CORNELL UNIVERSITY THE Sflomct Hetmnary Slibrarg FOUNDED BY ROSWELL P. FLOWER for the use of the N. Y. STATE Veterinary College 1897 Cornell University Library QR 121.R961896 Outlines of dairy bacteriology, a concis 3 1924 000 232 912 The original of tliis bool< is in tlie Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924000232912 OUTLINES OF Dairy Bacteriology A Concise Manual for the Use of Students in Dairying BY H. L RUSSELL Professor of Bacteriology University of Wisconsin MADISON, WIS. PUBLISHED BY THE AUTHOR 1896 COPYRIGHTED 1894 AND 1896 BY H. L. RUSSELL MADISON, "WIS. M. J. CANTWELI., PRINTER PREFACE TO FIRST EDITION. With the rapid advances made in modern dairying that are based directly upon the effect that bacteria have upon milk and its products, the need of an exposition of the principles that underlie bacteriology especially in reference to dairying is being more and more felt. Xo dairy course that purports keeping up-to-date can afford to omit some reference to this subject that is growing more and more im- portant. Having felt the need of some text-book that could be placed in the hands of my own students, the following outlines have been prepared primarily for the use of those who are making a special study of dairying, although the attempt has been made to free the subject matter as far as possible from technical expressions, so that it may aid those practical students of dairying that are forced by circum- stances to forego a course of dairy instruction. A brief glossary is appended of those technical terms that are nec- essarily employed that wiU be of aid to those that are not familiar with bacteriological expressions. I take this opportunity of expressing my thanks to Dr. S. JI. Babcock for valuable suggestions that he has kindly made in the preparation of the work. Madison, Wisconsin, November, 1894. PREFACE TO SECOND EDITION. The cordial reception with which these Outlines have been received justifies the purpose for which they were originally published. The recent advances in bacteriological research, and par- ticularly that branch devoted to dairy subjects, renders a frequent revision desirable, that the results of confirmed investigations may be brought up to date. In this edition a thorough revision has been made, par- ticularly in that part of the subject relating to the manu- facture of butter and cheese. Illustrations have also been added where necessary to elucidate the text, in the interest of those who are unable to improve the privileges of class room work. Acknowledgement is hereby made to the fol- lowing parties for cuts that are used in this edition: Wis- consin Expt. Station; J. H. Monrad, Winnetka, 111.; Phar- maceutical Keview Pub. Co., Milwaukee, Wis.; Bausch & Lomb Opt. Co., Kochester, N. Y. H. L. R Wisconsin Dairy School, December, 1896. INTRODUCTORY. Bacteriology is the youngest member of the sisterhood of those biological sciences that deal with life, in all of its forms and functions in their different relations. While it has a strictly scientific side, this subject has a greater prac- tical bearing than is found in many of its sister sciences. For a long time it was merely a protege of medicine; even now, the term bacteria is always associated in the minds of many with some dread contagious disease. But with further study, the effect of bacteria in many other practical lines is being shown, and the circle of its influence is steadily widening. Its methods have revolutionized the brewing industries; on the presence of bacteria depends the success or failure of many of the industrial arts, such as butter and cheese-mak- ing, many of the processes of tanning as seen in the leather industries, the manufacture of vinegar, and of wines, etc. Modern agriculture recognizes the effect of germ life in the various processes of fertilization by natural manures; in the accumulation of nitrogenous food in the soil as a result of the process of nitrification, and in the fixation of free nitro- gen of the air by members of the clover family. Often the bacteria are to us a- scourge; and very often, indeed, they come to us in the guise of a real friend and helper. More especially is this true with those bacteria that are associated with dairy products, for in both the but- ter and cheese industries, success is alone attained through the activities of certain favorable forms. a Introductory. The following pages attempt to show this group of organ- isms — so infinitely small yet almost infinitely powerful — in their relation to the dairy and dairy products. Knowledge in dairying like all other technical industries has grown mainly out of experience. The fact has often been learned, but the why of that fact is frequently shrouded in mystery. Modern dairying is attempting to build its more accurate knowledge upon a surer foundation, and in doing this is seeking to ascertain the cause of well established processes. To assist in tbis, bacteriology comes as a potent ally and is striving, together with chemistry, to broaden and strengthen the underlying principles upon which advance in modern scientific dairying is rendered possible. TABLE OF CONTENTS. PART I. Synopsis of the Bacteria in General. PAGE. Chapter I. Structure and Form 9- Chapter II. Physiology 14 Chapter III. Methods of Studying Bacteria 29- PARt II. Relation of Bacteria to Milk. Chapter IV. Bacterial Inlection of Milk and Methods of Prevention 3T Chapter V. Fermentations in Milk and their Treatment.. 61 Chapter VI. Disease-Producing Bacteria in Milk 83- Chapter VII. The Preservation of Milk with Especial Ref- erence to Pasteurization 94- PART III. Relation of Bacteria to Milk Products. Chapter VIII. Bacteria in Fluid Products of Milk 127 Chapter IX. Bacteria in Butter 136 Chapter X. Bacteria in Cheese Making 154- ERRATA. Chapter I, page 37, should read Chapter IV. Chapter IV, page 154, should read Chapter X. Reference 135 on last line on page 65, should read 125. PART I. GENERAL SYNOPSIS OF THE BACTERIA. CHAPTER I. STRUCTURE AND FORH. 1. What are bacteria ? — When Leeuwenhoek (pro- nounced Lave-en-hake) in 1675 first discovered these tiny, rapidly-moving organisms, that are now known as bacteria, he thought they were animals. Indeed, under a microscope, many of them bear a close resemblance to those minute worms found in vinegar that are known as "vinegar eels." The idea that they belonged to the animal kingdom con- tinued to hold ground until after the middle of the present century ; but with the improvement in microscopes, a more thorough study of these tiny structures was made possible, and their vegetable nature demonstrated. The bacteria as a class are separated from the fungi mainl}' by their method of growth; from the lower algae by the absence of chloro- phyll, the green coloring matter of vegetable organisms. 2. Structure of bacteria. — As far as structure is concerned the bacteria stand on the lowest plane of vegeta- ble life. They are one-celled organisms, therefore called unicellular. The inner structure of the cell does not differ essentially from that of many other types of plant life. It is composed of protoplasm, and is surrounded by a thin 10 Dairy Bacteriology. membrane that separates it from neighboring cells that are alike in form and size. In some cases a thick gelatinous capsule surrounds a single cell or a small group of cells. 3. Form and size. — Where a plant is composed of a single cell but little difference in form is to be expected. While there are intermediate stages that grade insensibly into each other, the bacteria may be divided into three main types, so far as form is concerned. These are spherical, elongated, and spiral, and to these different types are given the names, respectivelj', coccus, bacillus, and spirillum, (fig. l,a,b, c). A ball, a short rod, and a corkscrew serve as convenient models to illustrate these different forms. In size, the bacteria as a class are the smallest organisms that are now known to exist. There is considerable relative difference in size between the different species, yet in absolute amount this is so slight as to require the highest powers of the best microscopes to detect it. As an average diameter, one-thirty-thousandth of an inch may be taKen. The elongated forms vary in length from .0004 to .001 of an inch. If hundreds of individual germs could be placed side by side, their total thickness would not equal that of a single sheet of paper upon which this page is printed. 4. Arrangement of cells. — In many instances the cells remain isolated and detached from each other, but often they are massed together in definite combinations that have quite a permanent character. Thus they ma}' be united constantl}' in pairs like a dumb bell (diplococcus, fig. 1, d). If the developing cells continue to grow in only one direction, the result is the formation of a chain of cells (streptococcus). Where the cells divide irregularly instead Structure and Form. -11 of always in one plane, then clusters are formed like a bunch of grapes, {staphylococcus, flg. 1, e). The rod-shaped bacillus forms also show marked differ- ences in manner of growth. Sometimes the cells cohere in long chains (filaments), but usually the cohesion between the different cells is so slight that they are broken up into isolated segments or smaller groups. A few species grow in three directions of space thus forming a tiny packet of cells {sarcina). As a rule, the Fig. 1. Showing different types of bacteria, a, ft, c, represent the different type forms: a, coccus; 6, bacillas; and e, spirillum, d, diplococcus or twin coccus, e, a group of staphylococcus or bunch coccus, f. and g, different forms of bacilli; in g the small oval cells some of which are within the larger are spores, h, and J, bacterial cells showing cilia or motile organs. bacilli are the most common organisms in milk, although numerous coccus forms are also found, but in smaller num- bers. The sarcinas are relatively rare in milk. 5. Manner of reproduction. — There are two ways in which the bacteria propagate themselves. 1. The usual method is by direct division {fission) of the mother cell into two j'oung individuals of smaller size (fig. 1, /). A single cell elongates in the direction of its long axis, 12 Dairy Bacteriology. and a new cell wall is formed across the middle of this growing cell, making simultaneously, two smaller, although distinct cells. The cells in this vegetating or growing condition are readily susceptible to the action of harmful agents. 2. Under certain conditions some of the bacteria, notably the bacilli, reproduce in another way, viz., by the formation of spores. The bacterial spore is analogous to the seed of the higher plant; it enables the species to be perpetuated in unfavorable surroundings, and so live over from one season to another. All bacteria do not form spores, but those species that do reproduce in this way possess an ad- vantage, because, on account of the high resistance of these structures toward external conditions, (see 9, 2,) they are much more difficult to kill. This condition is usually called the latent or resting stage. There are two different types of spores known among the bacteria, viz., endospore and arihrospore. With the endosporous bacteria, the protoplasm of the cell undergoes a marked change, drawing itself together in a small round or oval mass within the cell, and finally ac- quiring a thick resistant shell that always has a bright glistening appearance, (fig. '^, g). So far as is known at present, only one spore is to be found in each cell, although not every cell actually does form a spore in its interior. Many forms, especially the cocci, do not form true endo- spores but the whole cell itself changes into an arthrospore by the thickening of the outer membrane. For the germination of spores certain conditions are re- quired just as with seeds. They will not develop under con- ditions that sometimes allow the slow growth of cells in the vegetative stage. This point is important, for if milk con- Structure and Form. 13 tains bacteria in the spore condition only, it is possible to keep these from germinating under conditions in which ac- tual growth would occur, if the germs were in a developing state. 6. Rapidity of multiplication. — The rate of growth of actively vegetating bacteria is in many instances perfectly astounding. With many species under favorable conditions, a single cell will divide in twenty minutes, and each of the daughter cells will repeat this operation in an equal length of time. This rapid rate cannot be maintained indefinitely for the bacteria soon limit their own development by the production of bye-products that are unfavorable to their own growth. The sour milk bacillus thrives readily in milk until the lactic acid that is formed by it exceeds a certain amount then grovfth ceases. If this acid is neutralized by the addition of certain chemicals like chalk, the germs will start again to develop and produce acid to the point of excess. There is a marked difference with different forms in their rate of growth, and the conditions under which de- velopment best occurs. 7. Power of movement. — Some of the bacteria are unable to move bj' any vital motion. When looked at under the microscope, they appear to have an oscillating, vibra- tory movement. This is not a true motility, but merely a physical molecular motion, called the Brownian move- ment. Many of the bacteria are able to really move, and this the}- do by means of extremely delicate threads of pro- toplasm {cilia) that are arranged in different ways around the cell wall, (fig. 1, h, i). As a rule the coccus forms are im- motile. The spirilla are motile, but the bacilli may or may not be. These cilia are so delicate that it requires 14 Dairy Bacteriology. special treatment to demonstrate their presence and classi- fication. 8- Glassiflcation. — In classifying or arranging the different members of any group of living objects, certain similarities and dissimilarities must be considered. These are usually those that pertain to the structure and form, as these are regarded as the most constant, With the bacteria these differences are so slight that they alone do not suflflce to distinguish distinctly one species from another. As far as these characters can be used, they are taken, but in addi- tion to them, many characteristics of a physiological value are added. The way that the organism grows in different kinds of cultures, the reaction of the same on the different media, and when injected into the animal body are also used in distinguishing one from another. CHAPTER II. PHYSIOLOGY. 9. Conditions essential for bacterial develop- ment. — The growth of bacteria like all other living organ- isms bears a direct relation to their external surroundings. Certain conditions are absolutely essential before life can develop. Other conditions though often advantageous are not of such vital importance. 1. Adequate and proper food supply. Concerning this there is a very great difference with different forms of germ life. Many of those that have developed the property of producing disease are very particular in the selection of their food. With those forms that are usually found in milk, such delicacy of choice is not common. Physiology. 15 The essential food elements that mu^t be present are nitro- gen, carbon and ox3'gen together with minute quantities of mineral elements. The nitrogen and carbon are more available when in the form of organic compounds than as simple inorganic salts. Albuminous or proteid substances are the best adapted for the nitrogen supply, while sugars are available for the carbonaceous part of their food. The nitrogenous element is, however, the most indispensable. Concentration of medium. If the fluid is too dense, bac- teria cannot grow; on the other hand, 'it is impossible to dilute a nutritive fluid to such an extent as to completely stop all growth. Thus, condensed milk or syrup is too concentrated to permit of bacterial growth, but if diluted considerably, it rapidly undergoes fermentation. Some of the water bacteria find even in redistilled water, a sufficient' food supply so that they are able to increase. Chemical reaction of medium. As a rule, bacteria prefer a slightly alkaline to an acid medium, but there are so many exceptions to tbis rule, that the value of it as a gen- eral statement is now much modifled. Those organisms that are normal inhabitants of milk are as a rule less sus- ceptible to slight variations in the reaction of the food medium than many others. 2. Temperature. A certain degree of heat is absolutely necessary before the spores of bacteria can germinate, just as seed grain will not sprout when the ground is too cold. As the temperature of a fluid increases, the rapidity with which the bacteria multiply also increases for a certain time. Bej'ond a certain point, however, a heat rigor sets in that destroys the activity of the protoplasm. Tljcre is, therefore, an optimumoi best temperature for growth and a 16 Dairy Bacteriology. minimum and maximv,m point as well, below and above which, development is impossible. These three cardinal growth points varj' considerably with different germs. The temperature limits of growth, i. e., the range be- tween the maximum and minimum points of development, are much wider with bacteria than with almost any other forms of living matter. For this reason, bacterial life is found under conditions that no other living organisms can endure. Several species are known that thrive easily at the freez- ing point Tyhile other forms have been described that have their optimum growth point in the neighborhood of 140° F. With the great majority of bacteria, especially those grow- ing in milk, the range is not so great. Most of them fail to develop at a point below 45°~50° F, while the maximum growth point does not exceed 105°-1]0° F., the optimum ranging from 80°-100° F. With disease producing forms that have adapted themselves to the conditions furnished by the animal or human body, the optimum growth point is usually high, approximating the blood temperature (98°- 100° F.). 3. Gaseous environment. To most forms of life, atmos- pheric air is a necessity, in order to supply the oxygen used by the plant or animal in its growth. With the bacteria, the great majority require the free access of air the same as other kinds of living organisms, and if denied this, fail to grow. Such bacteria are called aeroStc. Tbere are, however, quite a number toward which free oxygen acts as a direct poison. Only when they are surrounded with an atmos- phere other than air, such as hydrogen or nitrogen gas, can they grow. These forms are called anaerobic. All bacteria Physiology. 17 are not divided sharply into these two classes. While some of them are subject strictly to either one condition or the other, hence are called obligate aerobes or anaerobes, there are other forms that are seemingly indifferent to their gaseous surroundings. To this class, the name oi facultative or optional aerobe or anaerobe is given depending upon the relation of the germ to the oxygen supply. Most milk bacteria as well as the great majority of forms in general belong to the aerobic class, but there are quite a number that are able to grow without free oxygen. 4. Moisture. On a dry medium, bacteria can not grow, neither can the spores germinate. A certain amount of moisture is, therefore, 'necessary before bacterial growth can tate place. While they are unable to thrive in a dry atmos- phere, many of them are able to withstand a certain amount of desiccation without being greatly injured, es- peciallj' when in a spore-bearing state. The cholera germ is one of the most susceptible to drying, an exposure of a few hours being sufficient to kill it. 10. Value of milk as a bacterial food medium. — No substance undergoes decomposition changes more readily than milk. The presence of sufficient organic matter in proper proportions and in a dilute condition suffices to make it a perfect food for bacterial as well as mammalian life. While as a whole, milk is well suited for food pur- poses, its component parts differ in nutritive value. Ar- ranged in the order of their nutritive worth, they are as follows: 1. Nitrogen-hearing. This includes the different proteid materials (casein, albumen, etc.,) that are essential elements in any food supply. While the casein is now known to be 18 Dairy Bacteriology. almost wholly, if not entirely, in an insoluble condition, it is rendered soluble by many bacteria. Many species are able to curdle casein in a manner identical with that of rennet (47). 2. Carbon-bearing. The carbon-bearing compounds in milk constitute two classes, hydro-carbons and carbo- hydrates. Of the hydro-carbons butter fat is the most important, but as a nutrient for bacteria, it possesses but little value. It exists in the form of extremely fine globules, makiog an emulsion in the milk serum, but the carbon is in such a form that it cannot be utilized as food. Milk-sugar is the principal milk carbo-hydrate, and is a most important element in this fluid. A- large number of milk bacteria possess the ability of breaking down this com- pound into lactic acid and different gaseous products (45-46). 3. Ash constitmnts. The mineral elements of the milk, potassium, sodium, calcium, magnesium, etc., that are combined with phosphoric and sulfuric acids make up only a small fraction of the milk solids, yet these elements are essential to the growth of any kind of protoplasm, and ^are used by the bacteria in the formation of new cell material. 11. Belation of animal to bacterial processes.^ All kiuds of living matter are either constructive or de- structive, i. e., they either build up or teai apart organic material in the exercise of their vital activities. The green plant takes the inorganic elements of the soil and air, and under the influence of the chlorophyll of the leaves, in com- bination with the energizing power of sunlight, produces the organic matter of the plant. The nitrifying bacteria of the soil take the ammonia liberated in the decomposition of Physiology. 19 previously formed organic matter, and oxidize it into nitric acid, tlie salts of which are used by growing vegetalion. In this way the fertility of the soil is increased. Animal life, on the other hand, derives the energy neces- sary to sustain and develop itself from the b.reaking down of these complex substances that have been built up by the green plant. They are, consequently, destructive in their vital processes, tearing apart the organic material that has heen previously manufactured by other forms of life. Matter in nature is, therefore, constantly moving m a cycle from the inorganic to the organic; the organized material again to be resolved into inorganic elements by bacterial action, only to be subsequently reformed into organic com- pounds. 12. The role of bacteria in nature. — The great majority of bacteria like animal life, belong to a class whose function it is to disintegrate organic matter and resolve it into its- constituent elements. The value of this breaking down process is evident at a glance when we consider what would be the result, if all decay, putrefaction, and decom- position were at once arrested. Not only would the supply of carbonic acid gas be very soon absorbed, stopping the development of chlorophyll-bearing plants, but all nature would be clogged and soon buried under its own debris. Dead and dying vegetable and animal matter, unable to rot and decay would soon bury all under the constantly' accu- mulating mass. Owing to the energies of the lower types of plant and animal life, all of this dead effete matter is slowly consumed. The splitting of these materials into the simpler elements is largely due to the activities of the bac- teria. Almost without exception, they prey upon organized 20 Dairy Bacteriology. matter, absorbing sufficient energy for their maintenance, and at the same time, giving oflf the unused elements to form new combinations that are again absorbed in differ- ent forms. 1:{. Specialization of bacteria.— With the higher plant and animal forms, a certain concentration of energy along diflterent lines is quite frequently observed. The plant ■or the animal adapts itself to its environment, and in doing so, each group as a whole acquires certain characteristics that mark it in a greater or lesser degree. This specializing function is operative throughout the whole domain of nature. The cacti of the arid regions have adopted a pecu- liar concentrated method of growth that enables them to conserve their water supply. The Alpine flora betrays its native haunt by the manner in which it nestles and clings to the rock. Usually, specialization accompanies a complexity of structure, so that the higher developed plants and ani- mals show this characteristic more fully than those that be- long to the more primitive types. In the bacteria is seen a curious anomaly to this rule. Simple in structure as they appear to be, they surpass in functional variability many of the higher forms of plant and animal life. Ubiquitous in their distribution throughout the realm of nature, they have gradually adapted them- selves to the different habitats in which they have found themselves, and as a result, more or less well marked groups are to be found. Thus, there is a native bacterial flora of the mouth, another of the skin, and still another of the intestines. Again, there is a class known collectively as water bacteria, still another, that might with propriety be classified as milk bacteria. Those forms that have become Physiology. 21 habituated to certain conditions, may be said to be indige- nous to that habitat, while the presence of uncommon or in- troduced forms from some other source would be considered as adventive. Thus, the tubercle bacillus is adventive in milk even though it may be dei:ived from the udder direct, while the sour milk bacillus is a natural and therefore indi- genous organism. 14. Distribntion of bacteria. — Bacteria maybe con- sidered either from the standpoint of habitat, or according to the character of the changes they produce. If classified as to habitat they may be divided into air, soil, or water forms; if divided on the second basis, they may be grouped as nitrifying, disease-producing, or fermentative forms. 1. Soil bacteria. Discussing their distribution from the standpoint of the medium in which they live, an examina- tion of the soil shows that the superficial layers abound in myriads of bacteria. Only the surface layer is, however, rich in germ life. At the depth of a few feet they are either filtered out or find the conditions unsuited to their development. 2. Air bacteria. The bacteria found in the air are origin- ally from the soil beneath. In the atmosphere they are un- able to develop, but exist in a dried, latent condition. Their prevalence in the air is measured by the condition of the soil below and the movement of dust particles. For this reason they are more numerous in the air in summer than in winter; the atmosphere of cities contains a larger num- ber of them than country air. They are very prevalent in illy ventilated houses and out-buildings, particularly barns and stables, where dust from hay and dried manure particles fill the air (33). 