gassy due to yeasts, 186; in bitter cheese, 189; in canned butter, 159; kephir, 72.