The Project Gutenberg eBook, The Book of Cheese, by Charles Thom and Walter Warner Fisk

Transcriber's Note:

The original text contains a large number of words which occur in hyphenated and spaced forms with comparable frequency. Such inconsistencies have been retained in this version.

Page numbers out of sequence shown in brackets or in groups, are the actual pages numbers shown in print.

THE BOOK OF CHEESE

The Rural Text-Book Series

Edited by L. H. BAILEY

Carleton: The Small Grains.

B. M. Duggar: The Physiology of Plant Production.

J. F. Duggar: Southern Field Crops.

Gay: Breeds of Live-Stock.

Gay: Principles and Practice of Judging Live-Stock.

Goff: Principles of Plant Culture.

Guthrie: Book of Butter.

Harper: Animal Husbandry for Schools.

Harris and Stewart: Principles of Agronomy.

Hitchcock: Text-book of Grasses.

Jeffery: Text-Book of Land Drainage.

Jordan: Feeding of Animals. Revised.

Livingston: Field Crop Production.

Lyon: Soils and Fertilizers.

Lyon, Fippin and Buckman: Soils, their Properties and Management.

Mann: Beginnings in Agriculture.

Montgomery: The Corn Crops.

Morgan: Field Crops for the Cotton-Belt.

Mumford. The Breeding of Animals.

Piper: Forage Plants and their Culture.

Sampson: Effective Farming.

Thom and Fisk: The Book of Cheese.

Warren: The Elements of Agriculture.

Warren: Farm Management.

Wheeler: Manures and Fertilizers.

White: Principles of Floriculture.

Widtsoe: Principles of Irrigation Practice.

Fig. 1.—A cheese laboratory in the New York State College of Agriculture at Cornell University.

THE BOOK OF CHEESE

BY
CHARLES THOM
INVESTIGATOR IN CHEESE, FORMERLY AT CONNECTICUT
AGRICULTURAL COLLEGE
AND
WALTER W. FISK
ASSISTANT PROFESSOR OF DAIRY INDUSTRY (CHEESE-MAKING),
NEW YORK STATE COLLEGE OF AGRICULTURE
AT CORNELL UNIVERSITY

New York
THE MACMILLAN COMPANY
1918
All rights reserved

Copyright, 1918,
By THE MACMILLAN COMPANY.
Set up and electrotyped. Published July, 1918.
Norwood Press
J. S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.

PREFACE

Certain products we associate with the manufactures of the household, so familiar and of such long standing that we do not think of them as requiring investigation or any special support of science. The older ones of us look back on cheese as an ancient home product; yet the old-fashioned hard strong kind has given place to many named varieties, some of them bearing little resemblance to the product of the kitchen and the buttery. We have analyzed the processes; discovered microorganisms that hinder or help; perfected devices and machines; devised tests of many kinds; studied the chemistry; developed markets for standardized commodities. Here is one of the old established farm industries that within a generation has passed from the housewife and the home-made hand press to highly perfected factory processes employing skilled service and handling milk by the many tons from whole communities of cows. This is an example of the great changes in agricultural practice. Cheese-making is now a piece of applied science; many students in the colleges are studying the subject; no one would think of undertaking it in the old way: for these reasons this book is written.

This book is intended as a guide in the interpretation of the processes of making and handling a series of important varieties of cheese. The kinds here considered are those made commercially in America, or so widely met in the trade that some knowledge of them is necessary. The relation of cheese to milk and to its production and composition has been presented in so far as required for this purpose. The principles and practices underlying all cheese-making have been brought together into a chapter on curd-making. A chapter on classification then brings together into synoptical form our knowledge of groups of varieties. These groups are then discussed separately. The problems of factory building, factory organization, buying and testing milk, and the proper marketing of cheese, are briefly discussed.

Such a discussion should be useful to the student, to the beginner in cheese-making, as a reference book on many varieties in the hands of makers who specialize in single varieties, and to the housekeeper or teacher of domestic science. The material has been brought together from the experience of the writers, supplemented by free use of the literature in several languages. Standard references to this literature are added in the text.

No introduction to the subject of cheese should fail to mention the work of J. H. Monrad, who has recently passed away. Mr. Monrad never collected his material into a single publication, but his contributions to chees-emaking information, scattered widely in trade literature over a period of thirty years, form an encyclopedia of the subject.