22 Dairy Bacteriology. 3. Water bacteria. Water when exposed to the air in- variably contains a sufHcient amount of organic matter to serve as bacterial food. Some of its germ life is derived from dust, or the washings of the land, but many species exist in this element which are not to be found in soil. Stagnant pools rich in organic matter always teem with bac- teria and even running water often has large numbers. The ground water layer is normally free, as the bacteria are filtered out by passing through the intervening soil laj'ers. As a consequence spring water as it issues from the soil is relatively poor in bacteria, but quickly becomes contami- nated after it reaches the surface. Some of the highly in- fectious diseases, such as cholera and tj'phoid fever are often transmitted by means of a contaminated water supply. 15. Saprophytic bacteria. — If bacteria are consid- ered from the manner in which they live, they may be divided into two very unequal divisions, known as sapro- phytes and parasites. Those belonging to the first class sub- sist on dead organic matter, while the parasitic forms are able to thrive in living tissues of either vegetable or animal nature. The great majority of different forms belong to the first class, and it is undoubtedly true that this condition is more fundamental than the parasitic method of life. The saprophytes find their food supply in the vegetable and animal matter that has already ceased to live. They are the organisms concerned in the tearing down processes of nature and their beneficient function to the world about them is in their scavenger character as thej' make way with the ofial and debris of organic life. 16. Parasitic bacteria.— There is no sharp line separating the parasitic bacteria from those that live on Physiology. 23 lifeless matter. In all probability those forms that are now able to thrive in living tissue came originally from ancestors of a saprophytic type. They h.ave graduallj' adapted them- selves to the more restricted parasitic method of growth, just as many parasitic plants have been produced, and as a result of this specialization in function, they have often lost the power to thrive under conditions generally favorable to saprophytic forms. Thus, the consumption bacillus is re- stricted in its growth limits to a much narrower range in temperature than is usual with the ordinary forms of bacteria. In parasitism a marked variation in degree is to be noted. Some species, like leprosy, grow with only the greatest diffi- culty outside of their proper host. Such as this may be said to be an obligatory parasite. Then again, other forms like the colon or feces bacillus are normally saprophytes, but under certain conditions, assume a semi-parasitic mode of life, hecoravag \heiQtor&, facultative parasites,!, e., they possess at times the faculty of developing under parasitic conditions. 17. Fermentation. — Most of the saprophytic bacteria are concerned in the breaking down of organic matter, and consequently, they are often associated with the many dif- ferent phases that this process assumes as seen in putrefac- tion, fermentation, decomposition, or decay. These changes are of a complex character and vary much from one instance to another. The changes embraced under the special term, fermenta- tion, are characterized by such a prominent characteristic that they may well be considered separately. In fermenta- 24 Dairy Bacteriology. tion, complex substances are transformed by regular steps into simpler compounds. In this class are to be included a large number of fermen- tative changes that occur in milk, such as the production of lactic acid in the souring of milk, the formation of butyric acid, and others that will be mentioned later. In fermenta- tion, there are often large quantities of the fermentable substances changed and while a certain amount of the energy released in the breaking-down of this m aterial may be utilized by the bacterial cells, yet the disruptive changes set in motion by the living germs are out of all proportion to the results that are seen in the end. Fermentation, although one of the best known processes that occurs in nature is even yet a partial mystery. Our knowledge of the changes that fermentable solutions un- dergo during this process has been vastly augmented during the latter half of this century, but even yet, the problem has not been completely solved. The results of fermenta- tion have been made use of from time immemorial, but no adequate conception of the changes involved was ever recognized until this century. The earlier theories of this action were mainly chemical, and it was not until the im- portant studies of Pasteur were made that the relation of these changes to the action of living organisms was fully proven. He showed that fermentation was closely con- nected with the growth and multiplication of minute forms of organic life, and that the process was usually inaugura- ted by vital forces rather than by purely chemical activity. Previous to this the action of rennet upon milk had been known, and the difference between this fermentation and other types had been recognized by Schroeder and Dusch. Physiology. 25 They showed that the rennet ferment was unaffected by alcohol and other chemicals, while the other fermentation due to organisms was checked in these flaids. Pasteur em- phasized the distinction between these two sets of fermen- tative action and soon was able to classify these changes into two distinct types. 1. Organized ferments — those in which the change takes place as a result of the presence of a living organism. 2. Unorganized ferments — those in which the change is caused by a chemical substance, devoid of vitality, that is itself unchanged in the fermenting process. These unor- ganized, non-vital ferments are known as enzymes. Among the better known of these are rennet,,that has the power of coagulating milk; diastase, the enzyme that converts starch into sugar; pepsin and trypsin, the digestive ferments of the animal body. Some of these unorganized ferments can withstand a high degree of heat as well as the influence of deleterious chemicals such as alcohol and various salts without injury. Among the organized class of ferments are those that directly affect the character of sugar-containing fluids, such as the yeasts and most of the bacteria. A great many of these organized ferments accomplish their effect indirectly through the agency of the enzymes that they themselves excrete. For instance, many of the abnormal fermenta- tions in milk are caused by bacteria that are able to form various enzymes (47-49) • 18. Relation of bacteria to external conditions. — Bacteria, even in a vegetating stage possess a much greater resistance toward external forces than other forms of animal or vegetable life. When they are in a spore stage, this re- 26 Dairy Bacteriology. sistance is stLll further iacreased, A thorough knowledge of the effect of these exteroal forces is essential, lor it is by their action that we are often able to destroy undesirable forms of germ life. In considering these forces in their re- lation to bacteria, the subject may be divided into two parts, the influence of physical and chemical forces on bacteria. 19. Influence of physical forces. 1. Heat. The bacteria possesa a high power of resis- tance toward heat ; but this relation varies, depending upon the condition in which it is applied, whether it is in a dry or moist state. Moist heat is much more effective in destroying germ life than dry on account of its greater penetrative power. The temperature at which any form is killed is called its thermal death point. For the majority of germs in a spore-free, developing condition, a temperature from 130°-140° F. for ten minutes in a liquid medium is fatal. A shorter exposure necessitates a somewhat higher temperature. When ia the more resistant spore-stage, many of them are liable to withstand moist heat in the form of steam (212° F.) from one to three hours, and to destroy spores by dry heat, a temperature varying from 260°-300° F. is neces- sary for an hour or more. Germ life may be more rapidly destroyed in super-heated steam under pressure, a tempera- ture of 230°-240° F. for fifteen to twenty minutes being sufficient to kill most species. Many of the milk bacteria, like the sour milk germ, are very easily destroyed bj' heat, (67) as they do not form spores. Other forms like the hay and potato bacilli are difficult to eradicate on account ot the great resistance that their spores offer toward heat. High temperatures are by far the most efficient means that can be used to render any substance germ-free and are most often employed for this purpose. Physiology. 27 2. Cold. While the maximum temperature that the different spore-bearing species can stand has been accurately determined in many cases, the minimum temperature has in most instances never been reached. But little reliance is to be placed upon this method in attempting to free any sub- stance completely from bacterial life, although freezing is able to destroy a large percentage. Even an artificial de- gree of cold as low as — 220° F. for a day has been found to be insufficient to destroy some forms, especiallj- when in the spore stage. 3. Desiccation. Bacteria behave very differently when subjected to drying. The cholera germ dies in three hours if it is dried, while anthrax retains its virulence unimpaired for decades. Tuberculous sputum withstands drying and is often found to be infectious after the lapse of eight or nine months. Those species that form spores naturally resist desiccation much better than those that do not form these structures. 4. Light. The influence of light on bacterial growth has not been generally appreciated until within a few years. Exposed to the rays of direct sunlight, many forms are killed in a few hours. Even diffused daylight often exerts a powerful inhibiting effect, if the exposure covers any considerable length of time. 5. Pressure. The claim has been made that a pressure of a few atmospheres suffices to keep milk from spoiling, but experiments upon this question are as yet contradictory. Von Freudenreich and Schaffer found that under a pressure of ninety atmospheres, the development of certain forms took place at a normal rate. In studying the deep sea flora of 28 Dairy Bacteriology. the Mediterranean, the writer* found the same species of bacteria at the surface that were present on the sea bottom at a depth of 3600 feet, or under a diflference in atmos- pheric pressure of over one hundred atmospheres. 30. Inflaence of chemical forces. — A great many chemical substances exert a powerful effect upon bacterial protoplasm, although on the whole, bacteria resist the action of chemical poisons more successfully than other forms of vegetable life, Among the chemical substances that are destructive to bacteria are mercury in the form of corrosive sublimate, carbolic acid, the mineral acids, and alkalies, es- pecially the potash and soda groups. The value of the above chemicals in killing bacteria, either in the vegetative or spore stage, is so great that these substances are extensively used for disinfecting purposes where it is possible. 21. Antiseptics and disinfectants. — Antiseptics &v& substances that inhibit or restrain bacterial growth. Disin- fectants are those that kill bacterial life with which they come in contact. These two terms are often confused, but it will aid in a clearer comprehension of their exact meaning if the above distinction is made. All substances possessing disin- fecting power must of neceesity be antiseptic in their ac- tion, but all antiseptics are not disinfectants. Sugar or salt has a retarding effect on bacterial growth, yet these substances do not kill out the bacteria. The same is true of boracic acid and sundry proprietary compounds that are used to prevent the souring of milk. With many chemicals the difference in effect is accomplished by varying the degree of concentration, as for example, carbolic acid in the proportion of 1 : 400 is antiseptic toward the typhoid * Eussell, Zeit. f. Hyg. 11: 165. U LIBRARY. ;j y^ -*- >!y/ Methods of Studying JBacteria^\^lARy^^29 germ, while 1 : 200 is a disinfectant. While the value of these chemical disinfectants for hygienic and sanitary pur- poses is very great, they are of but little worth in the dairy, except in occasional instances (56). In many countries, stringent laws prohibit the use of any chemicals in milk intended for human food. CHAPTEE III. HETHODS OF STUDYING BACTERIA. 22. Necessity of bacterial masses for study. — Bacteria are so infinitesimally small that it is impossible to study individual germs separately without the aid of flrst- class microscopes. For this reason, but little advance was made in the knowledge of these lower forms of plant life, until the introduction of culture methods, whereby a single organism could be cultivated and the progeny of this cell increased to such an extent in a short course of time, that they would be visible to the unaided eye. 23. Culture Metliods. — The system of cultivating bacteria, known as the pure culture method, is based upon the supposition that the food medium in which the organ- ism is grown is first freed completely from all pre-existing forms of life, or in other words, is perfectly sterile. The pure culture processes of the bacteriologist may be said to be in a sense, refined methods of seeding, such as the agri- culturist employs. Just as the seed grain will in due sea- son bring forth a harvest after its kind, so any kind of bac- teria planted in a favorable food medium will produce a crop of its own. If the farmer's seed is foul, it shows in his crop, and the same is true with bacterial farming. But bacteria are so universally distributed that it becomes an 30 Dairy Bacteriology. impossibility to grow any special kind, unless the soil is first freed from all existing forms of germ life. To accom- plish this, it is necessary to subject the nutrient medium used for a culture to some method of sterilization, such as Fig. 2. Steam Sterilizer for the disinfection of glass and culture material by streaming steam. by heat or filtration, whereby all forms of organic life are thoroughly eliminated. Germ free culture material is kept in sterilized glass tubes and flasks, and is protected from outside infection by plugs of sterile cotton.' Material thus prepared, if protected from evaporation, will keep indefi- nitely, as the cotton acts as an effectual filter against the passage of any particles of matter. 24. Media used for cultures. — For culture media, many different substances are emploj-ed. In fact, bacteria will grow on almost any organic substance whether it is solid or fluid, provided the essential conditions of growth Methods of Studying Bacteria. 31 are lurnished (9). The food substances that are used for culture purposes are divided into two classes; solids and liquids. Solid media may be either permanently solid like pota- toes or they may retain their solid properties only at certain temperatures like gelatin or agar. These last are of utmost importance in bacteriological research, for their use, which was introduced by Koch, permits the separation of the dif- ferent forms that may happen to be in any mixture. Grela- tin is used advantageously because the majority of bacteria present wider differences in their appearance upon this medium than upon any other. It remains solid at ordinary temperatures, becoming liquid in the neighborhood of 70° P. Agar, a gelatinous product derived from a Japanese sea- weed, has a much higher melting point, and can be success- fully used, especially with those organisms whose optimum growth point is above the melting point of gelatin. Besides these solid media, different liquid substances are extensively used, such as beef broth, milk, and infusions of various vegetable and animal tissues. Skim milk is of especial value in studying the milk bacteria and maj^ be used in its natural condition, or a few drops of litmus solu- tion may be added in order to detect any change in its chemical reaction due to the bacteria, 25. Methods of isolation. — Suppose for instance one wishes to isolate the different varieties of bacteria found in milk. The method of procedure is as follows: Sterile gela- tin in glass tubes is melted and cooled down so as to be barely warm. To this gelatin which is germ free a drop of milk is added. The gelatin is then gently shaken so as to thoroughly distribute the milk particles, and is then poured 32 Dairy Bacteriology. out into a sterile flat glass dish and quickly covered. This is allowed to stand on a cool surface until the gelatin hard- ens. After the culture plate has been left for twenty-four Fig. 3. A gelatin plate culture showing appearance of diflfer- ent organisms from a sample of milk. Each mass represents a bacterial growth (colony) derived from a single cell. Different forms react differently toward the gelatin, some liquefying the same, others growing in a restricted mass, a, represents a colony of the ordinary bread mould ; b, a liquefying organism ; c, and d, solid forms. or thirty-six hours at the proper temperature, tiny spots will begin to appear on the surface or in the depth of the culture medium. These patches are called colonies and are composed of infinite numbers of individual germs, the result of the continued growth of a single organism that was in the drop of milk which was firmly held in place when the gelatin solidified. The number of these colonies represent Methods of Studying Bacteiia. 33 in general the number of germs that were present in the milk drop. If the plate is not too thickly sown with these germs, the colonies will continue to grow and iucrease in -w ../•a^ Fig. 4. Shows profile view of gelatin plate culture. Shaded part represents the gelatin medium in the covered glass dish; on the surface, different bacteria are developing; b, is a liquefying form that dissolves the gelatin while c and d grow on surface only and do not render gelatin soluble. size, and as they do, minute differences will begin to appear. These differences may be in the color, the contour and the texture of the colony, or the manner in which it acts toward gelatin. In order to make sure that the seeding is not too copious so as to interfere with continued study, an attenua- tion is usually made. This consists in taking a drop of the infected gelatin in the first tube, and transferring it to an- other tube of sterile media. Usually this operation is repeated again so that these culture plates are made with different amounts of seed with the expectation that in at least one plate the seeding will not be so thick as to prevent further study. To further study the peculiarities of different germs, the separate colonies are transferred to other sterile tubes of culture material and thus pure cultures of the various germs are secured. These cultures then serve as a basis for con- tinued study and must be planted and grown upon all the 34 Dairy Bacteriology. different kinds of media that are obtainable. In this way, the slight variations in the growth of diflferent forms are detected and the peculiar characteristics are determined, so KwwvwwvAH Hp^'^'^'^^ f-1 a \V Fig. 5. Pure cultures of different kinds of bacteria in gelatin tubes, a, growth slight in this medium; 6, growth copious at and near surface. Fine parallel filaments growing out into medium liquefying at surface; c, a rapid liquefying form; d, a gas producing form that grows equally w^ell in lower part of tube as at surface (facultative anaerobe); d, an obligate anaerobe, that develops only in absence of air. that the student is able to recognize this form when he meets it again. Methods of Slvdying Bacteria. 35 These culture methods are of essential importance in bac- teriology, as it is the only way in which it is possible to secure a quantity of germs of the same kind. 26. Use of the microscope in bacterial inyesti- gation.— The microscope is in constant demand through- out all the different stages of the isolating process in order to verify the purity of the cultures, For this purpose, it is essential that the instrument used shall be one of strong magnifying powers, (600-800 diameters) combined with sharp definition, so that these tiny organisms shall stand out clear and distinct. The microscopical examination of any germ is quite as essential as the culture characteristics; in fact, the two must go hand in hand. This examination reveals not only the form and size of the individual germ but the manner in which they are united with each other, and any peculiari- ties of movement that they may possess. In carrying out the microscopical part of the work, not only is the organism examined in a living condition but preparations are made by using solutions of anilin dyes as staining agents. These are of great service in bringing out almost imperceptible differences. The art of staining has been carried to the highest degree of perfection in baeterio- logj', especially in the detection of germs that are found in diseased tissues in the animal or human body. In studying the peculiarities of any special organism, not only is it necessary that these cultural and microscopical characters should be closely observed, but special experi- ments must be carried out along different lines, in order to determine any special properties that the germ may possess. Thus, the ability of any form to act as a fermentative organ- 36 Dairy Bacteriology. ism can be tested by fermentation experiments; the prop- erty of causing disease, studied by the inoculation of pure cultures into animals. A great many diflferent methods have been devised for the purpose of studying special character- istics of different bacteria, but a full description of these would necessarily be so lengthy that in a work of this char- acter they must be omitted.* * The following general works contains more or less complete descriptions of the various processes employed in studying bacteria ; Sternberg, Manual of Bacteriology, 1892; Frankel, Bacteriology. 1891; Wood- head, Bacteria and their pro3uct3, 1893; Salomonseo, Bacteriological Technique, 1889. PART II. RELATION OF BACTERIA TO MILK. CHAPTER I. BACTERIAL INFECTION OF HILK AND HETHODS OP PREVENTION. 27. Milk as secreted is sterile.— Under ordinary conditions milk always reveals bacterial life when examined in the proper manner. If, however, it were possible to secure the milk in exactly the same condition as it is se- creted, no microbes would be found in the milk from a healthy cow. Like other secretions of the body, it is normally sterile, but in the very process of withdrawing it from the udder, it usually becomes contaminated (31). 38. Contamination of milk. — The sources to which the bacterial contamination of milk may be ascribed vary greatly in different cases. Prom the time the milk is drawn until it is consumed in one form or another, it is continually subject to contamination from external sources. In the main these occur on the farm, but even in the factory, the opportunities for infection are present to a greater or lesser extent. Milk is so admirably adapted for bacterial growth, that the organisms that gain an entrance into the dairy are usually those that preponderate in the fluid. They have the advantage of the early seeding, and their rapidity of de- velopment tends to exclude other forms. 38 Dairy Bacteriology. 39. Influence of unclean dairy utensils.— Of first importance, are the vessels that are used during milking and also all storage cans and other dairy utensils that come in contact with the milk after it is drawn. By unclean utensils, actually visible dirt need not always be considered, although the presence of such in cracks and corners of pails and cans is often the case. Unless cleansed with especial care, these cracks and joints are apt to be filled with foul and decomposing material that suffice to abundantly seed the milk. A momentary application of hot or even boiling water or steam is insufficient to destroy the germ life that is always harbored in these joints. To be effectual, it is neces- sary to expose the vessels to the infiuence of high heat lor several minutes at least (19, 1). Steam is by far the best agent that can be used in clean- ing and sterilizing cans and other utensils. To be effective, it must be more or less confined so that it penetrates all joints and cracks. Dirty vessels must first be washed in water not too hot in order to remove the grease and dirt. They should then be rinsed in scalding water, and if possible turned over a steam jet for several minutes. Not only should the greatest care be paid to the condi- tion of the cans and milk pails, but all dippers, strainers, and other objects that come in contact with the milk must be kept thoroughly clean. Cloth strainers unless attended to are objectionable, for the fine mesh of the cloth retains so much moisture that they become a veritable hot-bed of bacterial life, unless they are daily boiled or steamed. Through negligence, vessels are often used that are ei^ther unfit or are in an improper condition for handling milk. A Infection of Milk and Methods of Prevention. 39 rusty milk can often spoils more milk than sufficient to pur- chase a new can. Wooden pails are no longer to be tolera- ted in a well-regulated dairy. Where possible, vessels should be made of pressed tin. If joints are necessary, they should be well flushed with solder so that they may be easily and thoroughly cleaned. Even where cans and vessels are in good condition and fairly well cleaned, the milk would remain sweet much longer if the vessel was previously sterilized by steam as is shown by the following experiment which was made in the month of August; Two covered cans were taken, one of which had been cleaned in the ordinary way, and the other sterilized by s'eam for thirty minutes. Previous to milking, the udder of the animal was thoroughly cleaned, and special precautions taken to avoid the raising of dust ; the first milk was also rejected. Immediately after milking, gelatin cultures were prepared with the following results : Number of germs per cc in sterile pail, 165. Number of germs per cc in ordinary pail, 4265. Time necessary to sour the milk in sterile pail, ^^honrsi$/L. Time necessary to sour the milk in ordinary pail,-^^iiours.2.cJ, A repetition of the same experiment in the winter showed nine hours difference in souring time, so that the factor of unsterilized vessels plays an important role in the fermen- tation changes that take place in milk. 30. Use of milk cans for transporting bye pro- ducts from factory back to the farm . — This practice is such a flagrant violation of the foregoing paragraph that its use cannot be sanctioned, unless under carefully scruti- nized conditions. The usual bye products of the fac- tory, skim milk (8*2) or whey (84) are invariably rich 40 Dairy Bacteriology. in bacteria, owing to the high temperature at which they are separated. If these fluids are returned to the farm in the factory set of milk cans, it often happens that they are im- perfectly cleaned, and if so, the milk is richly seeded with these organisms from the whey or milk. A large proportion of the taints and troubles that bother factories are due to this direct cause. A leading creamery- man recently gave me the information that during a certain season, all of the trouble that he had experienced with tainted milk came from a few patrons who insisted upon using their milk cans to carry back the skim milk to the farm. The responsibility of this defect should, however, not be laid entirely upon the shoulders of the producer. The factory operator should see that the refuse material does not accumulate in the waste vats from day to day and be transformed into a putrid mass. A dirty whey tank is not an especially good object lesson to the patron to keep his cans clean (85)- Extreme care in this respect must be taken for the safety of each individual . patron, as abnormal ferments that may prove difficult to eradicate are often transferred from one dairy to another. Suppose there appears in the dairy of A an infectious milk trouble, such as bitter milk (49)- This milk is taken to the factory and passes unnoticed into the general milk supply. The skim milk from the separator is of course infected with the germ, and if conditions favor its growth, the whole lot soon becomes tainted. If this waste product is returned to the diiferent patrons in the same cans that are used for the fresh milk, the probabilities are stronglj' in favor Infection of Milk and Methods of Prevention. 41 of some of the cans being contaminated and thus infecting the milk supply of other patrons. If the organism is ' endowed with spores so that it can withstand unfavorable treatment, this disease may spread from patron to patron simply through the infection of the vessels that are used for the transportation of the bye products. 31. Influence of first or fore milk. — A leading factor in the conr.amination of milk, the importance of which is rarely recognized, comes from the bacteria that gain access to the milk, by mixing the first or fore milk with Fig. 6. Section of udder showing relation of milk secreting tissue to milk duct fafter ThanhofFer). a, exterior opening of milk duct; b, milk cistern; c, milk sinus; d, glandular secreting tissue; e, sphincter muscle of teat. The bacteria are able to work their way up to the milk duct for a varying distance. the remainder of the milk. Even when the milking is most thoroughly done, there remains in the milk cisterns of the udder and in the teat, a few drops of milk that afford ample nutriment for the development of any germs that may gain access through the opening of the teat. Particles of flith and manure, if moist, easily adhere to the udder of the 3 42 Dairy Bacteriology. animal as she reclines on the ground or in the stable. If these filth particles come in contact with lower open end of teat, bacteria in this way gain access, and when once they ihave established themselves, they easily pass up into the milk duct and sometimes into the udder proper, as in the •case . of mastitis where inflamation sets in. Under these surroundings, bacteria find the most favorable conditions of development that are possible with perhaps the single exception that atmospheric air is wanting in some degree. Here they have a high temperature, approximating the body heat of the animal, so that the teat, if once infected, be- comes an ideal incubator. Under these conditions, a few germs will multiply enor- mously, so that if the fore milk is examined with reference to its bacterial contents many more germs will be found in this than is received in the later milk or in the strippings. The writer found in one instance under ordinarv condi- tions, 2800 germs in the fore milk while the average of total yield was 330 per cc. One marked characeristic of the bacterial flora of the fore milk is that the number of species is usually small, one or two kinds usually predominating to a large degree. This is due to the fact that these special forms are rapidly multiplying in this warm chamber. For this reason, their growth in the mixed milk is more rapid than those forms that are in a spore or dried state that come from the dust and air. G-enerally speaking, it may be said where a reason- able degree of cleanliness is found that those species that predominate in large numbers in any sample of milk are derived from the fore milk of the cow. Those that are pre- sent in relatively small numbers are due to contamination from the exterior. Infection of Milk and Methods of Prevention. 43 Under ordinary conditions, the germs that are commonly found in the teat are those that produce lactic acid, as these microbes find in milk their best medium for growth. It is, however, not impossible that noxious forms capable of producing serious changes may likewise obtain access, as is often experienced in a gargety or inflammatory condition of the udder. Bolley* in a series of thirty experiments made recently found twelve out of sixteen forms to be lactic acid produc- ing germs. In no case did he find gas producing bacteria present when the fore milk was secured by means of a sterile milling tube. How far these difflerent forms of germ life are able to penetrate into the healthy udder is as yet unknown. In all probability, the glandular tissue of the udder is not aflJected, although it is possible that microbes might work their way up the open channel of the teat into the udder proper. 32. Influence of animal and milker. — It is popu- larly supposed that much of the germ life that gets into the milk is absorbed in the food. For this reason fermenting food of any kind is regarded as unfit for use. While food undergoing decomposition is not a suitable nutrient material for any animal, the danger of impregnating milk is not due to the bacteria ingested with the food, but to a large extent, to those that adhere to the animal's coat, and subsequently fall into the milk. The hairy coat of the animal offers ex- ceptional facilities for the harboring of dust and dirt. It is, therefore, extremely rich in various forms of bacterial life that are derived from the particles of excreta that stick *As30. Ag. Coll. and Expt. Stations, 189.5, also Cent. f. Bakt. n, Abt. 1: 795, (1895). 44 Dairy Bacteriology. to the flanks and under parts of the animal when they lie down. These foreign particles are continually falling into the milk, and so adding bacterial life to this liquid. They may often be so numerous as to be easily recognized by the unaided eye. The amount of actual impurities that are to be found in milk, even after it is strained, will surprise the casual ob- server. From a large number of determinations of the solid impurities found in the market milk of different European cities, Renk* deduces the following rule: If a sample of milk shows any evidence of impurity settling on a trans- parent bottom within two hours, it is to be regarded as con- taining too much solid impurities. These solid particles, most of which are bits of manure, are always teeming with bacteria, especially with putrefactive and decomposition or- ganisms. Backhaus has recently estimated that the city of Berlin consumes daily, 300 pounds of excrement in its milk supply- Improper stable conditions greatly favor the amount of filth that may adhere to the animal. The high feeding that is practiced at present produces a softer and more liquid manure, and one in which putrefactive organisms are found in greater numbers owing to the higher nitrogenous contents of the dejecta. Then, too, the animal serves to transmit to the milk, organisms in stagnant waters, as the cow is often covered with slime and filth that adheres to her as she wades through stagnant mud holes. The filth with the myriads of germs that it contains, dries on the coat of the cow and is easily displaced by any movements of the animal or *Kenk,Cent.f. Bakt. 10 : 193. Infection of Milk and Methods of Prevention. 45 milker. In these ways, innumerable bacteria are always present on the animal's coat, and are constantly being brushed into the milk as it is milked. The taints in milk which arise from the use of certain foods such as garlic, onions, cabbages, some weeds, and in some cases, eveil from ensilage, are due t6 an entirely diflfer- ent set of causes. In these cases, the food materials contain certain volatile substances, that are thrown off from the body in the excretions and secretions. As a result the milk becomes charged with the odor, and their presence is easily detected upon the withdrawal of the same. What is true of the animal is likewise true concerning the person of the milker. Clothed in dust-laden garments that he has in daily use, he himself is covered with innumerable bacteria in a dried condition, and by means of the move- ments that are made during the milking, numerous dust particles are dislodged that bear their quota of germs into the rich nutrient fluid. A large amount of this filth and dirt can be easily avoided, but even the best cleaning in a dry manner of the particles of excreta and loose hairs from the animal's coat only diminishes in part the number of germs. Bacteria are dislodged from a dry surface with great ease, but from a thoroughly moistened surface, however rich in germs, little chance for development is had. Applying this course of treatment to the cow, a very marked diminution in the amount of bacterial life that finds its way into milk will be noted. The fianks and under parts of the animal, especially the udder, should be carded to remove loose hairs, then thoroughly moistened with water, and then sponged off so there is no drip from the surface. If the coat is thoroughly 46 Dairy Bacteriology. damp, the myriads of germs that are invariably present, even under the most favorable conditions, will be quite eflfeotually held in place. The hands of the milker should also be cleaned thoroughly with soap and water, just previous to milking. The habit of moistening the hands with a few drops of milk just before milking is to be deprecated from every standpoint, but es- pecially so, when considered from our present point of view. A pinch of vaseline on the hands will enable the milker to obtain a firmer grasp, and at the- same time, any scales or dirt rubbed from the teat would be held by the vaseline. Its healing e£fect on chapped or sore teats would also be helpful. It is worth while to have the milker clothed in a suit kept for this purpose, especially the upper portion of the body. An outer garment could easilj' be slipped over the regular working clothes. This garment should be laundried at frequent intervals. In order to show the contaminating effect of this factor, the experiment described below teaches a valuable lesson. A cow that had been pastured in a meadow was taken for the experiment and the milking done out of doors, so as to eliminate as much as possible, the influence of the germs in the barn air. Without any special precaution being taken, the cow was partially milked, and during the operation a covered glass dish containing a thin layer of sterile gelatin was exposed for sixty seconds underneath the belly of the cow in close proximity to the milk pail. The udder, flank, and legs of the cow were then thoroughly cleaned with water, and all of the precautions referred to before were carried out and the milking then resumed. A second plate was then exposed in the same place for an equal length of Infection of Milk and Methods of Prevention. 47 time; a control also being exposed at the same time at a distance of ten feet from the animal and six feet from the ground to ascertain the germ contents of the surrounding air. Prom this experiment the following instructive data were gathered. Where the animal was milked without any special precautions being taken, there were 3,250 bacterial germs per minute deposited on an area equal to the exposed top of a ten inch milk-pail. Where the cow received the precau- tionary treatment as suggested above, there were only 115 germs per minute deposited on the same area. In the plate that was exposed to the surrounding air at some distance from the cow, there were sixty-five bacteria. This indicates that a large number of organisms from the dry coat of the animal can be kept out of milk if such simple precautions as these are carried out. 33. Influence of air. — The influence of the stable air is also a factor of considerable importance. This germ life, although always present in the air, does not live and develop in this medium, (14., 2,) but in all oases is derived from the dust and dirt of the stable. In some cases patrons are not allowed to strain the milk, so that the cleanliness of the same may be better judged by the factory operator. The use of dry fodder, like hay, straw, or corn stalks, and the custom of bedding the animals with coarse refuse and litter adds greatly to the number of germs to be found in the barn air. Tais material covered with dust is filled with b%cteria o? the hay bacillus type that are very resistent or- ganisms. Unless the manure is removed at frequent intervals, the finer particles dry out quickly, and so contribute their quota of orgiinisms to the air. 48 Dairy Bacteriology.. Several epidemics of milk troubles have been traced to this cause. Weigmann mentions a case where the milk had a soapy flavor, (52), due to the presence of the specific organisms with which it had become contaminated that had been derived from the straw that was used for bedding. In another case, it was present in the hay and doubtless gained access to the milk by the dust that was raised during feeding. It is, of course, impracticable to attempt to free the barn entirely from dust, but much could be done with a little forethought. The common practice of feeding during the milking increases materially the germ life in the air of the barn, if the fodder is of a dry character. Moistened food is not so objectionable on this account, and if it is deemed necessary to feed during milking, this can be substituted for the dry coarse fodder and so lessen materially the germ content of the air. If it is necessary to use dry fodder, the danger can be minimized by feeding some time before milking, or after the milk has been removed from the barn. 34. RemoTal of milk from barn and straining. — The common custom of allowing the milk to remain in the barn until the milking is completely finished, and also the straining of the milk in the stable are entirely contrary to the teaching of the preceding pages. If the mUk is removed to a separate room, and for this purpose in a well organized dairy, a separate room should be provided, then the constant deposition of germ life from the infected barn air will be largely excluded. Still more important is it to strain the milk in a separate room arranged for this purpose. The process of straining exposes so much surface to the air that it should be done under conditions that do not favor the Infection of Milk and Methods of Prevention. 49 easy access of organisms as is the case when this operation is done in the stable. The commonly accepted idea that the milk more easily absorbs taints when strained in the barn is not strictly true, for when the milk is warmer than the air it does not absorb volatile substances. The explanation of this statement lies doubtless in the fact that the milk is- more easily infected when strained in the stable, and that the taints are the result of the growth of these bacteria derived from the atmosphere, although it must be admitted that even fresh milk has a peculiar odor, that when retained in the milk under certain conditions, is the cause of pronounced taints. 35. Aeration of milk. — Practical experience has de- monstrated the advantage of aerating the milk as soon after milking as possible. A satisfactory scientific explanation of this advantage has not as j'et been made. The influence of aeration is coupled so closely with a reduction in tempera- ture that it is difficult to assign to each the proportionate benefit that it deserves. It is, however, difficult to see how this process has any particular effect on the bacterial life in the milk. Where milk is used for cheese, the beneficial eflfect of this treatment is most apparent. 36. Combined eflfect of foregoing precautions.— The following experiment was made to determine the actual difference in germ content of milks secured under different conditions. In this work, the precautionarj' measures were carried out by an ordinary workman, and are in no sense so refined as to be impractical for daily use. The milk was received in steamed pails; the udder of animal before milking was thoroughlj' carded, and then moistened with water so as to prevent dislodgment of dirt. 50 Dairy Bacterwlogy . o o ■a ^1 O O c _o '+J c o CJ C o CO so . ^1 §^ '^ nj -O ■" u o o< +J S -o "o ca (n tn 15 Cii a o a c 5/ to a ^-M "73 ^\'-: J^\l»°^ °-SfQ,^X^'''^i'V, 150° F.) By the action of L ' of ^'&^<.'» ^V^'^^' the heat, the fat aggrega- ' ;vV/?.. .^'■'? -^J^J-'/^^j^llFt" .'.l tio-n is broken down, the "^-^ .^.* b' '"^ ' '^"^^'Itl*'°°' ° ^ v-'^ Vv globules being homogene- ^ , ■^"°i'-lJs.'^'':°'^°P'^'--- J •'S-'^J "^ ously distributed through- .-/»■, °:, »',•,"„ fS'q ^'ij. 'A'^jiy'^l'^^'Ji out the serum. "f ' 'n''" '"'^-ji >^£'.v vo"" ''■.•• '<^*°-Jte"'v :o 'rfS?*..-?- . -X- V,- •«a.o4-.' • .-'... 1. Thinner body. The consistency or body of milk or cream is dependent upon two factors; the viscosity imparted by substances in solution, and the effect due to substances in suspension. In cream the fat plays the most important 100 Dairy Bacteriology. part in determining the consistency. This is due more es- pecially to the state of aggregation of the fat globules. In normal milk or cream, a large proportion of the fat is grouped together in tiny clots or masses in fig. 8. If this milk or cream is heated to a temperature exceeding 150° F, these fat aggregations are broken down and the fat globules are homogeneously distributed throughout the whole serum, as in fig. 9.* 2. Cooked or scalded taste. If milk is heated to 160° P., it acquires a cooked taste that becomes more pronounced as the temperature is further raised. The cause of this change is not well known. Usually it has been explained as being pro- duced by changes in the nitrogenous elements in the milk, particularly in the albumin. Kecently, Thoerner t has pointed out the coincidence that exists between the appearance of a cooked taste and the loss of certain gases that are expelled by heating. He finds that the milk heated in closed vessels from which the gas cannot escape has a much less pro- nounced cooked flavor than if heated in an open vessel. This taste is permanently acquired at about 160° F., so that all sterilized milks possess it more or less. 3. Fermentative changes. Milk or cream normally sours, but heated milks usually do not undergo this typical fer- mentation. As a rule the casein curdles, but little or no acid is produced. This fermentation is due to the action of ren- net and digesting enzymes (47), that are secreted by bacteria that resist the influence of heat. As a rule the lactic acid organisms are destroyed by temperatures that are employed in pasteurizing, 4. Action toward rennet. Heated milks are changred in * Babcock & Russell, 13th Kept. Wis. Stat., 1896. p. 73. t Thoerner, Chem. Ztg 18 : 845. Preservation of Milk. . 101 their chemical nature as is shown by the action of rennet. Normally, rennet extract quickly coagulates milk, but if it is first heated, the action is much retarded. This condition is brought about by a change in the calcium salts in the milk induced by heat. 66. Sterilizatioa of milk. — Sterilization as applied to methods of preserving milk means the use of heat at or above the boiling point. A single application even under considerable pressure, and therefore, at a high temperature, often fails to render milk perfectly sterile. Particularlj', is this true during the summer months, at a time when the milk contains a larger percentage of spore-bearing organisms. Although the commercial methods of preparing sterilized milk do not usually render the milk perfectly germ-free, yet the organisms that remain, are so impaired in their vitality that they are unable to develop for a considerable length of time. As a consequence, sterilized milks have a better keeping quality than pasteurized. For export trade and for use where a long keeping quality is desirable, this is advan- tageous, but the fact that they have a more or less pro- nounced cooked taste militates somewhat against their gen- eral use. A number of different machines have been devised for the pupose of using high heat with and without pressure. In some cases the milk is treated in bulk and bottled or canned afterwards as in the Kleeman method; then again, in other cases, the milk is first bottled, and then subjected to the sterilizing process. In this latter method no danger of after- infection occurs. The details of the treatment vary con- siderably, but the most successful methods may be reduced to the following general classes: 102 Dairy Bacteriology. 1. Single sterilization at a high temperature under pres- sure. 2. Intermittent multiple sterilization without increased atmospheric pressure. The first method necessitates the use of a digestor or autoclave so that the steam may be confined as in a boiler, thus increasing the pressure, and consequently the tempera- ture. A single application of heat for one to two hours un- der a pressure of 250° F. usually will destroy all evidence of life. Milk so treated is materially changed in its physical and chemical composition. In the second class, the intermittent method of applying heat is emploj'ed. This rests upon the principle discovered by Tyndall that bacteria in the vegetating stage succumb to heat more easily than in the spore stage, and that if treated to a killing temperature and then placed under con- ditions that permit of spore germination, these newlj^ sprouted spores are readily destroj-ed by a second applica- tion of heat. While intermittent sterilization at the boiling point gives the most successful results, yet the pro- cess can be carried out in a fairly successful way by the use of heat at a somewhat lower temperature. This temperature must exceed the thermal death point (19, 1) of bacteria in a vegetative condition. Such a method of treatment is em- ployed in the preparation of Dahl's milk that has been so extensively used in Europe. In some machines the two methods of treatment are combined as in the newly pat- ented device of Neuhauss, Groenwald, and Oehlmann where the heat is used intermittently, but the last sterilization em- ploys pressure. All milks treated by high heat have the characteristic boiled or cooked taste more or less pronounced, although Preservation of Milk. 103 this is less marked where the milk is heated under such conditions that the dissolved gases are not allowed to escape. The various kinds of apparatus lor this purpose so far de- vised, are as a rule expensive, and can only be used economi- cally where a large amount of sterilized products are handled. 67. Pasteurization of niilli.— Pasteurization is the use of heat at a temperature ranging from 140° — 175° F., and is usually applied for only a liinaited length of time. The process was first used extensively by Pasteur (from whom it derives its name), in combatting the various maladies of beer and wine. Its importance as a means of increasing the keeping quality of milk was not generally recognized until a few years ago; but the method is now growing rapidly in favor as a means of purifying milk for commercial pur- poses from germ life of all sorts. The method does not de- stroy all germ life in milk; it affects only those organisms that are in a growing, vegetative condition. 68. Advantages and disadvantages of pasteurized products. — In comparison with ordinary normal milk and cream, pasteurized products enjoy the following advantages: Increased keeping qualitj', due to the annihilation of fermentative bacteria. So important is this that from the economic standpoint alone, pasteurization would be justi- fiable. If properly carried out, the pasteurizing process ensures the treatment of the milk in such a way as to destroy the seeds of contagious disease, thereby rendering the milk supply pure and wholesome. Pasteurized products in common with other milk products that have been heated to any considerable temperature are 104 Dairy Bacteriology. thinner than normal products possessing the same amount of butter fat. This reduction in consistency is coincident with, and dependent upon, a physical change in the mUk (65, 1). In comparison with sterilized milk, the following points should be noted: Pasteurized milk does not usually have the keeping quality of sterilized milk, because the germ-destroying agent is not applied- for so long a time and at such a heat. However, for ordinary consumption where it is to be used in a few days, pasteurized milks are satisfactory. Pasteurized milk and cream show no apparent change in physical characteristics as taste, or color. Sterilized milks have a more or less cooked taste that is objectionable to many. The relative digestibility of pasteurized or sterilized milks, and also their relation to heated milks is yet an un- settled question. As determined by artificial digestion ex- periments, the difference, if any, is very slight. For children, sterilized milk has been used much more than pasteurized, especially in Grermany, but here in this country, the sterilzing process has not made much head- way. There is some evidence to show that the continued use of highly heated milks, like sterilized or boiled, has a tendency to produce certain disorders of the system.* 69. Restoration of "body" of pasteurized cream. The diminished "body" of pasteurized cream has been a seri- ous drawback to its wide spread introduction. This reduc- tion in consistency is dependent upon the change induced by heat on the aggregation of fat globules into microscopic * V. Slarck, Milch Ztg , No. 4, 1895. Preservation of Milk. 105 clusters or clots. This objection has been recently overcome by Babcock and the writer. * They find that the addition of extremely minute quantities of lime in solution, causes the fat globules to cluster around each other, and this mechani- cal aggregation restores the body. In order to dissolve a large quantity of lime in a small volume so as not to change the relation of the milk solids to the serum, they use cane sugar as a solvent. This fluid called " viscogen " is added to cream in the proportion of about one part to one A B Fig. 10. Showing relative consistency of pasteurized creanr (A) compared with cream after being treated with viscogen (B). * Wis. Expt. Stat., Bull. 54; also 13th Rept Wis. Stat., 1896, p. 81. T 106 Dairy Bacteriology. hundred and fifty. This alkaline viscogen should never be added to such an extent as to make the cream neutral. The amount necessary is so small that it is often less than the variation in lime content between two samples of milk from various sources. The marked difference in consistency between the pas- teurized and the treated cream is seen in fig. 10. This sim- jple method of determining the relative consistency of two ; samples of cream is made as follows : Take a perfectly -clean piece of glass (plate or picture glass is preferable, as it is less liable to be wavy). Drop on one edge two or three drops of cream at intervals of an inch or so. Then incline piece of glass at such an angle as to cause the cream to flow down surface of glass. The cream having the heavier body or viscosity will move slower as is seen in figure 10, B. If several samples of each cream are taken, then the aggre- gate length of the same may be taken, thereby eliminating slight differences due to condition of glass. 70. Details to be observed in pasteurization.— While the pasteurizing method is simple in itself, yet in order to be uniformly successful in the operation, certain conditions must be observed in the treatment both before and after the heating process. In order of their occurrence they are as follows : 1. Selection of best milk for pasteurizing purposes. While any kind of milk or cream may be treated according to the pasteurizing process, yet the best results will only be se- cured if some control is exercised over the character of the milk used for the purpose. Just as a can of tainted milk may spoil a whole vat full for cheese-making, so a small quantity of milk that is either dirty or foul may ruin a large quantity for pasteurizing. Preservation of Milk. 107 Milk that is evidently dirty should be rejected (32) for this use as the presence of visible dirt is a sure indication that the fluid is highly contaminated by the bacteria de- tached from the dirt particles. The best results are ob- tained where the milk is first purified by passing it through a cream separator and then remixing the cream and skim- med parts. The age of the milk and the conditions under which it has been kept should also be carefully noted. Old milk is almost always richer, not only in bacterial germs but in the latent spore forms as well; so the fresher the milk, the fewer bacteria it will have under general conditions, and therefore, the pasteurizing process will be the more com- plete. 2. Rapid method of selecting milk. The true standard for selecting milk for pasteurization Should be to determine the actual number of bacterial spores that are able to resist the heating process, but this method is impractical under com- mercial conditions. The following method while only approximate in its re- sults will be found helpful. Assuming that the age or treatment of the milk bears a certain relation to the pres- ence of spores, and that the acid normally increases with an increase' in age or temperature, the amount of acid present may be taken as a rough index of the suitability of the milk for pasteurizing purposes. Biological tests were carried out in the author's laboratory* on milks having a high and low acid content, and it was shown that the milk with the least acid as a rule is the freest from spore-bearing bacteria. This acid determination can be quickly made by employ- ing the Farrington alkaline tablet such as is used in cream * Shockley, Thesis, Univ. of Wis., 1895. 108 Dairy Bacteriology. ripening. An arbitrary standard of 0.2 of per cent, acid has been found to give good results in actual practice. Details of using acid test. This can be done at the weigh can where the milk is received, and the milk selected for the pasteurizing process can then be run into a separate receiver. Fig. 11 shows the apparatus necessary to make this selection. An extended use of this test shows that 0.2 per cent, acidity may be selected as [irstOuiji^ 60unce^ottle. Measure Fig. 11. Apparatus for testing the approximate acidity of milk or cream. Bottle holding eight ozs. of water in which eight tablets (Farrington alkaline tablets) should be dissolved. Meas- ure for measuring the proper amount of alkali solution and milk to be used in making the test. Common cup in which the two fluids are mixed. Preservation of Milk. 109 a standard. If this is done a definite quantity of the alkali solution (one dipper full of the measure shown in diagram) may be poured into white cup. To test any given sample of milk, ada Ij ij wo as much milk to alkali solution in cup and then shake thoroughly. If pink color persists, even slightly, it shows that the milk does not contain sufficient acid to neutralize the alkali in the test solution. If on the other hand, the color wholly disappears, the milk retaining its usual color, then it contains more than the standard per cent, of acid and should be rejected for pas- teurizing purposes.* 3. Temperature and time in pasteurization. The necessary temperature in pasteurizing may be subject to considerable variation. The minimum temperature must exceed the ther- mal death point (19, 1) at which the milk bacteria are des- troyed. As the tubercle bacillus is sometimes found in milk, and as it one of the most resistant organisms in its vegetative state that is known, the thermal death point of this germ serves as a minimum standard for efficient pas- teurizing. The determination of this point depends on the following conditions : 1. The temperature of heat used. 2. The length of exposure to the heat. With either condition fixed, the same result is accomp- lished more rapidly by an increase either in temperature or duration of exposure, so that an exposure for a longer time at a lower temperature is quite as efiective as a shorter ex- posure at a relatively higher temperature. With the great majority of bacterial species that have been individually tested as to their thermal death point, * For further details of this pTOce?s, see Wis. Expt. Stat., Bull. 52. 110 Dairy Bacteriology. 140° P. for ten minutes has been found fatal. This degree of heat suffices to kill all the disease-producing bacteria that are found in milk with the exception of the tubercle bacillus. According to Porster,t heating thirty minutes at 149° F., fifteen minutes at 155° F., or ten minutes at 167° F. suffices to destroy this germ. The maximum temperature that «an be employed is just below the point at which the milk will permanently acquire a cooked flavor. The appearance of this peculiar flavor can- not be detected with absolute accuracy, but it is not far from 158° F. It should .be observed, however, that a tem- porary cooked taste appears in milk before it reaches the above point, but if it is not held at this high temperature for any considerable length of time, it -loses it when chilled. Above 158° P. or thereabouts, this temporary condition be- comes permanent. As the maximum pasteurizing temper- ature is limited by this condition, it becomes necessary to lengthen the time of pasteurizing to the fifteen minute period in order to have a fatal eflect on the tubercle germ. Allowing a margin of safety, the period of time in pasteur- ization is then placed at twenty minutes and the temperature at 155° P. It is not enough to merely heat the milk to the requisite temperature, but it must be kept at this point for the proper length of time. 4. Chilling milk after heating. The pasteurizing process kills the bacteria in the milk that are only in a developing condition; the spores are able to resist this temperature. If the milk is allowed to cool without the use of artificial chill- ing, it will take several hours for it to lose the heat that t Forater, Hyg. Eund., No. 15, 1893. Preservation of Milk. Ill has been imparted to it by the pasteurizing process. Dur- ing this time the temperature for the most part, will be such as to favor rapid bacterial development, so that the spores that are left after the heating process will rapidly germinate and grow. After they have once germinated, they are able to develop under conditions in which it would be impossible lor them to sprout. The practical applica- tiou of this principle is, then, that pasteurized milk changes quite rapidly, unless it is immediately and thoroughly chilled after it is heated. Its keeping quality is therefore largely dependant upon the storage temperature after heating. The experiments of Bitter* indicate that when stored at 86° P., pasteurized milk will remain sweet from six to eight hours longer than raw milk. At 77° F., ten hours; at 73° F., twenty hours; at 58° F., from fifty to seventy hours. In our experience, pasteurized cream when kept in an ordinary refrigerator usually remains sweet for a period of four to six days, and sometimes even longer. The relation of the chilling process to the growth of bacteria is shown in Fig. 12. 71. Pasteurizing apparatus. — Apparatus adapted to pasteurization of milk and cream differs according to the purpose for which it is used. If it is desired to pasteurize these products for direct consumption, the treatment differs som^irhat from that when pasteurizing for butter-making is the object in view. Equipment necessary for pasteurizing for the former use may be divided into two general classes. 1. Apparatus of limited capacity designed for private family use. 2. Apparatus of sufficient capacity to pasteurize on a commercial scale. * Bitter, Zeit. f. Hyg., 8: 240, 1890. 112 Dairy Bacteriology. 72. Domestic apparatus. — Pasteurization can be easily and efflcientlj- done in a limited wa}- with the addition of an ordinary thermometer to the common utensils found fflSTEUuizi-flq- rtnptRflTURf THCRHAL r- Jtf^THpoinTl- A\flxinun GROWTH ]>oi.nT BuDODHt^T - QRowTHpoinj 2H1-3 Fig. 12. Diagram showing temperature changes in milk sub- jected to the pasteurizing process and the relation of the same to the growth of bacteria. Bacterial development occurs between 10 — 43° C; the most rapid growth being designated by deeper horizontal shading. The black line aA indicates the temperarure changes in pasteuriz- ing; a to b the heating process, b to c the maintenance of a proper temperature, and c to A the cooling. The necessity of rapid cool- ing of the milk to a temperature below the germinating point is apparent in order to retain the good effects of pasteurization. Preservation of Milk. 113 in every kitchen. Fig. 13 indicates a simple contrivance that can be readily arranged for this purpose. The following simple suggestions indicate the different steps of the process : 1. Use onlj' fresh milk for this purpose. 2. Place milk in clean bottles or fruit cans, filling to a uniform level. (If pints and quart cans are used at the same time, an inverted dish or piece of wood will equalize the level.) Set these in a flat bottomed tin pail and fill with warm water to same level as milk. An inverted pie tin punched with holes will serve as a stand on which to place the bottles during the heating process. 3. Heat water in pail until the temperature reaches 160° F. ; then remove from source of direct heat, cover with a Fig. 13. Simple pasteurizer for family use. A covered tin pail serves as a receptacle for water and milk. To measure the temperature of the water, remove cover and insert thermometer in water. 114 Dairy Bacteriology. cloth or tin cover and allow the whole to stand for half an hour. 4. Kemove bottles of milk and cool them as rapidly as possible without danger to bottles and store in a refrigerator. 73. Apparatus for commercial pasteurization. — There are many pieces of apparatus that have been de- signed for the purpose of pasteurization. The great ma- jority of these have originated in Germany where this method is largely used for the preservation not only of milk but such bye products a.s skim milk and whey. The first devices for this purpose to be used with milk were those of Thiel and Pesca that were introduced early in the eighties. In these the milk was warmed for only a short time as it flowed over a heated surface, and then cooled by passing it over a cold corrugated surface. These machines were introduced primarily to increase the keeping quality of the milk without any reference to its contents from a hy- gienic point of view. At present there are a large number of different machines intended for the pasteurization of milk and other liquids that have been put upon the European market. In many cases, there is a marked similarity, so that it will be quite unnecessary to describe in detail the different designs. The greater part of them are included in two general classes. 1. Those in which a thin sheet of milk is allowed to flow over a surface that is heated by either hot water or steam. These are arranged so that the milk usually flows in a con- tinuous stream by gravity over a corrugated surface, or bj' centrifugal force. The milk is heated only while it is in contact with the heating surface, and is, therefore, not under direct control unless drawn off in special tanks and kept at a definite temperature. Preservation of Milk. 115 To cool the milk, cold or ice water is geuerally run through a similar type of machine. Among the more prominent machines of this class are those of Thiel, Kuehne, Lawrence, and Hochmuth, The centrifugal machines of Lefeldt and Lentsch of Grermany, and that of Monrad in Fig. 14. Motirad's centrifugal pasteurizer with a Baer cooler attached. this country, utilize centrifugal force in distributing a thin layer of milk over a heated revolving drum. By regulating the speed, the milk may be kept in the machine until it reaches any desired temperature, but it must be held in a heated storage tank for the requisite length of time to des- troy all pernicious organisms. Monrad heats his machine with direct or exhaust steam. The inner surface of the drum is, therefore, inevitably covered with a layer of coagu- lated albumen where the milk has been heated above the scalding point. •> 2. Those in which a reservoir is used to hold the milk during the heating process. This space is usually sur- 116 Dairy Bacteriology. rounded bj' an exterior shell that contains the heating agent, steam or steam heated water. The more efficient machines of this class are provided with an agitator in the milk reservoir so as to hasten the equalization of temperature in the inner chamber, and at the same time keep the milk in motion in order to prevent the coagulation and the adher- ence of the proteids on the wall of the vessel. The larger number of these machines are arranged for a continuous delivery, the milk flowing in at the lower end and displacing that already pasteurized which flows out above into a cooler. In some of them the agitator even " Fig. 15. Fjord's Pasteurizing machine. D, outside wooden jacket; c, milk chamber; k, agitator run by gearing at a. Milk enters at m through b. If direct steam is used, it enters at e ; if exhaust, at f. Condensed water e.scapes at g. Preservation of 31ilk. 117 mixes the fresh supplj- with that which has already beea heated, so that the efficiency of the process is much lessened. Among the more prominent of these machines belonging to this class are those of Ahlborn and Fjord that are composed of two cylinders, the milk reservoir being the inner one. The milk is kept in constant motion by means of a rotating vane in the milk chamber. The apparatus of Dierks and Moellman has a series of closely set rotating brushes in the milk that act upon the heating surface so as to prevent the collection of coagulated protelds. The majority of these machines are subject to the very serious objection that the product they deliver is not always uniformly heated. Where it is constructed for continuous delivery, the length of exposure must necessarily be quite limited, and as the temperature ought not to exceed 160° E., it very often happens that the pasteurizing process is not efficient. In the case of the sour milk organisms, from the hj^gienic standpoint, it is of little moment, but to insure absolute freedom from disease germs, the temperature and the time of exposure must be thoroughly under control. Very few of the machines intended for this purpose have been sub- jected to a rigid bacteriological test and the lack of this has allowed the introduction of many designs that may be adapted for the pasteurization of bye products intended for animal food, for which purpose they were originally de- signed, but thay certainly do not deliver a product that can be relied upon for human food. in. Essential conditions in pasteurizing appar- atus. — A pasteurizing apparatus to be a success must ful- fill certain conditions as follows : 118 Dairy Bacteriology. 1. Efficiency in operation. To operate efficiently, any ma- chine must be thoroughly under control as to length of exposure and temperature to which the milk is heated. A satisfactory product can only be obtained where a definite quantity of material is heated for a definite length of time at a definite temperature. A fulfillment of these conditions necessitates the use of the intermittent type of apparatus, or continuous apparatus arranged so as to practically conform to the discontinuous process. Not only must the milk be uniformly heated, but the heat should be applied so that there is no danger of scorching the proteid elements of the milk. This precludes the use of apparatus using direct steam as a heating agent. 2. Easily cleaned. To facilitate cleaning thoroughly, all parts of the apparatus that come in contact with the milk should either be removable or easily accessible in their fixed condition. Dirt and filth is very apt to collect where it is impossible to see just how thoroughly the apparatus is cleaned. Vat corners and edges should be thoroughly flushed with solder. 3. Simplicity in construction. This requirement is essen- tial as complicated designs are not only harder to keep clean, but they are necessarily more expensive. 4. Economical in use. Economy in use is dependent upon the absolute cost of operation compared with relative out- put of product. The continuous type of machines invariably has a greater capacity, and therefore, the relative cost per gallon is less than in intermittent apparatus. In many •devices, however, both the heating and cooling appliances are used in a wasteful manner, only a small percentage of ■energy being utilized. Preservation of 31 ilk. 119 5. Safety from reinfection. It is not necessary to have a machine constructed -so as to exclude the air entirelj', but there should be no opportunity for germ laden dust to get into the apparatus. In most pasteurizing machines it is necessary to have a separate heater and cooler. Sometimes, two machines of the same construction are used, one for the heating and the other for the cooling of the product. This arrangement is, however, not an economical one, as the physics of the two operations vary somewhat. 75. Wisconsin Dairy School apparatus. — The ap- paratus for pasteurization that has been devised by the author* has been constructed primarily from the hygienic standpoint. In this apparatus, the attempt has been made to so treat milk and cream for direct consumption that it is positively freed from any disease-producing bacteria that might be present in the regular supply. The various details of the process as carried out on a commercial scale are as follows: 1. Pasteurizing. 2. Cooling by cold water and ice. 3. Sterilizing cans, dippers, bottles, etc. 4. Bottling. 1. The pasteurizing vat is built on the reservoir principle, allowing the heating of a definite quantity of milk. This vat is long and narrow, and is surrounded by a water cham- ber, on the bottom of which is placed a row of perforated steain pipes. To facilitate the heating of the milk, both the milk and water reservoirs are supplied with agitators having a to and fro movement supplied from a belt power. The milk stirrers are perforated so as to give an up and * Eussell, Bull. 44, Wis. Expt. Stat. 120 Dairy Bacteriology. down movement, thus preventing the cream from rising to the top of the vat during the operation. The milk is with- drawn from the vat by means of a stop cock that is placed inside of the water chamber. This cock has a circular bore so that when open, the outlet tube presents a continuous passage that can be easily and thoroughly cleaned. Placing the stop-cock within the water chamber prevents the accumu- lation of unpasteurized milk in the outlet tube at a point not reached by the heated water, and where it would con- taminate the whole vat when the milk was withdrawn. igi ig) 0" -P ^ ',M///^/,'.'.'^.'iylyj/y/. w,-/.'/^//^. \ 5A M.E. N-O Fig. 16. Diagrammatic side view of pasteurizer: i. y, — wall of milk reservoir; o. v. — wall of outside water reservoir; m. c. — milk chamber; w. c. — water chamber; i. s. — Stirrers in milk cham- ber attached by r to gearing; o. s. — stirrers in water chamber at- tached by rod r to gearing; p. — pulley for belt power; c. — crank for hand power; water and steam introduced at w. p.; milk drawn oif through s. c. which is operated by lever /.; temperature taken by thermometer at tber. 2. A large part of the expense in pasteurizing milk on a commercial scale is in the rapid cooling of the heated Preservation of MUh. 121 '0-V. Fig. 17. End view of pasteurizer showing twin vats. Water chamber w. c. is filled with water while milk is placed in in- ner chamber m. u. Water and steam admit- ted to vant through w. p. and distributed by vents F. The agita- tors in milk and water chambers are i. s. and o. s. respectively; /. v. and o. v., the walls of milk and water reservoirs, if. p. the support for the milk chamber. product. Under ordinary conditions, water can be used in part, but usually tlie temperature of the water supply is higher than that desired in the chilled product, so that the use ol ice is imperative. To economically cool milk, it is best to use water to extract a' part of the heat at first, re- FiG. 18. Surface view of pasteurizer. Milk chamber m. c. surrounded by w. c. water and reservoir. Gearing run by pulley p. or crank c. carrying stirrers s. in inside chamber, p,' pin that is removable permitting the detachment of agitating machinery in milk while the outside stirrers may be kept in mo- tion, s. side view of milk agitator. 122 Dairy Bacteriology. serving the ice for the latter part of the process. The milk may l)e drawn off in ordinary shot gun cans, and these set in running water and finally in ice water, but this method is somewhat crude. Fig. 19. Sectional view of water cooler using either cold water or water and ice combined. Another method is to provide two separate coolers so as to utilize water and ice most advantageously. The water cooler is made of two cylindrical shells between which the milk flows (m. c.) in a thin sheet. The inner cylinder (w. c) has a current of cold water circulating through it, while the apparatus is immersed in a rectangular tank that is filled either with flowing cold water or broken ice. The milk comes into the apparatus at (i. m.) and is thus spread out into a thin sheet in (m. c), where it loses its heat quickly and then flows out through milk pipe (o. m.) at the other end of apparatus. The cylinders fit together with a ground joint or with a gasket, the connections for the milk being made with a sink plug so that they are detachable and easily cleaned. The ice cooler is made up of an inner chamber that is filled with ice. The milk as it comes from the water cooler flows down one side of this ice box that is corrugated, and is held in a larger chamber (m) in which the inner ice box is placed. In this way all of the absorptive energy of the ice is utilized as it must be spent on cooling the surrounding Pretervation of Milk. 12:- milk that can be held in the chamber as long as desired. If desired this milk reservoir can be immersed in another box fiUed with ice water (w), in which case the milk is sur- rounded on both sides by a chilling surface. The milk chamber is prevented from reinfection by a loose fitting, cover. 3. Sterilizing oven is a large galvanized iron box fur- nished with steam pipes on bottom, having vents at regular intervals. This permits the steam to be thrown directly into the cans thereby increasing its sterilizing effect. If a Fig. 20. Sectional view of ice cooler. Ice is on inside of milk -chamber m. Milk flows in at r. and remains in reservoir as long as desired. Withdrawn at s. 124 Dairy Bacteriology. closed chamber is used, the steam can be employed under pressure, securing the desired disinfection in a shorter space of time. All cans, pails, dippers, bottles, and other utensils that come in contact with the milk should be thoroughly steamed to render them germ-free as possible. W.5. S.TP S.T.'PV. Fig. 21. Steam sterilizer for sterilizing bottles, cans, and' other utensils, st. p., steam pipe; st. p. v., vent of same; w. s., wire shelf to support utensils; c, stop cock for condensed water. 4. In bottling the product it is necessary to keep the milk protected from reinfection. It may be bottled from a large can with a bottom faucet, or on a large scale with commer- cial bottling machines that fill several bottles at once. The best bottles for the purpose are those that have a plain pulp- cap. All metal fastenings or stoppers are dirt catchers and; Preservation of Milk. 125 are likelj- to get out of order. It is our practice to heat pulp caps in paraffia, therebj- rendering them more pliable, and at the same time sterilizing them. Bottles sealed with hot caps in this way are tightly closed. Better results will be secured if bottled cream is kept over night on ice than where it is delivered immediately after being chilled. The retarding effect of chilling on growth is thus emphasized. In delivering pasteurized pro- duets it is always necessary to use care in handling to pre- vent the cream and milk from being warmed up, and thus inciting into activity the latent spores. G<6- Bacteriological study of pasteurized milk.^ In an extended bacteriological examination of pasteurized Fig. 22. Showing diminution of bacteria in pasteurized milk. Black square represents number in raw milk, tiny white square number left after pasteurization. 126 Dairy Bacteriology. milk and cream that had been prepared on an ordinary com- mercial scale at the Wisconsin Dairy School Creamery, it was found that 99.8 per cent, of the bacterial life in the raw- milk or cream was destroyed by the heat employed i. e., 155° P. for twenty minutes duration. This indicates that the number of resisting spore forms in milk that is properlj' selected is very small. The bacteria that are de- stroyed are mainly lactic acid-producing forms, while those that resist the pasteurizing conditions are either organisms that seem to have no apparent effect on the milk, or those that curdle it with the production of certain chemical fer- ments like rennet or trypsin. * While these organisms capable of resisting the pasteuriz- ing temperature are fermentative germs, it is also important to determine if they have the power of forming toxic sub- stances that exert a harmful effect on the animal bodj'. Fluegget has found several forms in sterilized milk that are able to produce highly toxic substances. A similar study has been made under the writer's direction on the bacteria isolated in pasteurized milk under commer- cial conditions. Shockley J studied thirteen different forms found in pasteurized milk, but all of them with a single ex- ception failed to cause any disturbance when inoculated into white mice and rabbits. In the single instance, fatal results were only obtained when large quantities were inoculated. * Kussell, 12th Wis. Expt. Stat. Eept., p. 160, 1895. t Fluegge, Zeit. f. Hyg., IJ : 272, IfDl. I Shockley, Thesis, Univ. of Wis., 1896. PART in. RELATION OF BACTERIA TO MILK PRODUCTS. CHAPTER VIII. BACTERIA IN FLUID PRODUCTS OF MILK. 77. Bacteria in cream due to mechanical causes. Cream whether secured by the gravity process or by a cream separator is invariably richer in bacteria than the skim milk of the same age. A sample of milk might contain less than 100,000 germs per cc. in the skimmed part, while the bacterial contents of the cream layer in the same, would be several millions for the same unit of volume. This is largely due to the filtering out of the microbes during the process of creaming. It is a well-known fact in sewage filtration that the addition of some chemical that will cause the precipitation ot certain organic elements always present in the sewage, will eliminate, by far, the majoritj' of bac- teria in the settling of this precipitate. The same principle seems to be operative in the process of creaming, except that the fat globules instead of sinking to the bottom rise to the surface. Gravity -raised cream is usually richer in bacteria than separator cream, but this is because it is ma- terially older when gathered. In full milk separated by the centrifugal method there are three well marked products, — the skim milk, the cream, and 128 Dairy Bacteriology. the slime that adheres to the separator bowl. If milk is thoroughly mixed just previous to separating and examined with reference to its bacterial life just before, and then im- mediately after the separation these three materials are ex- amined in a similar way, the following results will be obtained: The slime which is composed of particles having a greater specific gravity than the milk is thrown out on the edge of the revolving fluid by centrifugal force. This material, if examined microscopically will be found to contain large quantities of foreign matter as well as innumerable bacteria. The cream will almost always contain a larger number of bacteria than the skim milk. Popp and Becker found in a sample of whole milk, containing 73,000 germs per cc, the following germ contents after it had been separated: Cream, 58,275 germs; skim milk, 21,700 germs, and the separator slime 43,900 per cc. This centripetal movement of a large number of germs with the cream indicates that thej' adhere to the tiny fat globules, for this peculiarity in distribution can hardly be explained on the ground of their specific gravity, as they remain in the skim milk in considerable numbers in spite of the great centrifugal pressure. 78. Relation of tubercle bacillus to separator slime. — According to Scheurlen* and Bang, t tubercle bacilli, if present in a milk are largely thrown out with the slime in the separating process. Moore X found in milk ar- tificially infected with tubercle bacilli that the separating process diminished them to such an extent that they could not be determined microscopicallj', but when this separated * Scheurlen, Arb. a. d. k. Ges. Amte, 7 : 269, IS91. t Baug, Land. Wocb.f. Sch, Hoi., 1894, p. 47. X Moore, Year-book of U. S. Dept. of Agri., 1893, p. 4.32. Bacteria in Fluid Products of Milk. 129 milk was inoculated into guinea pigs, they succumbed. This indicates that while the removal was considerable, yet, it was not complete enough to justify the use of this method for the purification of infected milk. Coupled with this peculiar relation of the tubercle germ to centrifuge slime, is the fact that tuberculosis among swine is much more preva- leut in Denmark and North Grermany where the centrifugal process in creaming is extensively used, and where, until re- cently, .the swine were fed the uncooked separator slime. Ostertag* has pointed out this condition, and has drawn at- tention to the numerous cases of intestinal tuberculosis in hogs. 79. Bacterial changes in cream. — As germ life is so abundant in cream, it is to be expected that this product will be the seat of manifold changes of a fermentative character. Such is, however, not usually the case, as the cream is so much richer in butter fat — a milk constituent that is not well adapted as a food element for bacteria. (10, 2). Any fermentative changes are of course undesirable where cream is intended for consumption in its natural form, because they sooner or later result in the formation of vari- ous products that render it worthless for table use. Although cream is numericallj', much richer in bacteria than milk, j-et the changes due to bacterial action are so much slower that the later product usually apoils sooner than cream. For this reason, cream will sour sooner when it remains 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 increase the keeping quality of the prod- * Ostertag, Milch Ztg, 2!! : 672. 130 Dairy Bacteriology. uct, and is another argument in favor of the separator pro- cess. 80. Bacterial rariations by different creaming methods. — The method used in creaming has an important bearing on the- liind as well as the number of the bacteria that are to be found in the cream. The difference in species is largely determined by the difference if temperature used in the various processes, while the varying number is gov- erned more by the age of the milk. Primitive gravity methods. In the old shallow setting pro- cess, the temperature of the milk is relatively high, as the milk is allowed to cool naturally. This comparatively high temperature favors especially the development of those forms whose optimum growing point is near the air temper- ature. B}' this method the cream layer is exposed to the air for a longer time than any other, and consequently, the contamination from this source is greater. Usually, cream obtained by the shallow setting process will contain a larger number of species, which fact is accounted for mainly by this exposure. Cream secured by this process contains more acid than that which is obtained in any other way. Modern gravity methods. In the Cooley process, or any of the modern gravity methods where cold water or ice is used to lower the temperature, the conditions do not favor ihe growth of a large variety of species. The variation in the cream will depend largely upon the way in which the milk is handled previous to setting. If milked with care, and kept so as to exclude outside contamination, the cream will be relatively poor in bacteria. Onlj- those forms will de- velop in abundance that are able to grow at the low tem- perature at which the milk is set. Cream raised by this Bacteria in Fluid Products of Milk. 131 method is less frequently infected with noxious forms than that which is creamed at a higher temperature. Centrifugal method. Separator cream should be freer from germ life than that which is secured in anj' other way. It should contain only those forms that have found their way into the milk during and subsequent to the milking, for the cream is ordinarily separated so soon that there is but little opportunity of infection, if care is taken in the handling. As a large part of the infection of fresh milk is due to the contamination from the fore milk (31) which usually has but a few species, separated cream, if handled with caution, will contain mainly those species that are to be found in abundance in the milk while it is still fresh. Where cream is obtained by the separator process, it is always prudent to cool the cream, as it must be heated pre- vious to separating in order that the centrifugal process may be effective in removing all of the fat. 81. Factory bye products. — While the bye products in the manufacture of butter and cheese are in certain sense waste products, yet, all of them possess sufficient nutrient value to warrant their use as human food or in animal feeding. When handled carelessly, however, their nutritive value is much lessened by the continued fermentations that occur in them. All of these products are rich in bacteria owing either to their age or the treatment they have under- gone in the manufacture of butter or cheese. It is, there- fore, all the more essential that they should be kept in such a manner as to check the continued development of germ life within them. 82. Skim milk. — ^ Skimmed milk varies mach in its bacterial content, depending upon the way in which the full 132 Dairy Bacteriology. milk has been treated. ^lilk from which the cream has been removed by the shallow setting process is usually very rich in germs, and often has so much acid that it is easily recognized by the taste. Where the cream is gathered by the aid of ice water, the temperature is reduced to such an extent that the skimmed part is relatively poor in bacteria. Separator skim milk is treated in such a radically different way that it is bacteriologically quite a different product. The skimmed part is separated from the fat when the milk is only a few hours old so that the opportunity, for germ growth has been relatively slight. The conditions under which it is secured are such that unless certain precautions are taken, the product will be badly contaminated in a very short time. The warm skim milk radiates heat so slowly that the organisms present have the best facilities offered them for growth (fig. 12). It is for this reason that separator skim milk sours so rapidly after the cream has been removed. This difficulty can be obviated in two ways: 1. Rapid and efficient cooling below the growing point, of the contained bacteria. 2. Pasteurization and subsequent cooling. Where skim milk is used immediately for feeding pur- poses, it is perhaps hardly worth while to pasteurize, for, if it is properly cooled, it will remain sweet for several hours. In Europe, pasteurization of skim milk is extensively pract- iced, but a considerable part of the product is there used , lor human food. 83. Buttermilk. — Buttermilk contains a large amount of casein and sugar, and is, therefore, of considerable value for feeding purposes. It is usually Aery lich in bacteria, sometimes containing even more than ripened cream. Bacteria in Fluid Products of Milk. ] 33 Pammel* found 1,700,000 germs per cc. in buttermilk, while the organisms in butter were less than half a million. This high content is due to its age, for while it is in the ripening cream, bacterial growth is taking place rapidly, and when the buttermilk is strained from the fresh butter, the major- ity of the organisms are washed out. 84. Wliey. — This is another bye product of cheese making that has considerable value for feeding purposes. Like separator skim milk, it is drawn at a temperature that greatly favors bacterial development. Compared with the germ content of the raw milk, whey possesses fewer organ- isms than skim milk or buttermilk kept under similar condi- tions, as a larger part of the bacteria present in the milk are caught in the curd as it is coagulated by the rennet. The presence of these in whej', if left to cool naturally, soon sours the product, as the milk sugar is further converted into various acids. Both whej- and skim milk for feeding- purposes should be carefully handled in order to get the most out of them. If the ferments are allowed to develop, the sugar is changed into various acids, the albumen under- goes putrefactive changes, and much of the feeding value is lost. 85. Cleanliness of whey vats.— The vats for factory- bj'e products are often made of wood; consequently, they are difficult to clean thoroughly, even if that procedure was attempted. If not carefully cleansed and sterilized by steam each day, the particles of milk or whey that adhere to the walls, quickl}' sour, and so infect the material that is stored in the vat on the succeeding daj'. In this way the vat becomes a center of infection. Often, too, this already * Pammel, Bull. 21, Iowa Expt. Stat. 134 Dairy Bacteriology. contaminated waste product is allowed to stand in cans after it is taken back to the farms until it is thoroughly soured. Such fermented food has only a minimum value, as much of its nutritive worth is gone. Yats for these bj'e products should be constructed from galvanized iron and arranged so that the fluids may be drawn into the cans by gravity instead of i)umping material into the same. The vats and waste pipes should be carefullj' scalded each day as much as any other part of tbe factory. Fig. 23. A Swiss cheese factory showing the careless method in which the whey is handled. The trouble arising from sour whey and sour milk is made still worse by the practice often seen of returning these highly contaminated fluids to the farm in the same set of cans that are used for the transportation of milk (30). Filthy whey vats and cans are responsible' for much of the tainted milk that is seen in factories. It is Bacteria in Fluid Products of Milk. 135 necessary that the cheese-maker should have his waste vat free from sour and half fermented whey before he can charge all of the blame upon the patron who brings bad milk. The accompaniyng sketch illustrates the manner in which the whey is distributed and cared for in many factories that make Swiss cheese particularly. The hot whey is run out through the trough from the factory into a large trough that is placed over a row of barrels, as seen in the foreground. Each patron thus has allotted to him his proper share, which he removes day by day. No attempt is made to clean out these receptacles, and the inevitable result is that thej- become a foul, stinking mass in hot weather. This material is usually carried home in the same set of cans in which the milk is brought, with the result that the new milk is im- pregnated with these foul breeding organisms. That such a custom should have grown up in the Swiss cheese industry where all possible precautions are taken to insure the milk being kept as good as it can be, illustrates the necessity of bacteriological principles being instilled into this phase of the dairy industrj^ 86. Heating whey to preserve it. — In some facto- ries, sterilizing or pasteurizing the whey is employed in order to preserve its full nutritive value. To do this, a steam pipe is usually introduced directly into the whey vat. This method enables one to raise the temperature of the whey -to any desired point, but in order to secure the full advantages of this heating process, it is necessary to cool it quickly to a point at which bacterial growth will be mater- ially retarded. This cooling process as a rule is too expen- sive when the intrinsic value of the sterilized whey is 136 Dairy Bacteriology. considered. The same practice is followed to some extent in some creameries that return the skim milk. Such a course is hardly necessary where the whej' vat is arranged as suggested in the foregoing paragraph and where it is thoroughly scrubbed and scalded each day. CHAPTEK IX. BACTERIA IN BUTTER. 87. Sweet and sour cream butter. — The good qualities of butter are so dependent upon the relation of bacteria to cream, that in order to understand the subject aright, it is necessary to consider, first, their effect in cream. If butter is made from fresh sweet cream, it has a delicate, although unpronounced flavor that differs somewhat from the usual product. With the great bulk of the commercial product, the cream is allowed to stand for a certain length of time, during which it undergoes a series of fermentations technically known as "ripening." American consumers have become accustomed to the peculiar properties of sour cream butter, and as yet, there is but little demand for the sweet cream product. The keeping quality of the latter is relatively poor compared with that made from ripened cream, so that it is only adapted for immediate consumption. The germ content and the quality of ripened cream butter varies materiall}-, depending upon the way the cream is handled. In dairy butter making, the cream is usually gathered by the gravity process, and is ripened in various ways. In butter made by the creamery process, two methods of secur- Bacteria in Fluid Products of Milk. 137 ing cream are in vogue. The more primitive method is where the cream alone is gathered from the individual patron, who holds the same until it is nearly ready for churning. The more modern method is where the whole milk is brought to the creamery, and the cream taken from the same in a relatively fresh condition. In the one case, the fermentative changes are well underway when it reaches the central station, and as the cream is secured from a num- ber of different persons in the ripened condition, it is inevitably handled in a great variety of ways. Where cen- trifugal cream separators are used, the cream is secured «weet, thereby permitting a supervision of the ripening process at the central station where the butter is made. This control of the ripening process exerts a profound influence on the character of the fermentation that takes place, and naturally modifies the product. The uniformity -of methods employed necessarily has a tendency to render the product more uniform. 88. Ripening of cream for sour cream butter. — If fresh sweet cream is allowed to stand for a day or two at ordinary temperatures, a marked change is to be noted in its condition. As it increases in age, there is a noticeable ■development of acid; and with this acid, a peculiar aromatic odor and flavor of a mild, pleasant type is produced that is very characteristic. While the acid and flavor are produced simultaneously, still they are not necessarilj^ associated. ■Conn* has recently shown that good flavors and fine aromas may be had from organisms that are unable to form lactic :acid. Butter possessing this delicate flavor commands a * Conn, Cent. f. Bakt., II Abt., S 138 Dairy Bacteriology. higher price in the market, so that this condition is eagerly sought after by all first class butter makers. The source from which this delicate property is derived is not definitely known. However, this much is certain, that the production of the flavor depends upon the activity of the bacteria that are in the cream, and, that if these are excluded, the flavor is lacking. This change has been ex- plained in two different ways. Storch* holds that the flavors are produced from the decomposition of milk sugar, and the absorption of the volatile flavors of the butter fat. Conn t believes that the nitrogenous elements in the cream function as food materials from which are formed various decomposition products, among which is the desired aro- matic substance. The change is unquestionably a complex one, and cannot be explained as a single fermentation. If the steps of the ripening process, as it is ordinarily carried on, are carefully noted, it will be observed that the con- ditions under which the cream is kept during this change are those that favor the development of germ life. As at present conducted, this fermentation is somewhat a chance matter. Experience has taught that cream ripened under certain conditions produces first class butter, but the cause of the change is entirely ignored. Ordinarily the product is good, as scrupulous care and attention to details usually succeeds in producing a satisfactory product. Very often, however, the butter is of an inferior quality, and the reason of this deterioration, a m3-stery. With our present knowledge concerning the fermentations that occur in milk, and which are subsequently transferred to the cream, the causas that often produce this inferior * Storeh. Nogle Unders. over Floed. Syrning, 1890. t CoDD, Kept. Storrs (Conn.) Stat., 1893, p. 66. Bacteria in Fluid Products of Milk. \ 39 grade ought to be apparent. Where no control is exercised over the kind of fermentation that goes on in the raw ma- terial, it is a wonder that the product made under such con- ditions is as good as it is. As no especial attention is paid to the manner in which milk is secured and kept, many dif- ferent kinds of bacteria inevitably gain access to the milk and set up their fermentation changes. These processes are going on simultaneously, and it is mere chance whether those producing favorable products are in the ascendency or not. Most of the bacteria found in ripening cream do not seem to have any material effect on the butter. A limited number of species have the property of producing decomposition products that are either positively desirable or undesir- able. Fortunately, the objectionable species are relatively rare, if the cream is handled with any special degree of care, so that as a rule, the quality of the butter is not un- savory, although it may be wanting in that -'high, quick" flavor that commands an extra price in the market. How- ever, even in the best of creameries and dairies, the product is far from being uniform in quality, and much loss is occa- sioned by this variability of the output in value. 89. Methods of ripening cream. — Several methods of ripening cream are in vogue in different sections of the countrJ^ 1. Natural ripening. The simplest and oldest method is- where the cream is allowed to ripen without any special con- trol, the only artificial aid being a regulation in a crude way of the temperature. The whole process is a let-alone one. The ripened product varies much in degree of ripeness; also, in the kind of fermentation, depending upon the various 140 Dairy Bacteriology . species of bacteria tliat have iiappened to gain access to the milk. The results attained by these cruder methods are usually fair, but simply because the bulk of the organisms normally preseat in ordinarily clean milk are such as are unable to produce marked flavors of an undesirable character. By this method, the rate of ripening is often irregular und to overcome this, starters have been introduced as a means of hastening and rendering more uniform the ripen- ing process. 2. Addition of natural starters. By the use of these starters, dairymen have practically attempted, in a way, to select mixed bacterial growths to assist in the ripening of cream. In doing so they have advanced materially' from the standpoint of the previous method, and are also able to exclude in this way, undesirable fermentative changes. For starters of this sort, several different materials are used, all of which are liquids rich in bacteria. a. Buttermilk starter. For a starter a small quantit}' of fresh butter milk is added to sweet cream and kept at a tem- perature favorable for bacterial growth. The amount used as a starter does not exceed a small per cent, of the entire volume of cream to be ripened. In this way. the fresh cream- is seeded with a large number of organisms from a favorable ripening of an earlier date. b. Sour cream starter. Sour cream is sometimes used, but it is not considered as desirable as butter milk, because the fermentation in the old cream has usually passed be- j'ond the proper degree of acidity and is liable to be " off " in flavor. c. Skim m,ilk starter. In using skim milk, the milk is taken from the mixed skim milk of the factory, or what is Bacteria in Fluid Products of Milk. 141 much better, a single cow is chosen and the milk from this animal is handled with care; after the cream has been re- moved, the skimmed part is kept at a good ripening tem- perature so as to sour properly for use as a starter. In this way there is usually a less number of foreign bacteria in the milk than where the starter is derived from a mixed supply that is considerably older. It is very evident, that while the use of these natural starters is a step in the right direction, yet it does not over- come all possible difficulties; for, after all, any starter of this nature is usually a mixed one, and may possess unde- sirable organisms that effect noxious changes in the ripening cream. With the use of these natural starters any trouble would be perpetuated from day to day. In such instances, it is often necessary to introduce a new starter from some other factory that is known to be iree from such contamina- tion. But the whole process is at all times an empirical one, and while it is susceptible of certain modification that would make it very much more efficient, j-et it can be handled, and often is handled, in such a way that no benefit is derived from its use. With a bacteriological knowledge of these changes that go on in milk and cream under various conditions, it is possible, however, to use a starter of this sort so that it will be of much value for its purpose. If sterilized vessels are used in which to develop the starter; if the greatest of care is taken in the selection of a pure fluid that is known to be associated with a first class product; if this is ripened at a uniform temperature; and, if it is at all times handled so as to exclude any contaminating factor from the outside, then a starter of this sort maj' be perpetuated by continued re- inoculations and will be quite uniform in its action. 3. Addition of artificicd starters {bacterial cultures). A 142 Dairy Bacteriology. much more scientific method of ripening cream has been introduced within the last few j-ears and is now being ex- tensively used in Scandinavian and German dairies. It con- sists in the use of cultures of certain bacteria that have the ability of producing a desirable ripening change in cream. In 1890 Storch, the Danish scientist, took up this subject with reference to dairying, and in studying the effect of the different organisms present in cream, he succeeded in ob- taining a form that produced th"? proper ripening, and with it a delicate pleasant aroma. His investigation opened up a new field for research which has been diligently prosecuted by several Danish investigators, Weigmann in Germany, and Conn in this country. 90. Methods of using Ibaeterial cultures.— Theo- reticallj', this method is an advance over the use of natural starters, inasmuch, as the bacteria that are added to the cream are known to be of a kind capable of producing a proper flavor. By cultivating the different species of bac- teria in sterilized cream, the value of each for flavor produc- tion may be determined. If onlj- proper species are selected for seed, the course of fermentation is under control. Usually but a single species is added, although some of the cultures on the market are prepared by mixing several dis- tinct Jorms that have been selected on account of their de- sirable qualities. When first put on the market, these culture starters were sold in a liquid form in which state they did not retain their vitality for a sufQaieat length of time. Thej' were also quite easily contaminated if the contents of the bottle were not completely used. At present, most of the dealers send them out in a dry powder form, in which condition, the objections mentioned above do not obtain. Bacteria in Fluid Products of Milk. 143 1. In pasteurized cream. The u?e of a pure culture starter presupposes a previous treatment of the cream so as to de- prive it of Its pre-existing bacterial life. If, after this is accomplished, the proper liinds of bacteria are introduced, the desired fermentation will take place. The following de- tails are usuallj- recommended where a pure culture starter is used: First, the dry powder must be added to a small amount of milk that has first been pasteurized in order to develop an active growth from the dried material. Second, the cream to be ripened must be pasteurized so as to destroy the developing organisms already in the same, and so prepare the cream for the addition of the pure starter. Third, the addition of the developing starter to the pas- teurized cream, and the holding of the cream at such tem- perature as will induce the best development of flavor. Fourth, the propagation of the starter from day to day. A fresh lot of pasteurized milk should be inoculated daily with some of the pure starter of the previous daj"^, not the ripening cream containing the starter. In this way, the puritj' of the starter is maintained for a considerable length of time. 2. In unpasteurized cream. The majority' of pure culture starters are used in the above way, but in some instances they are* added directly to the sweet cream. When this is done, the full effect of the culture cannot be obtained, for the different species that are able to grow in the cream ex- ert to some extent an influence on the flavor. Such a method of procedure has been extensively advocated in this country, but the experience of dairymen in Denmark is op- posed to this method. 144 Dairy Bacteriology. 91. Pasteurizing cream for butter.— The best re- sults in the use of a pure culture starter are obtained where the cream is first pasteurized. To perform this process, it is not necessary that it should be carried out in the same stringent manner as it is when it is done to preserve milk, or to free it from possible disease-breeding bacteria. Many of the taint-forming organisms succumb at a relatively low temperature like that which destroys the sour milk bacteria, so that a heating of the milk for a short time accomplishes the desired end. The temperature that is used varies con- siderably, ranging from 140°-175° F. In many creameries in Europe the milk is pasteurized before separation. la this way the skim milk is pasteurized which is a necessary procedure where tuberculosis is so prevalent that the disease is often spread by means of this bye product. For this purpose, machines having a continuous flow are the most practicable as their capacity is much greater. 92. Essential requisites in pure culture starters. In studying the effect of these various pure cultures in cream ripening, it was found that two problems presented themselves for solution. Not only is it necessary that butter should have this delicate pleasant aroma, but it is quite as essential that the product should have a good keeping quality. If this is lacking, the value of the butter is much lessened, for it must be consumed immediately, and consequently, only a local market can be supplied with such a product. Weigmann * found that some species of bacteria pro- duced the requisite aroma, while others gave it a good keeping quality. It seems that these essential character- * Weigmann, Landw. Woch. f. Scbl. Hoi., 1^0.2 1890. Bacteria in Fluid Products of M!lk. 145 istics are not easily found in the same species; that where one is present, the other Is usually lacking. The desirable requisites in a starter of this sort are : 1. Rapid growth at a comparatively low temperature (60°-'75° F.) so as to ripen the cream quickly. 2. The production of " high flavor " and good aroma in the butter. 3. The production of butter with good keeping qualities. Quite a large number of organisms have been isolated that fulfill some of these conditions, but, as j-et, only a few have been found that meet all of these requisites in the highest degree, the chief difficulty being that the flavor is not strong enough to meet the demands of the American market. The researches of Conn* indicate that the proper- ties of flavor and aroma may be entirely independant of each other. As a rule the acid forms produce good flavor, although some forms not producing acid likewise yield good flavors. Aroma, on the other hand, appears to be formed by the decomposition of the albuminous material under the action of peptonizing organisms. The query is often made how is it with the numerous fermentations occurring in milk, that butter under natural conditions- should be as fine as it usually is? For, if the mechanical processes are properly carried out, the product is usually of fairly good quality. The reason is doubtless due in part, to the fact that butter fat does not readily absorb flavors. Again, most of the bacteria found in ripening cream exert little or no influence on the flavor or aroma of the butter. Under normal conditions, but few species are positively bad, many aflect it but indifferently, while those that enhance its quality are by no means numerous. * Conn, Cent. f. Bakt. 11 Abt. 2 : 415. 146 Dairy Bacteriology. 93. Advantages claimed for pure culture starters. 1. Flavor and aromi. These are the factors of greatest im- porfcance in butter, the market value of the product depend- ing more on them than any other condition. In the use of a bacterial culture for a starter, the attempt has been made to introduce organisms into the cream that would enhance these properties, the idea being that the action of a desirable ferment would produce these characteristios to a greater degree than where the fermentation was not con- trolled in any way. 2. Uniformity of ■product. Even the best butter-makers fail, now and then, to secure a product always up to the standard. This failure is largely conditioned upon the var3'ing germ content in milk, incident to changes in meth- ods of feeding, ways of handling, etc. With the use of a pure starter, these difficulties can be largely overcome, as the fermentative process must of necessity be the same un- der all circumstances. 3. Keeping quality of product. Butter made from pasteur- ized cream to which a good, pure starter has been added will keep much better than the ordinary product, because, if properly handled, the butter will not contain those organ- isms so commonly present in milk that produce the " oflf flavors " as the butter increases in age. 4. Elimination of butter defects. Incidentally-, the im- provement in flavor, etc., would have the effect of eliminat- ing troubles that are often a serious menace to successful butter-making. These difficulties are in part due to bac- teria that have the propertj- of forming undesirable aromatic substances that are imparted to the butter (98, etc.). Also, certain defects in butter not caused bj' the action of living organisms such as the presence of leek or wild onion odors Bacteria in Fluid Froducts of Milk. 147 may be materiall}' diminished by first, pasteurizing the cream, and then using a culture starter. Doubtless, the volatile substances that are the cause of these troubles are driven off by the heat employed in pasteurizing. 9t. Present status of pure culture starters. — Within the last few years, the use of pure culture starters has been greatly extended in Europe, particularly in Den- mark and north Germany. The acid starters (lactic fer- ments) are used most extensively, and usually the cream is previously pasteurized, except in farm dairies where the conditions of milking can be more rigidly controlled, and a better milk secured. The favor in which this system is held may be seen from the fact that in 1891 only 4 per cent, of the butter exhibited at the Danish butter exhibitions was made from pasteurized cream with a culture starter. In 1895, the percentage had increased to 86 per cent.,* and almost universallj-, the prizes are conferred upon samples made by the more modern method. It must be borne in mind, however, that most of the foreign markets to which the bulk of the Danish butter is sent demand a relatively mild flavor — much milder than is desired in our American markets. For this reason the same degree of success cannot be ex- pected if these foreign cultures are applied under the con- ditions as they exist here in this countr}'. In this country, the use of pure cultures in pasteurized cream has not as yet met with general favor, although in unpasteurized cream they have been quite extensively applied. It is a question whether the flavor of butter made * Friia, Bull. Danish Expt. Station, 1896. 148 Dairy Bacteriology. from properh' handled cream, in which, doubtless, a number of different organisms participate, can be intensified or not. The evidence at present is conflicting, but it is not improb- able that further search may reveal organisms endowed with higher flavor-producing qualities than any that have yet been tried. 95. Bacteria in butter. — As ripened cream is nec- essarily rich in bacteria, it follows that butter will also contain germ life in varying amounts. But as the butter fat is not well adapted for bacterial food, the number of germs in butter is usually less than in ripened cream. Sweet cream butter is naturally poorer in germ life than that made from ripened cream. G-rotenfelt* reports in sweet cream butter, so-called " Paris butter " only 120-300 bacteria per cc, while in sour cream a larger number was found, viz., 2,000-55,000 germs per cc. Pammelt found from 125,000-730,000 per gram, while LafarJ found in butter sold in Munich from 10-20,000,000 organisms per gram. The germ content of butter on the outside of a package is much greater than it is in the middle of a mass; this doubtless being due to the freer access of air favoring the growih of aerobic forms. 96. Clianses in germ content of butter. — The bacteria that are incorporated with the butter as it flrst " comes," undergo a slight increase for the flrst few days. The duration of this period of increase is dependent largely upon the condition of the butter. If the buttermilk is well worked out of the butter, the increase is slight and lasts for * (Jrotenfelt-Woll, Prin. Mod. Dairy Praetics, p. 244, t Pamm.el, Bull. 21, Iowa Expt. Stat., p. 801. X Lafar, Arch. f. Hyg., IS: 1, 1891. Bacteria in Fluid Products of Milk. 149 a few days only, while the presence of so nutritious a medium as buttermilk affords conditions much more favor- able for the continued growth of the organisms. While there may be many varieties in butter when it is fresh, they are very soon reduced in kind as well as number. The lactic acid group of organisms disappear quite rapidly; the spore bearing species remaining for a somewhat longer time. Butter examined after it is several months old is often found to be almost free from germs. This fact is important, for in considering the after changes in " stored " butter, the relation of these changes to the germ life would naturally be considered (97). In the manufacture of butter there is much that is dependent up:)n the mechanical processes of churning, washing, salting, and working the product. These processes do not involve any bacteriological principles other than those that are incident to cleanliness. The cream, if ripened properly, will contain such enormous numbers of favorable forms that the access of the few organisms that are derived from the churn, the air, or the water in washing will have little effect, unless the conditions are abnormal. 97. Rancid change in butter. — Fresh butter has a peculiar aroma that is very desirable 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 the peculiar aroma that it possesses when first made, yet a gilt-edged product should retain its good keep- ing qualities for some length of time. All butters, however, sooner or later undergo a change that render them worth- less for table use. This change is usually a rancidity that 150 Dairy Bacteriology. is observed in all stale products of this class. The cause of this rancid condition in butter has been attributed to the action of living organisms, particularly, those that form butyric acid, to the influence of light, of air, etc. In rancid butter, butyric and allied acids are always found, and it was supposed for a long time that the change was a butj'ric fermentation inaugurated by some of the butyric organisms that are found so commonly in milk and cream. Ritsert* found that sterile butter became rancid in three days if exposed to the action of the light, while unsterilized, normal butter exposed under similar conditions did not change in five months, if the air was excluded from it. Duclauxf has proven that the rancid change is largely a chemical action that takes place in butter fat where it is exposed to light and oxygen; that it is not necessarily inaugurated by the vital functions of any special kind of bacteria. While the change goes on in most cases in a purely chemical way, there are, however, certain organisms that are able to hasten this process if they are present in the butter, t For this reason a soft butter containing con- siderable quantities of buttermilk, and therefore rich in nitrogenous material, undergoes a rapid change and quickly becomes rancid. 98. Defects due to abnormal bacterial fermen- tations. — Besides the rancid change that is so common in butter there are various other defects that occur from time to time. In the majority of instances, these faults are * Ritsert, Inaug. Diss., Berne, 1890. f.Duclaux, Le lait. p. 34. X Von Klecki, Cent. f. Bakt., 15 : 354, 1891; also Sigismund, Arch. An. Kahr., 6 : 42, 1891. Bacteria in Fluid Products of Blilk. 151 purely mechanical, but in some cases the first cause of the trouble can be traced either directly or indirectly to the action ol some undesirable bacteria. Often, the "off" flavor in the butter is due to some abnormal fermentation that has been going on in the cream or even in the milk before the cream was separated. The conditions under which butter is made are subject to such fluctuations in ripening as to temperature, kind, and degree of ripening, that it is almost impossible to make the product uniform. These variations have a marked eflect on the organisms concerned in the process, and when the ripening is left to chance, it is surprising that noxious flavors do not develop more frequently in the cream and be transmitted directly to the butter. The great majority of these "off" flavors are not sufficiently marked to have a verj' decided character, j'ec they are easily detected by the taste, and at once diminish the market value of the product very materially. There are besides these indefinite troubles that may be perpetuated in the butter from the milk or cream, or may originally appear in the butter after it is made, several other serious defects that are more sharply characterized by the production of some peculiar propertJ^ Some of these defects or " diseases " in butter have been traced to a biological cause. i)9. Lack of flavor. — One of the most common troubles is where there is lack of flavor in the product. Often this may be due to improper handling of the cream in not allow- ing it to ripen far enough, but sometimes it is impossible to produce a high flavor. The lack of flavor in this case, is due to the absence of the proper flavor-producing organ- isms. This condition can usually be overcome by the 152 Dairy Bacteriology. addition of a proper starter (89)- Tiie relation between flavor and desirable bacteria is very intimate, and troubles of this kind usually arise, because the proper forms commonly lound in the cream have been supplanted by other species that do not possess the ability of forming these aromatic substances so necessary in sour cream butter. 100. Putrid Butter. — This specific butter trouble lias been observed in Denmark, where it has been thoroughly studied by Jensen.* Butter affected by it rapidly acquires a peculiar putrid odor that ruins it for table use. Some- times, this flavor may be developed in the cream previous to churning. Jensen found the trouble to be due to several ■different putrefactive bacteria. One form which he called Bacillus fcetidus lactis, a close ally of the common feces "bacillus produced this rotten odor and taste in milk in a very short time. Fortunately, this organism was easily killed by a comparatively low heat, so that pasteurization of the cream and use of a culture starter (89, 3) quickly elimi- nated the trouble, where it was tried. 101. Turnip-flaTored butter. — Butter sometimes acquires a peculiar flavor recalling the odor of turnips, rutabages, and other root crops. Often this trouble is due to feeding, there being in several of these crops, aromatic substances that pass directly into the milk (55); but in some instances the trouble arises from bacteria that are able to produce decomposition products, the odor and taste of which strongly recalls these vegetables. 103. " Cowy " odor in butter.— Often there is to be noted in milk a peculiar odor that resembles that of the -cow stable and sometimes the odor of the animal itself. * Jensen, Cent. f. Bakt., 11 : 409, 1891. Bacteria in Fluid Products of Milk. 153 Usually this defect in milk has been ascribed to the absorp- tion of impure gases bj^ the milk as it cools, although the gases and odors naturally present in fresh milk have this peculiar property that is demonstrable by certain methods of aeration. Occasionally, it is transmitted to butter, and recently, Pammel* has isolated from butter, a bacillus that produced in milk the same peculiar odor so commonly pre- sent in stables. 103. Lardy and tallowy butter.— The presence of this unpleasant taste in butter may be due to a variety of ■causes. In some instances, improper food seems to be the source of the trouble; then again, butter exposed to direct sunlight bleaches in color and develops a lardy flavor.t In addition to these, cases have been found in which the defect has been traced to the action of bacteria. Storch t has ■described a lactic acid form in a sample of tallowy butter that was able to produce this disagreeble odor. 1 04-. Oily butter. — Jensen has isolated one of the causes ■of the dreaded oily butter that is reported quite frequently in Denmark. The specific organism that he found belongs to the sour milk bacteria. In twentj'-four hours it curdles milk, the curd being solid like that of ordinary sour milk. There is produced, however, in addition to this, an unpleas- ant odor and taste resembling that of machine oil, a pecu- liarity that is transmitted directly to butter made from affected cream. 105. Bitter butter. — Now and then butter develops a bitter taste that may be due to a variety of different bac- » Pammel, Bull. 21, Iowa Expt. Stat., p. 803. t Fischer, Hyg. Eund, 6 ; 573. J Storch, 18th Kept. Danish Agric. Expt. Stat., 1890. lO 154 Dairy Bacteriology. terial forms. In most cases, the bitter flavor in the butter is derived primarily from the bacteria present in the cream or milk. Several of the fermentations of this character in milk (49) are also to be found in butter. In addition to these defects produced by a biological cause, bitter flavors in butter are sometimes produced by the milk being impreg- nated with volatile, bitter substances derived from weeds, etc. CHAPTER IV. BACTERIA IN CHEESE HAKINQ AND IN NORMAL CUR= INQ OF CHEESE. 106. Necessity of bacteria in cheese making. — If bacteria are desirable in butter making, they are an absolute necessity in the manufacture of cheese, for without the intervention of microbes to perform certain chemical changes, the casein would remain unchanged, and cheese would have but little nutritive value. Cheese making, like all other industries, especially those pertaining to the dairy, is a vocation based upon experience, and, while a large num- ber of methods for difierent products have been more or less perfected, all of them have been evolved in an empirical manner. The why and the wherefore of many of the operations that are traditional in cheese making are even now just beginning to be understood. The application of bacteriological ideas to this branch of dairy industry is timely and seems to promise more than ordinary results. As yet, this relation is very imperfectly understood. A few investigators here and there in Europe, have shown the bearing of this new biological science to this subject, but the full details are by no means elucidated. The subject Bacteria in Cheese Making. 155 as applied to our American methods of manufacture has been investigated even less, so that as yet, it is only a field of promise. Nevertheless in spite of the lack of exact knowledge, the application of bacteriological facts in a general way will be pertinent under this head. 107. Different methods of cheese making.— From the same kind of milk, a great variety of different cheeses can be prepared, depending upon the treatment of the milk during the manufacture, and the way in which the cheese is handled after it is made. Cheese is made by precipitating the casein of the milk in either of two ways, the fat in either instance being caught in the coagulated casein. 1. By adding rennet to the milk, which has the power of coagulating the casein as in cheddar cheese. 2. By allowing acid to develop in the milk, until the casein is precipitated, as in sour milk or cottage cheese. Cheese made by the addition of rennet is divided into two general classes, soft and hard, depending upon the treatment it receives daring the manufacturing process. This difference in texture is produced largely by the way in which the curd is handled. There is also a marked differ- ence in the way in which the different kinds of cheese are cured. 108. Development of bacteria in making cheese. Cheese as manufactured in this country is made almost entirely in large factories, where the milk of many patrons is collected daily for the purpose. Under these conditions, it is usually well seeded with bacterial germs as some of the milk is from twelve to fifteen hours old. As a rule, the 156 Dairy Bacteriology. sour milk bacteria predominate so that the milk ripens rapid- ly, a Gondition to be desired in the proper manufacture of Cheddar cheese. This ripening of the milk is caused by the production of lactic acid from the milk sugar, under the influence of certain kinds of bacteria (45). The proper •degree of acidity is usually determined by means of a ren- net test. In this test, the relation of the acid to the curd- ling power of rennet is shown. Milk maj' be " oflf " in flavor where the normal milk :Organisms are supplanted by other forms of a less desirable character (125); or it may be simply "slow" where the growth of lactic acid bacteria and the consequent develop- ment of acid has been checked by low temperature. JEither of the above conditions may be remedied, wholly or in part, by the addition of proper starters. With manj- of the gas producing bacteria and the abnor- mal alkaline ferments, the acid in the milk does not develop, even though it is held for a considerable length of time. In these cases, as well as where the milk is too sweet through lack of development of bacterial life, the addition of a proper lactic acid starter will hasten the production of acid and put the milk into better condition for typical cheddar cheese making (116). 109. Bacteria in rennet.— The addition of rennet to milk has no appreciable effect on the development of the'bacteria in the same. 'The idea has been advanced that the bacteria in the rennet extract exert a considerable iafluence on the cheese; this view is however, hardly tenable, in spite of the fact that the rennet usually contains large numbers of organisms, for the relative amount of rennet that is added is so small that this does not perceptibly in- crease the germ content of the milk. Bacteria in Cheese Malcing. 157 It is possible that undesirable bacteria maj' be introduced into the milk by means of the rennet where the same is contaminated with noxious forms; but these cases are relatively, rare. The way in which the curds are handled from the time they are cut until they are ready for the press has a pro- found effect on the character of the cheese. A "high scald" tends to shrink the curds, thus expelling the water with the result that the cheese is hard and dry. A development of considerable acid as determined by the hot iron test also has a similar physical effect on the cheese. Such cheese ripen less rapidly than where more moisture is incorporated with the curd. 110. Green cheese. — If the proper amoant of acid has been allowed to develop, a cheese fresh from the press will present a close uniform texture, that in the "green" con- dition is tough and elastic. This rapidly changes in a cheese as it ripens and with this change there is a marked increase in the amount of bacterial life (114). Where insufficient acid is developed in the milk or in the curds before they are put to press, the cheese usually has an open porous texture. The numerous cavities that aie to be noted arise from the fact that the curd particles have not been closely matted together. This matting or cementing of the curd masses is due to their partial solution under the liquefying influence of the acid produced by the bacteria. The acid is by no means the only substance capable of pro- ducing this essential change. Any chemical, either acid or alkaline in its nature that has a solvent action on casein will bring about the same result. Where these "mechanical holes" are present ^.s in a sweet curd cheese, gas is almost 158 Dairy Bacteriology. invariably developed, as the gas producing bacteria that are quite susceptible to acid are not inhibited under these con- ditions. This gas finds its way into the ready made mechanical holes, greatly distending them as in Fig. 24. Fig. 24. Showing " mechanical holes " in a sweet curd cheese and the distention of the same by gas. L shows appearance of a sweet curd cheese immediately after it is taken from the press. The "mechanical holes" are produced by the failure of the curd particles to cement owing to the absence of acid. P shows appearance of a duplicate cheese four days after re- moval from press. The gas produced by bacterial germs fills up and distends these "mechanical holes," thus causing a marked distortion in shape of cheese (huflSng). 111. Physical changes in ripening.— When a green cheese is taken from the press, the curd is tough, firm, but elastic. It has no value as a food product for immediate use, because it lacks a desirable flavor and is not readily digestible. It is nothing but unchanged casein and fat. In the course of time, a deep seated change occurs. Physi- Bacteria in Cheese Making. 159 cally this change is demonstrated in the modiflcation that the curd undergoes. Gradually it breaks down and becomes plastic, the elastic, tough material being changed into a softened mass. This change in texture of the cheese is also accompanied by a marked change in flavor. The green cheese has no distinctively cheese flavor, but in course of time, with the gradual change of texture, the peculiar flavor incident to ripe cheese is developed. The characteristic texture and flavor are susceptible of considerable modification that is induced not only by varia- tion in methods of manufacture, but by the conditions under which the cheese are cured. The amount of moist- ure incorporated with the curd materially aflfects the physical appearance of the cheese, and the rate of change in the same. The temperature at which the same is ripened, likewise, the moisture content of the surrounding air, also «xert a marked influence on the ripening product. To some extent the action of these forces is purely physical, as in the gradual loss by drying, but they also aflect the course of the biological changes to a certain extent. 112. Chemical changes in ripening. — The marked physical change that occurs during ripening also records a profound chemical action in the composition of cheese. Coincident with the softening of the curd, comes a change in the condition of the casein. The hitherto insoluble casein is graduall}- transformed into soluble substances. This chemical phenomonon is a breaking down process that is analogous to the peptonization of proteids, although in addition to peptones, numerous other intermediate products are likewise formed. According to Duclaux,* bacteria pro- * Duclaux, Le Lait, p. 113. 160 Dairy Bacteriology. duce these changes by tbe ferments that they secrete. The ferment causing this change, he calls casease, the casein after it is changed, caseone. This gradual conversion of the insoluble nitrogen into n soluble form marks the change in its digestibility. _ The milk sugar originally present in the milk and incorporated in the curd rapidly disappears, this- change occuring with greater rapidity in a typical cheddar than in a sweet curd cheese. The fat and the ash consti- tuents in the curd suffer but little change. It should be borne in mind that the changes that make the cheese valuable as a food product are mainly dependent upon the modification that the proteid nitrogen undergoes. The importance of this should be borne in mind in discus- sing the relation of living organisms to the curing process. 113. Relation of bacteria to curing changes. — That bacteria are concerned in these ripening changes is now an admitted fact. Other factors, undoubtedly, modify the course of the process, but the influence of living organisms, either directly or indirectly, is a potent cause in determin- ing the progress of the change involved. If bacteria are excluded by the action of physical or chemical agents, a cessation of the curing changes occurs immediately. Adametz * added to green cheese, various disinfectants as. creolin, thymol, etc., and as a rule the ripening of the same did not progress. Cheese made from sterilized, or even from pasteurized milk in which not all of the living organisms are destroyed, fail to ripen naturally, although at times, some of them break down with a sharp bitter flavor. In soft cheese the curing changes occur much more rapidly than in the hard, flrm cheese. In these the molds. s Adametz, Landw. Jahrb., 18 ; 228. Bacteria in Cheese Making. 161 and other fungi, as well as bacteria, play an important role, and the curing process extends slowly from the rind to the center. The changes in the firm cheese are much slower in their action, so that the hard type of cheese often require several months before they are fit for use. With this kind of cheese the bacteria are the principal agents in the curing process. When different varieties of cheese are made from milk in the same locality, the germ content of even the ripened product has a - marked similarity as is illustrated by Adametz's work* on Emmenthaler, a hard cheese, and Schweitzer Hauskase, a soft variety. Of the nine species of bacilli and cocci found in mature Emmenthaler, eight of them were also present in ripened Hauskase. Whether this breaking down or peptonizing process is due to the influence of a single organism or not, is not yet definitely settled, but from our present knowledge, it is more than likely that it is to be ascribed to the action of the lactic acid organisms as a class and not to anj' one special form. In the hard, firm cheese, the change goes on throughout the whole mass simultaneously, and has no reference to the free access of oxygen. Whether the properties of both flavor production and peptonization of the casein are present in the same organ- ism or in different organisms has not yet been determined. It seems highly probable from Preudenreich'st experiments that they may be caused by two different forms, because he found that if the air was excluded, the cheese broke down but the flavor was bitter and abnormal. The authorj has * Adametz, Loc. cit. t Freudenreieh, Landw. Jahr. d. Schw., : 62, 1892. J Jlussell, 13th Wis. Expt. Stat., p. 110. 162 Dairy Bacteriology. obtained practically the same result in the addition of certain bacteria to pasteurized milk. In this case the casein was completely broken down, but the cheese lacked in flavor, while those made from pasteurized milk without any starter failed to ripen. The addition of undesirable bacteria to milk has a marked effect on the flavor of the cheese. Milk so handled is practically the same as tainted milk. The suggestion has been made,t and it seems very likely indeed, that the good quality of cheese is sometimes dependent more upon the character of the germ life than it is upon the kind of feed to which the cows have access. Our domestic Swiss cheese, or even cheese of this class made in Germany, rarely have the peculiar flavor that is found in the product imported from the Swiss valleys. For centuries this brand of cheese has been made in that country, until the factories and dairies have become stocked with the right kind of germs, capable of producing the desired fermentation. These germs are so prevalent that the milk is infected with them under ordinary conditions; consequently, the ripening changes bring about the produc- tion oi the peculiar flavor that is so highly prized. Groten- felt* stales that in his studies on the Swiss tj-pe of cheese made in Finland, he found but very few of the species that Adametz described as occuring normally in the same kind of cheese made in Switzerland. The testimony of Wisconsin makers of Swiss cheese is that the flavor of the Swiss product is very much better than it used to be when the industry was first started. Possibly the dairies con- tributing the milk are permeated with the desirable organ- isms to a larger extent than formerlj'. The question is, * Grotenfelt, Prin. Mod. Dairy Prac, p. 269. f Baumann, Landw. Vers. Stat,, 4*2 : 214. Bacteria in Cheese Making. 163 however, not 3'et settled, and extensive investigations will have to be made both from the bacteriological and the feed- ing standpoint before definite conclusions can be drawn. 114. Bacterial flora of cheddar cheese. — In study- ing the bacterial flora of cheddar cheese at different stages of the curing process, a peculiar change is found compared with the typical conditions usuallj'' seen in milk. The changes in germ content may be grouped as follows : First, there is a marked decrease in numbers, lasting for a day or so. This is then followed by an enormous numeri- cal increase that is entirely produced by the marked development of the lactic acid forms. Synchronous with this increase, the peptonizing and gas producing bacteria gradually disappear. The physical symptons of ripening begin to manifest themselves about this stage and continue to increase in intensity until the texture of the cheese is entirely changed. This rapid development of lactic acid organisms is finally followed by a gradual decline that con- tinues, in an old cheese until the number of bacteria is reduced to a mininum. The conditions that bring about this series of changes is not yet well understood. But so characteristic is it of the bacterial development in the cheese, that it would seem to bear an important relation to the ripening process, with which it is more or less coincident.* This condition has been found to exist not only with cheddar cheese here in this country but in Canada t and England.! FreudenreichI has also found the same series of changes in the Emmenthaler (Swiss) cheese. * KuBsell. 13th Eept. Wis. Expt. Stat., p. 95. t Unpublished data from Prof. F. C. Harrison, Guelph, Canada. X Lloyd, Bath & West of England Reports, 1894. ^ Freudenreich, Landw. Jahib. d. Schweiz, 4 ; 17; 5 : 16. 164 Dairy Bacteriology. The overwhelming development of the lactic acid type of. organisms and the rapid disappearance of other phj'siologi- cal classes are the predominant points to be noted in the bacterial flora of the firm type of cheeses. 115. Explanation of curing changes. — Just how to explain the cause of the ripening changes in cheese has long been a vexed question in this branch of dairying. As noted above (112) the changes observed are chemical as well as physical, but the physical appearances are likewise explain- able on the basis of the chemical transformations. Naturally enough, a purely chemical explanation was satisfactory and ■was accepted for a long time. With further study, it became evident that micro-organ- isms bore a very intimate relation to the changes produced, and the labors of Duclaux showed the causal relationship existing between bacteria and the changes noted in cheese. Thus the biological theory of cheese ripening came to be established. The principles of this explanation are now so thoroughly proven that there is no doubt as to general truth of the statement that the ripening of cheese is dependent (directly or indirectly) upon the life processes of bacteria. As to the exact nature of the organisms concerned in the process, no settled views can yet be enunciated. The theory that has received the widest acceptance is what maj' be called the. peptonizing theory. 1. Peptonizing theory. This explanation is based uport the action of certain bacteria present in the milk that have the power of dissolving the casein, producing as a result of this decomposition, peptone-like substances. It is known that a similar change takes place in the cheese — that the Bacteria in Cheese Making. 165 casein is gradually changed into soluble form, but the proof that the organisms in the milk capable of producing this change also operate in the cheese has been surmised rather than demonstrated. If these digesting organisms function as casein transformers, we should naturally expect to find them frequently, at least, if not predominating in the ripen- ing cheese, but such is not the case. In tj'pical cheddar cheese they rapidly disappear (114) although in the moister, softer varieties, they persist for considerable time. Coincident with this gradual disappearance is the rapid rise in numbers of the lactic acid bacteria. This marked development occurs synchionouslj' with the changes that are incident to ripening, and has led to the view that to this type of bacterial life, this series of changes is to be ascribed. 2. Lactic acid hypothesis.. One is hardly warranted in asserting that this is a theory, for it has not j-et been sufficiently proven that this type of bacteria are causally connected with the ripening changes. Considerable evi- dence, however, is accumulating, and while the proof cannot be regarded as conclusive, yet it certainly has less objection to it than the preceding explanation. The following facts are to be noted in this connection. The very rapid increase in lactic acid bacteria that is coincident in point of time with the ripening changes in the cheese. Cheese made from pasteurized milk fail to ripen nor- mally, but if a pure lactic ferment is used as a starter the course of the ripening goes on in the usual manner. How the lactic acid bacteria are able to function as casein peptonizers is not yet proved, but the evidence is very strong that such is really their function, although the sub- ject has not been sufficiently investigated to enable a satis- factory explanation to be made. 166 Dairy Bacteriology. 116. Use of starters for cheese-making.— Inas- much as flavor and texture of cheese are to a great extent modified by bacterial action, the question naturally arises as to the effect of starters that have been selected for their favorable qualities. In common practice, a small quantity of buttermilk or sour milk is often added to sweet milk to hasten the ripening process. This is merely adding a bac- terial culture like the addition of ordinary starters in butter- making (89). The starter is usually selected by taste and smell, and the purity of its contained germ life is only con- jectured. If its germ character could be safely determined, a starter of this sort would doubtless be as efficient as any. Culture starters. Some attempts have been made to use pure cultures of selected bacteria for starters but the inher- ent difficulties of this method are many. They could be added to unpasteurized milk without any trouble, but the best effects of a favorable starter are to be seen in a medium relatively poor in germ life. When milk is pasteurized, its. relation to rennet is materially changed. Instead of mat- ting with a close meaty texture, the curds are of a mushy consistency. Owing to the destruction of the sour milk bac- teria, lactic acid is not formed and as a result the curds do not " string" on the hot iron or mat together on the racks. The use of these selected starters in unpasteurized milk will often prove to be very helpful, especially if the starter selected is a pure lactic ferment, — one that is well adapted for rapid development, not only in the ripening milk but in the curing cheese. From extended experiments made at the Wisconsin Dairy School, the author* has found that a pure culture of the * 13th Kept. Wis. Expt. Stat., p. 108, 1896. Bacteria in Cheese Making. 167 predominating lactic acid organism found in ripening cheese is a valuable aid in cheese-making in several ways. The time required to make cheese is naturally curtailed by the use of an acid starter, as it is not necessary to wait so long lor the development of the acid. Cheese made with such a starter are more uniform in quality and are some- what better in flavor, although this difference is but slight wher^ the milk is in good condition. In the summer months, where factory men are bothered with " gassy milks," the use of such a starter is of great aid in repressing these gaseous fermentations. 117. Normal ripening of some foreign brands of cheese. — Although most cheese made in America are Cheddars, still the amount of foreign kinds that are pro- duced is constantly increasing, especially with such varie- ties as Emmenthaler (Swiss), brick, and limburger. Many of the foreign cheeses are soft and difier much in their ripen- ing characteristics from the hard type. While the firm hard tj-pe of cheese made with rennet are ripened almost solely by the action of bacteria, most of the soft varieties are dependent upon a fungi to assist in the ripening process. In some cases this fungous growth gains access to the cheese in purely a chance manner; then again, the cheese is seeded with the requisite organisms to produce the de- sired fermentation. 118. Swiss or Emmenthaler cheese. — Swiss is the most common European representative of the hard type of rennet-made cheese, and differs mainly from the cheddar type in that it is made from a sweeter curd, i. e., a curd hav- ing but little acid. When green it always contains a certain amount of sugar and from the fermentation of this material, ] 68 Dairy Bacteriology. gas is formed which results in the production of the Swiss holes or " ej-es " in the cheese. The formation of these eyes is considered a part of the normal ripening process. Von Freudenreich,* Weigmann t and Baumannt have isolated organisms that are concerned in the formation of these gas holes. They showed that if the gas-producing germ is pre- sent in the milk in abnormal quantities, a too vigorous fer- mentation may take place, resulting in the production of a spongy structure (125). Adametz? found in Emmenthaler cheese from six to eight different species of bacteria present. A marked numerical increase in bacteria took place (90,000 per gram in green cheese to 850,000 in ripe), as the cheese increases in age. This multiplication is mainlj' in lactic acid forms (117). 119. Roquefort cheese. — In Eoquefort cheese, a French cheese made from goat's milk, the essential agent in the curing change is the ordinary blue-green bread mold, Peaicillium glaucum. This organism is cultivated first in a most careful manner on specially prepared bread. It is then dried, pulverized, and when the curds are put in the molds, it is sprinkled over the surface of them, thus mix- ing the fungus organism thoroughly with the fresh curd. A.% ordinary temperatures this organism develops too rapidly, so that the cheese to ripen properly must be kept sX a low temperature. The town of Roquefort is situated in a limestone country, in a region full of caves, and it is in these natural caves that most of the ripening is done. These caverns are always very moist and have a tempera- * Freudenreich, Ann. d. Microg.. 2 : 353. t Weigmann, Landw. Wooh. f. Schl. Hoi., 1890, p. i X Bauiaann, Landw. Vers. Stat., 42 : 181. g Adametz, Landw. Jahrb., 18 : 227. Bacteria in Cheese Making. 169 ture ranging from 35° to 44° F., so that the growth of the fungus is retarded considerably. The rapidity of the ripen- ing varies somewhat. In summer it is more rapid than in winter, but the flavor is somewhat impaired by this rapid curing. The spread of the mold throughout the ripening mass is also assisted in a mechanical way. The partially matured cheese are run through a machine that pricks them full of small holes. These slender canals allow the mold organism to penetrate the whole mass more thoroughly; the moldy straw matting upon which the ripening cheese are placed helping to furnish an abundant seeding of the de- sired germ. 120. Stilton cheese. — Stilton cheese, a very rich cream cheese made in England, is another of these fancy cheeses that is ripened by the aid of a fungus growth. To- distribute the desired mold in the fresh cheese, plugs are taken from ripened Stiltons, and the cores are interchanged so it is in a sense an inoculation process. 121. Brie clieese. — Brie cheese is one of the most important of the soft cheeses that is made in Prance. It is made from half skimmed or whole milk. The ripening process is a peculiar one and the cheese must be handled with great care, otherwise undesirable fermenta- tions are sure to set in. There is no artificial seeding of the curd in the Brie cheese manufacture, the maker depend- ing upon the germ content of the ripening cellar for the proper organism at the proper time. So necessary is it that the ripening room should be impregnated with these fungi and bacteria that when new curing cellars are constructed, the maker washes the walls and floor of the new room with scrapings from an old factory. In this way, he unwittingly 11 170 Dairy Bacteriology. introduces the ferments necessary for the maturation of the green product. Brie cheese are ripened on mats and soon after they are placed in a relatively cool ripening cellar, the surface of the cheese becomes coated with a thick growth of the common mold, Penicillium glaucum. The cheese are turned often so as to render the growth of the mold as uniform as possible; the object being to cover the cheese ■wiih a thick white felting of the moldy growth, to diminish drying. The temperature must be kept at a low point, otherwise the mold will fruit, in which condition the white fungus turns to the blue green stage. The growth of this fungus merely prepares the soil for a more essential organism. As the cheese goes to the curing room it is very acid and consequently favors the growth of fungi. The mold organisms slowly change the reaction of the cheese until it is neutral. With this change in reaction, there appears in the course of a few weeks another coating beneath the blue felting. At first this is white, but it soon changes to a red color. This slimy coat below the mold layer is made up of diverse species of bacteria and fungi that are able to grow after the acid is consumed by the blue mold The red coating acts upon the casein, also producing an alkaline reaction that is unfavorable for the growth of the blue mold. Two sets of organisms are therefore, essential in the ripening process, one preparing the soil for the fer- ment that later produces the requisite ripening changes. The process as carried on is largely empirical and if the red coat does not develop at the proper time, the maker re- sorts to all sorts of devices to bring out the desired ferment. The appearance of the right form is dependent, however, upon the proper reaction of the cheese, and if this is not suitable, the wished-for growth will not appear. Bacteria in Cheese Making. 171 122. Neufchatel cheese. — Neufchatel is another of the highly prized French soft cheese that is ripened largely \>y means of a fungus. A white mold covers these little cheese in the course of a few days after they are pre- pared. As this develops, they are stored in a ripening cel- lar for two or three days when they become enveloped with a blue growth that is merely the fruiting stage of the white felted coat. In all of these soft cheeses where the ripening goes on under the influence of fungi, the change takes place from -the outside toward the center. There is always an outer layer that is changed to such an extent as to be unfit for food. In the inner part, the casein is transformed by the slow diflFusion of the ferment throughout the whole mass. 123. Susceptibility of clieese to serm diseases. — Cheese more than butter is subject to undesirable fermenta- tions incited by bacteria because it is so much better adapted for germ growth on account of its better and more available food supply. Then too, the method of curing is so often without control, especially with many of those methods that employ fungi as agents in the curing process. This inability to check the ripening at the proper moment often results in the decomposition processes being carried too far and in this way unpleasant flavors are developed.* 124. Indefinite faults in cheddar cheese. — Besides a number of more or less distinct fermentations of diverse character, ill-defined defects in flavor and odor are often to be noted. Cheese possessing these may be thoroughly ripened and therefore digestible, but their market value is * Adametz has* brought together in hia little book entitled. The causes of the abnormal ripening of cheese, a large amount of data that pertains especially to ^European conditions. 172 Dairy Bacteriology. much diminished by reason of these improper flavors. In many cases, troubles of this sort are undoubtedly traceable to fault}^ manufacture, but more often, the defects arise from the presence of some fermenting organism that is the cause of the unpleasant flavor. Our knowledge of the in- fluence that the difl'erent species have in the ripening changes of cheese is still too meager to enable us to trace in all cases, these undesirable conditions to their proper source. The great majority of the taints observed in the factory are due to the abnormal development of some of these forms capable of evolving unpleasant or even putrid odors. Most of them are seeded in the milk before it comes to the factory and are due to careless manipulation of the milk while it is still on the farm. Others gain access to the milk in the factory, owing to unclean conditions of one sort or another. Sometimes the cheesemaker is able to overcome these taints by vigorous treatment, but often they pass on into the cheese only to detract from the market value of the product. In studying the effect of the different organisms found in milk in their relation to cheesemaking, we have isolated a number of different species that are concerned in the pro- duction of these "off" flavors in the curd. If these organ- isms are seeded in pasteurized milk and the infected milk made into cheese, they develop odors that are often to be noted in factories troubled with taints. These indefinite "off"' flavors and odors are often insufficiently pronounced to be given a name, consequently, it will be impossible to- describe them under an}' well defined head. 125. "Gassy" fermentations in cheese. — One of the worst and at the same time most common troubles in Bacteria in Cheese MaMng. 173 cheese making is where the cheese undergoes a fermenta- tion marked by the evolution of gas. The presence of gas is recognized by the appearance either of spherical or lens shaped holes of various sizes in the green cheese; often they appear in the curd even before it is put to press. Usually in this condition, the curds look as if they had been finely punctured with a pin, and are known as "pin-hole" curds. Sometimes the gas holes are larger, even approach- ing the large round so-called Swiss holes. When the gas is abundant, the holes are more apt to be restricted in size. The formation of gas may continue to such an extent that the curd even floats on the surface of the whey before it is removed. These " floating curds " are permeated through and through with gas bubbles, giving the curd an appear- ance of a coarse sponge. If " gassy " curds are put to press in this condition, an abnormal change usually occurs within a few days. The fermentation goes on in the green cheese causing it to swell or "hufl"," until it may be considerably distorted. The gas diffuses with some difficulty in the new cheese so that the mass rapidlj' swells. The fermenta- tion may be so energetic as to actually cause the cheese to crack, owing to the pressure of the contained gas. In the severe types of this gaseous fermentation, the product is rendered worthless, but even where the development is not so marked, the flavor of the cheese is impaired and the market value diminished. The difficulty occurs at almost any season of the year, but the trouble is most frequently observed in the late summer months. So common are these difficulties that they may be regarded as the worst cheese troubles with which our cheese-makers have to contend. Cheese-makers have by continual practice learned how to handle milks that develop these abnormal gaseous changps, 174 Dairy Bacteriology. and b}- the rapid development of acid in the whey, to hold in check this disease. Salt also has a retarding influence on the development of gas in cheese. It not only helps to expel the moisture in the curds, thereby rendering them drier and less liable to rapid fermentation, but it also has an inhibitory effect on the gas producing bacteria materially retarding their rate of growth. -■I ■-■(.-' ■ 5 ■~^ r- Fig. 25. Showing effect of salt on texture of cheese. Upper row made with 0, 1%, and 3 lbs. of salt per 100 lbs. curd in Nos. 1, 2, and 3 respectively; lower row, 0, 2, and 3 lbs. of salt for same amount of curd. With gassy milks, it is verj' diflScult to make what is known as a sweet curd cheese. The open texture of such cheese (110) is almost sure to permit huffing. The devel- opment of acid renders the acid curd cheese firmer, and at the same time, lessens the possibility of copious gaseous fermentations. Bacteria in Cheese Making. 1*75 With some of these gas-producing germs we have found it impossible in many cases to develop acid rapidly, unless a copious starter of sour milk or some pure lactic acid germ was ddded(116). The cause of the difficulty has long been charged to vari- ous sources, such as lack of aeration, improper feeding, re- tention of animal gases, etc., but in all these cases, it was nothing more than a surmise. Verj' often the milk does not betray- any visible symptom of fermentation when re- ceived, and the trouble is not to be recognized until the process of cheese-making is well advanced. Kecent studies from a biological standpoint have, how- ever, thrown much light on this troublesome problem; and, with the cause of these fermentative changes more fully re- cognized, it is quite probable that improved methods of handling milk will be rapidly introduced that will enable the maker to exclude these troubles. 126. Relation to living germs.— The formation of gas, either in the curd or after it has been put to press, is due entirel}' to the breaking down of certain elements, such as the sugar of milk, under the influence of various living germs. This trouble is then a type fermentation, and is therefore, much more widely distributed than it would be if it was caused by a single specific organism. There are present in all milks, numerous bacterial forms that are capable of producing a varied series of fermentations in which different gases may be given off, either as chief pro- ducts or as bye products. Among these organisms are a large number of the bacteria, although yeasts and allied germs are often present in milk and are likewise able in some cases to set up fermentative changes of this sort. In these 176 Dairy Bacteriology. cases the milk sugar is decomposed in such a way as to give off CO, and H, and in some cases, alcohol. According to Guillebeau a close relation exists between those germs that are able to produce an infectious inflam- mation (mastitis) in the udder of the cow and some forms capable of gas evolution. Several epidemics of " gassy " milk have been traced directly to animals suffering from an acute inflammatory condition of the udder in which it has been shown that the organisms producing this disease were the direct cause of the gas production in the milk. Gas producing bacteria are so numerous that they are almost always present in any sample of milk, especially if it is a little old. Bolley and the writer isolated six different species from a single sample of summer milk. Even in winter we have found them present in milk in such numbers as to require special care in the manufacture of cheese. Under normal conditions, where care is taken in the hand- ling of the milk, they are not usually found in large num- bers, and when thus numerically restricted, are doubtless kept under subjection by the competition of numerous other bacteria in the milk. These fermentations are very often observed in foreign kinds of cheese, especially those that are made after the sweet-curd process. Adametz* has collated data on the European forms that have been isolated, and has found nineteen bacterial species (five cocci, fourteen bacilli) and eight varieties of yeast-like fungi that are gas-producing organisms. If pasteurized milk is seeded with a pure culture of any of these gas-forming organisms and made into cheese, an intense fermentation is always to be noted. This often * Adametz, Die Ursachen u. Erreg. d. abnorm. Reif. Vorg. b. Xaeae, p 27. Bacteria in Cheese Mahing. 177, appears in the curd, sometimes even before the whey is drawn. The cheese when taken from the press develops gas rapidly, causing it to swell and a cross section at it at this stage will show a spongy structure. If the cheese is left intact, the fermentation may progress to such an extent that the pressure of the contained gas will cause the rind to split open. This fermentation does not last for more than a few da3's; then the swelled cheese sinks as the gas slowly diflfuses throughout its mass. An analysis reveals the presence of carbonic acid and hydrogen gases. 127. Gaseous fermentations in Swiss cheese. — From the researches of v. Freudenreich, Baumann, and Weig- mann, it is known that certain gas-producing forms invari- ably play a role in the normal ripening of Emmenthaler cheese. Sometimes Swiss cheese is devoid of these gas holes, or " eyes," in which case, they are said to be " blind." Such cheese may ripen well but their market value is dimin- ished on account of the absence of these round holes. The size of the gas holes as thej'^ appear in abnormal cheese is by no means constant. Neither is the character of the hole dependent upon any specific germ. If the gas- forming bacteria are plenty and quite evenly distributed the holes will be small and numerous. This is the condition seen in " pin- hole '' curds; Swiss cheese of this sort is called Nissler cheese. Where the organisms are less frequent and develop in small groups, then the " eyes " are much enlarged. As the different organisms varj' in their intensity of gas for- mation, this too, modifies the size of the gas holes to a marked degree. 128. Detection of tainted milk. — Taints in milk are not easy to recognize unless they are quite sharply pro- nounced. 178 Dairy Bacteriology. The necessity of identifying them is, however, so great, that each lot of milk brought to the factory should be sub- mitted to the closest scrutiny; for the addition of a single can of bad milk to the general supply is often sufficient to spoil the whole mass. It is generally difficult to recognize the presence of taints in milk, especially when it is cold. If the milk is first warmed, the volatile odors are given off quite readily and any peculiarity is more easily noticed. 129. Fermentation tests for '•tainted" niillf. — Very often it happens that the milk is not so bad that the receiver is able to detect its true condition at the weigh can. In these instances a fermentation test should be applied. The test is based upon the fact that if milk is held at a moderately high temperature, approximating the blood heat (S'Z P°), the bacteria in it will develop very rapidly, producing such fermentative changes as they are capable of. As a rule the taint forming organisms and those that are generally undesirable, grow more rapidly at the blood heat then they do at a lower temperature. A fermentation test, therefore, brings the noxious bacteria to the fore and if they are present in any considerable degree in a milk, their presence will be detected by the application of this test. The usual way in which the test is applied is merely to place a small quantity of milk in a glass cylinder or jar. These samples are then incubated in a warm place and after a period of a few hours, the course of the fermentation is noted. As usually carried out, no special attempt is made to free the vessels used for the purpose from all kinds of bacteria. Often too, the samples are left uncovered so that there is an opportunity for infection of milk from without. Bacteria in Cheese Making. 179 Even these crude methods, however, have been found serviceable in the past, but with the application of a few bacteriological principles, the method could be greatly im- proved. The following suggestions bear upon this point : 1. See that the glass tubes are first sterilized by heat so that no organisms will be introduced other than those found in the milk. 2. To each tube after it is filled with milk add an equal quantity of rennet and note relative rapidity of curdling. Milks that curdle very rapidly or very slowly are abnor- mal. To determine this it is necessary to use an equal quantity of milk for each satnple, and also equal quantities of rennet extract, so that the external conditions affecting the rate of curdling will be alike in all cases. 3. Protect tubes by plugs of cotton or some other cover- ing from the dust. 4. Incubate the tubes at a constant warm temperature for several hours, preferably about 95° P. 5. Note especially any rents or vertical splits in the curd- led milk. This betrays the passage of gas bubbles, and the presence of gas can often be recognized in this way before it becomes apparent to the eye. It must be borne in mind that a fermentation test is really a culture experiment in bacteriology. It necessitates, therefore, cleanliness from a bacteriological point of view in every way in order to insure accuracy in interpreting results. 130. Wisconsin curd test. — Still another modifica- tion may be mentioned that is applicable for the detection of improper milks. In this a considerable quantity of milk is taken in a sterilized glass jar. Rennet is added and after 180 Dairy Bacteriology. the casein is set, the curd is cut and warmed. This process permits the expulsion of the whej- and as the curds shrink, the whey is poured off. The mass of curd is allowed to re- main on bottom of bottle, and after it is incubated from 6-12 hours, the condition of the curds is examined as to flavor, texture, presence of pin holes, etc. In this way practically, a curd test is made from the milk supply of each patron and any peculiarity easily detected. The test requires no longer time than the preceding method, and at the same time it brings out points regarding the flavor and texture of curds that would be impossible to get in any other way. 131. Bitter cheese. — Bitter flavors are sometimes developed in cheese especially where the ripening process has not been fully completed, or where improper tempera- tures have been maintained for a considerable length of time. Several organisms associated with this abnormal fermentation have been noted. Guillebeau * isolated several forms which he connected with udder inflammation that were able to produce a bitter substance in cheese. Von Freudenreich t has recently described a new form. Micrococcus casei amari (micrococcus of bitter cheese) that was found in a sample of bitter cheese. This germ is closelj' related to Conn's micrococcus of bitter milk (19). It de- velops lactic acid rapidly, coagulating the milk and pro- ducing an intensely bitter taste in the course of one to three days. When milk infected with this organism is made into cheese, there is formed in a few days, a decomposition product that imparts a marked bitter flavor to the cheese. It is peculiar that some of the organisms that are able to « Guillebeau. Landw. Jahr., 1891, p. 27. t Freudenreich, Fuehl. Laodw. Ztg., 43: 3G1. Bacteria in Cheese Making. 181 produce bitter products in milk do not retain tbis property when tiie milk is worked up into cheese. 132. Putrid or rotten cheese. — Sometimes cheese undergoes a putrefactive decomposition in which the texture is profoundly modified and various foul smelling gases are evolved. These often begin on the exterior as small cir- cumscribed spots that slowly extend into the cheese chang- ing the casein into a soft slimy mass. Then, again, the in- terior of the cheese undergoes this slimy decomposition. The soft varieties are more prone toward this fermentation than the hard, although the firm cheeses are by no means exempt from the trouble. The " Verlaufen " or " running " of limburger cheese is a fermentation allied to this. It is where the inside of the cheese breaks down into a soft semi- fluid mass. In severe cases, the rind ma}' even be ruptured, in which case, the whole interior of the cheese flows out as a thick slimy mass having sometimes a putrid odor. The conditions favoring this putrid decomposition are usually associated with an excess of moisture, and an abnormally low ripening temperature. 133. Pigment clianges in cheese. — Occasionally with hard cheeses, but more often with the softer foreign varieties, abnormal conditions are noted that express them- selves in the production of various pigments in the diflferent sorts of cheese. The production of these variously colored pigments are due mainly to the action of bacteria, yeasts, and molds. More freqentlj' they are merely super- ficial, and affect only the exterior layers of the cheese. 1. Red cheese. In chedder cheese, red spots are occasion- ally noted that may be merely on the surface, but some- times extend throughout the cheese. In some instances. 182 Dairy Bacteriology. bacteria have been found in large numbers in these spots, but as yet no specific germ has been isolated. Eed streaks in cheese are sometimes traced to other causes than bacteria. In one instance in Canadian cheese the author found that the edges of the curd particles were covered with an orange pigment. This discoloration must have occurred after the •curds were cut and before they were put to press, but no definate reason could be assigned for the production of this pijjment. 2. Blue cheese. Blue discoloration in cheese sometimes arises from copper and iron salts derived from manufactur- ing utensils. Especially is this liable to occur with foreign cheese where utensils of such a character are used in the manufacture of these cheese. De Vries* has described a blue disease in Edam cheese that he ascribes to the action •of bacteria. It appears first as a small blue spot on the inside, increasing rapidly in size until the whole mass is •often affected. 3. Black cheese. Black cheese have now and then been noted, especially with limburger products. This appearance is caused by the copious growth of difierent forms of low fungi, mainly those that spread out in tiny threads, like the molds. In one instance a yeast-like form has also been isolated. So far as known, troubles of this sort are not caused by bacteria. 1 24. Poisonous cheese. — The production of poison- ous ptomaines in cheese is perhaps more common than with almost any other food product. Vaughan,t to whom the most of the work on this subject is to be credited, reports over 300 cases of cheese-poisoning in two years. It seems * De Vries, Mllch. Ztg., 1888, Nob. 44, 45. t Vaughan, Zeit. f. physiol. Chemie, 10 : 146. Bacteria in Cheese Making. 183 to be very much more common here in America than it is in Europe. Vaughan has isolated from numerous samples a highly poisonous alkaloid that he calls tyrotoxicon. This ptomaine has also been frequently demonstrated in milk, cream, and ice cream. The poisonous substances formed in the milk are probably produced through the agency of putrefactive organisms that gain access to the milk. Although no specific form has been isolated. The poisons secreted in the milk are transferred to the cheese with their virulence often unimpaired, so that serious complications follow where the infected material is used for human food (61). Vaughan states that cheese that reddens litmus paper rapidly should be regarded as suspicious. In normal cheese, this reaction occurs very slowly. Danger from this source is to be noted particularly with the so- called sour milk cheese, as the methods by'which these are made give the most favorable opportunity for the develop- ment of poisonous decomposition products. 135. Prevention of cheese disases. — In attempting to treat the various defects or diseases found in cheese, no uniform method can be applied in all cases. Excluding faults due to manufacturing methods, the most of the troubles traceable to bacteria originate in the milk before it comes to the factory. In some instances the maker can repress or subdue the in- fluence of these undesirable ferments by changing his m eihod s somewhat to suit the varying conditions, such as the rapid development of acid in gassy fermentations. But what he must insist upon under all circumstances is that the milk shall be handled by the patron in such a way as to exclude as far as possible all conditions favoring the access of any 184 Dairy Bacteriology. organisms into it. If this is done and pains is taken to point out to each individual patron the way in which his milk becomes infected, then the maker may expect to reap the reward for his teachings in the improved conditions of the milk. GLOSSARY. Adventive. — Species introduced from foreign sources. Aerobes. — Bacterial organisms requiring free oxygen for growth. Anaerobes. — Bacteria growing without free oxygen. / Albuminous. — Substances containing albumen (white of eggs). Anthrax (splenic fever). — A contagious animal disease character- ized by blood poisoning. , Antiseptic. — Substances capable of restraining bacterial develop- ment. Arthrospore. — Spore formed from the whole mother cell. Bacillus (plural, bacilli). — Straight, rod-like bacteria. Carbohydrates. — Organic material like sugars and starches. Casease. — The enzyme capable of converting casein into soluble compounds. Caseoue. — Casein that has been rendered soluble by enzymes. Cell. — The simplest form unit of vegetable or animal life. Chlorophyll. — The green coloring matter in leaves of green plants. Chromogenic. — Bacteria capable of forming color products. Cilia, (singular, cilium). — Tiny, whip-like protoplasmic append- ages of cells, serving as locomotor organs. Coccus, (plural, cocci). — Bacteria having a spherical shape. Colony. — The progeny of a single germ growing in an isolated mass. Constructive. — Organisms able to build up organic material from simpler compounds or elements. Culture Starters. — Starters for use in butter or cheese-making that are developed from a selected bacterial culture. 186 Glossary. Destructive. — Organisms whose function it is to break down or- ganic tissue. Disinfectant. — Any substance able to destroy germ life. Endospores. — Spore formed in a mother cell from a part of its protoplasm. Bnzymes. — Unorganized chemical ferments not endowed with life. J'acultative. — Forms that possess the faculty of growing under Taried conditions. Pission. — Division of a cell by direct partition. Germicide. — Any substance capable of destroying germ life. Hydrophobia. — A highly contagious disease commonly found in dogs, wolves, etc., but communicable to man. Hydrocarbons. — Organic material like the fats and oils. Indigenous. — Bacteria normally found in any given place or habitat. Mother cell. — A typical vegetative cell capable of reproduction. Obligate. — Bacteria that are obliged to grow under certain con- ditions. Optimum growth temperature. — The best temperature for the growth of any form. Parasites. — Living organisms subsisting on living matter. Pasteurization. — The use of heat from 140° to 165° F. as a germ destroyer. Pathogenic. — Bacteria able to produce disease in living tissue. Pepsin. — Enzymes capable of digesting proteids. Peptonizing. — The conversion of insoluble proteids into a soluble form (peptones). Proteids. — Complex nitrogenous substances of an insoluble nature. Protoplasm. — The living substance of organic tissues. Glossary. 187 Ptomaine. — A complex nitrogeneous product of bacterial growth. Pure culture. — A bacterial growth of a single species in a sterile medium. Saprophytes. — Living organisms subsisting on dead organic matter. Silex.— The quartz element in rock (pure sand). Spirillum (plural, spirilla).— CurTcd, or bent cylindrical bacteria. Spores. — The latent or resting stage of certain types of bacteria. Sterilization.— The use of heat in the neighborhood of 212° F. as a germ destroyer. Thermal death point. — A degree of heat sufficient to destroy any species. Toxicogenic. — Bacteria producing toxic or poisonous effects. Trypsin. — The active principle of the pancreatic secretion. Tyrotoxicon. — A poisonous ptomaine isolated frommilkorcheese. Unicellular.— Plants or animals composed of a single cell. Viscogen. — A solution of lime dissolved in sugar used in restoring the consistency of cream that has been heated above 145° F. INDEX. Action of bacteria in natuie, 19. Aeration of milk, 49, 79. Aerobic bacteria, IG. Air, bacteria in, ^1. Air, infection of milk from, 47, Alcoholic fermentation of milk, 72. Anaerobic bacteria, 16. .Antiseptics, influence on bacteria, 28. Animal, influence of, on milk, 43. Aphthous fever disseminated by milk, 92, Apparatus for pasteurizing milk or cream; commercial, 114, domestic, 112; es- sential requisites of, 117 ; Fjord's, 116; Monrad's 115; Wisconsin Dairy School, 119. Arthrospores, 12. Bacillus, definition of, 10. acidi lactici, 64; cyanogenus, 75; foe- tidus lacUs. 152; lactis erythrogenes, 75; prodigiosus. 74; tuberculosis, 84. Bacteria: action in nature, 19; arrange- ment of cells, 10; centrifuge slime, 128; classification, 14; cultures, 29; discovery of, 9; external conditions affecting, 25; form, 10; in rennet, 156; methods of isolating, 31; necessity of, in cheese, 154; motility of, 13; na- ture of, 9; parasitic, 22; rapidity of multiplication, 13; reproduction, 11; saprophytic, 22; size of, 10; speciali- zation of, 20; structure of, 9; study of, 29. distribution of: air, 21; Boston milk, 59; cheese, 163; milk of Madison, Wis., 59; milk of Middletown, Conn., 59; milk, 57; soil, 21; water, 22. of disease: anthrax, 92; aphthous fever, 92; cholera, 90; diphtheria, 91; scarlet fever, 91; toxic, 93; tuberculosis, 89; typhoid fever, 89. physiology of: essential conditions of growth, 14; food supply, 14; tempera- ture, 16, 26, 35; gaseous environment, 16; moisture, 17, 27. m cream: mechanical causes for, 127; centrifugal, 131; changes in, 129. Bacteriological study of pasteurized milk; 125. Bitter butter, 153. Bitter cheese, 180. Bitter milk fermentation, 68. Black cheese, 182. Blue cheese, 182. Blue milk, 75. Boracic acid in milk, 96. Brie cheese, 169. Butyric acid fermentation, 67. Butter: bacteria in, 148; pasteurized cfeam, 142; ripening cream for, 137; sour cream, 136; sweet cream, 136; value of pure cultures in making, 146. Abnormal changes in, 150; bacterial fer- mentations, 150; bitter, 153; co\vy, 352; lack of flavor, 151; lardy, 153; oily, 153; tallowy, 153; tuinip flavored, 152. Buttermilk: germs in, 132; starter, 140. Carbolic acid, 82. Coccus, definition of, 10. Cheese; bacteria a necessity in, 157; Brie, 169; cottage, 155; development of bacteria in, 155; bacteria m green, 157; effect of salt on texture, 174; Emmenthaler, 167; faults in, 171; gassy fermentations in, 172; Roque- fort, 168; Stilton, 169; Swiss, 167. Cheese ripening; bacterial changes in, 160; chemical changes in, 159; pep- tonizing theory of, 164; physical changes in, 158. 190 Index. Chemical- disinfectants^ 81, 95, Cholera in' milk, 90. Classification of bacteria, 14. Cold, influence of, on bacteria, 27. Color fermentations in milk, 74. Cooling pasteurized milk, 110, 322. Contamination of milk, ',M; through dis- ease germs, 83. Corrosive sublimate, 82. Cream, b.^cterial changes in, 129; me- chanical causes for bacteria \u, 127. Cream, pasteurized: bacteria in, 126; for butter, 144; restoration of consistency of, 104. ripening, methods of: artificial starters, 141; buttermilk, 140; natural, 139; natural starters, 140; skim milk, 140; sour cream, 140. Creosote, 82. Culture media, 3'"*. Culture methods, 29. Desiccation, influence of, on bacteria, 27. Detection of unwholesome milk, 79. Diagnosis of infective and non-infective changes in milk, 79. Digestive fermentations of milk, 66. Dilution of tuberculous milk, 38. Disease germs : aphthous fever^ 92; cholera, 90; contamination of milk by, 83; diphtheria, 91; foot and mouth disease, 92; scarlet fever, 91; tuberculosis 84; typhoid fever, 89. Endospores, 12. Electricity as a milk preservative, 97. Essential requisites in pasteurizing ap- paratus, 107. External conditions in relation to bac- teria, 14. Factory infection, sources of, 56. Factory bye products: buttermilk, 132; skim milk, 131; whey, 133. Ferments, organized, 2^; unorganized, 25. Fermentation of organic matter, 23. Fermentation test, 178, 179. Fermentations, milk: alcoholic, 72; bitter, 68; blue, 75; buty- ric, 67; curdling, 66; digestive, 66; gaseous, 55, 172; in boiled milk, 100; kephir, 73, kumiss, 72; lactic acid, 63; red, 74; ropy, 70; slimy, 69; soapy, 74; violet, 76; yellow, 76. Filtrat'on of milk, 97. Fjord's Pasteurizer, 116. Flavor in butter, 151, 152, 153. Food supply of bacteria, 14. Fore milk, 41. Fungi in cheese ripening 167. Garget, infectious, in milk, 92. Gaseous environment, influence of, on, bacteria, 16. Gaseous fermentations in milk, 65; in cheese, 172, 177. Growth, essential conditions for, 14. Germination of spores, 12. Heat, influence of, on bacterial growth, 26. Heat influence of, in preserving whey, 135. Hydrogen peroxide in milk, 96. Infection of milk: air, 47; animal, 43: fore milk, 41; unclean dairy utensils, ?8. Infectious diseases in milk, 79. Isolation of bacteria, methods of, 31. Kephir, 73. Kumiss, 72. Lactic acid fermentation, 63, 160. Lardy butter, 153. Light, action of, on bacteria, 27. Media, influence of, on bacterial growth,. 15, 30. Methods: of isolation, 31; of culture, 29. Micrococcus casei amari, ISO. Micrococcus Freudenreichii, 71. Microscope, use of, 35. Milk: a bacterial food medium, 17; as sec- reted, sterile, 37; comparative value,, pasteurized and sterilized, 103; cholera in, 90; diphtheria, 91; infection in fac- tories, 56; number of bacteria in, 57; pasteurized, 103; poisonous, 93; scar- let fever, 91; tuberculous, 84; treat- ment of infectious, 55; typhoid fever in, 89. Milk, bye products of: buttermilk, 132; skim, 131; whey, 133. Index. 191 Milk contamination, sources of; air, 47; animal, 43; dairy utensils, 38, 56; disease germs, 83; fore milk, 41, ice, 57; milker, 43; transportation cans, 39; water, 57. Milk fermentations: alcoholic, 72; bitter, 68; blue, 75; butyric, 67; colored, 74; curdling, 66; digestive, 66; gaseous, 65; lactic acid, 63; red, 74; soapy, 7Jl; slimy, 69; treatment of, 76. Milk heated, characteristics of, 99. Milk pasteurization, details in: acid test- ing, 108; rapidity of cooling, 110; selec- tion, 106; temperature, 109; time, 109. Moisture, influence of, on bacteria, 4, 19. Motility of bacteria, 13. Multiplication of bacteria, 13. Natural ripening of cream, 139. Neufchatel cheese, 171. Nisslei cheese, 177. Non-infectious changes in milk, 79. Odor, cowy,in butter, 152; in milk, 79. Oily butter, 153. Parasitic bacteria, 22, Pasteurization of milk, 103: bacteriological study of, 125; details of, 106-112; rela- tion to tubercle bacillus, 89; relative ad- vantages compared with sterilization, 103. Pasteurizing apparatus; commercial, 114; domestic, 112; essential requisites in, 117. Fjord's, U6; Monrad's, 115; Wisconsin Dairy School, 119. Paste'irized cream: for butter, 144; res- toration of consistency, 104. Pcnicillium glaucum, 169, 171, Pigment fermentations in cheese, 171; in milk, 74. Poisonous bacteria in milk and cheese, 93, 182. Preservation of milk: chemical agents, 95; physical agents, 96. Pressure, effect of, on bacteria, 27. Pure cultures, 33. Pure culture starters, 141, 166. Putrid butter, 152. Putrid cheese, 181. Rancidity in butter, 149. Red cheese, 181. Red milk, 74. Rennet, action of, on heated milk, 100. Rennet, bacteria in, 156. Rennet in cheese, 155. Reproduction of bacteria, 11. Ropy milk, 70. Saccharomyces giutinis, 75. Salt, effect of, on cheese texture, 174. SaUcylic acid in milk, 96. Saprophytic bacteria, 15. Scarlet fever disseminata by milk, 91. Separator slime: bacteria in, 127; tubercle bacilh in, 123. Size of bacteria, 10. Sk'm milk: as a bye product, 131; germs in, 131; a starter, 140. Slimy milk, 69. Soapy milk, 69, 48. Soil hacter.a, 21. Sour cream butter, 136. Sour cream starters, 140. Specialization of bacteria, 20. Spirillum, definition of, 10. Starters, in cream ripening, HO; in cheese, 166. Sterilization; of media, 30; milk, 91. Stilton cheese, 169. Structure of bacteria, 9. Suittir as a distntectant, 82. Sweet cream butter, 136. Taettemjolk, 70. Tainted milk, detection of, 177. Tallowy butter, 153. Temperature, eflfect on bacteria, 15, 26, 51. Temperature effect on milk, 51, 97. Test, acid, 107. Test, fermentation, 129 Treatment of milk fermentations, 76. Treatment of milk in factories, 40. Tubercle bacillus in milk, 84, 87; in sep- arator slime, 128. Tuberculin test, 86. Tuberculosis, bovine, 85. Turnip flavored butter, 152. Typhoid fever, 89. Tyrotoxicon, 94, 183. 192 Index. Water bacteria, 22. Whey: a bye product, 133; cleanliness of vats, 133; germs in, 133; heating to preserve, 135. Wisconsin Dairy School pasteurizing ap- paratus, 119. Viscogen, use of, in restoring consistency of pasteurized cream, 105.