Bulletins of the Agricultural Experiment Stations and United States Department of Agriculture have been quoted extensively, with citation of the sources of the material. Personal assistance from Professor W. A. Stocking and other members of the Dairy Department of Cornell University, and C. F. Doane of the United States Department of Agriculture, is gladly acknowledged.

Students cannot learn out of books to make cheese. They may, however, be aided in understanding the problems from such study. To make cheese successfully they must have intimate personal touch with some person who knows cheese. Sympathetic relations with such a teacher day by day in the cheese-room are essential to success in making cheese which, at its best, is one of the most attractive of food-products.

The Authors.

TABLE OF CONTENTS

THE BOOK OF CHEESE

CHAPTER I

GENERAL STATEMENT ON CHEESE

Cheese is a solid or semi-solid protein food product manufactured from milk. Its solidity depends on the curdling or coagulation of part or all of the protein and the expulsion of the watery part or whey. The coagulum or curd so formed incloses part of the milk-serum (technically whey) or watery portion of the milk, part of the salts, part or all of the fat, and an aliquot part of the milk-sugar. The loss in manufacture includes a small fraction of the protein and fat, the larger proportion of the water, salts and milk-sugar.

1. Nature of cheese.—Milk of itself is an exceedingly perishable product. Cheese preserves the most important nutrient parts of the milk in condition for consumption over a much longer period. The duration of this period and the ripening and other changes taking place depend very closely on the composition of the freshly made cheese. There is an intimate relation between the water, fat, protein and salt-content of the newly made cheese and the ripening processes which produce the particular flavors of the product when it is ready for the consumer. This relation is essentially biological. A cheese containing 60 to 75 per cent of water, as in "cottage cheese" (the sour-milk cheese so widely made in the homes), must be eaten or lost in a very few days. Spoilage is very rapid. In contrast to this, the Italian Parmesan, with 30 to 32 per cent of water, requires two to three years for proper ripening.

The cheeses made from soured skim-milk probably represent the most ancient forms of cheese-making. Their origin is lost in antiquity. The makers of Roquefort cheese cite passages from Pliny which they think refer to an early form of that product. It is certain that cheese in some form has been familiar to man throughout historic times. The technical literature of cheese-making is, however, essentially recent. The older literature may be cited to follow the historical changes in details of practice.

2. Cheese-making as an art has been developed to high stages of perfection in widely separate localities. The best known varieties of cheese bear the geographical names of the places of their origin. The practices of making and handling such cheeses have been developed in intimate relation to climate, local conditions and the habits of the people. So close has been this adjustment in some cases, that the removal of expert makers of such cheeses to new regions has resulted in total failure to transplant the industry.

3. Cheese-making as a science has been a comparatively recent development. It has been partly a natural outgrowth of the desire of emigrant peoples to carry with them the arts of their ancestral home, partly the desire to manufacture at home the good things met in foreign travel. Its development has been largely coincident with the development of the agricultural school and the science of dairy biology. Even now we have but a limited knowledge of a few of the 500 or more varieties of cheese named in the literature. It is desirable to bring together the knowledge of underlying principles as far as they are known.

No technical description of a cheese-handling process can replace experience. Descriptions of appearances and textures of curd in terms definite enough to be understood by beginners have been found to be impossible. It is possible, however, to lay down principles and essentials of practice which are common to the industry and form the foundation for intelligent work. Cheese-making will be a science only as we depart from the mere repetition of a routine or rule-of-thumb practice and understand the underlying principles.

4. Problems in cheese-making.—Any understanding of these problems calls for a working knowledge of the very complex series of factors involved. These include the chemical composition of the milk, the nature of rennet and character of its action under the conditions met in cheese-making, the nature of the micro-organisms in milk, and the methods of controlling them, their relation to acidity and to the ripening of the cheese. To these scientific demands must be added acquaintance with the technique of the whole milk industry, from its production and handling on the farm through the multiplicity of details of factory installation and organization, to those intangible factors concerned with the texture, body, odor and taste of the varied products made from it. Some of these factors can be adequately described; others have thus far been handed on from worker to worker but have baffled every effort at standardization or definition.

5. History.—The recorded history of the common varieties of cheese is only fragmentary. Practices at one time merely local in origin followed the lines of emigration. Records of processes of manufacture were not kept. The continuance of a particular practice depended on the skill and memory of the emigrant, who called his cheese after the place of origin. Other names of the same kind were applied by the makers for selling purposes. The widely known names were thus almost all originally geographical. Some of them, such as Gorgonzola, are used for cheeses not now made at the places whose names they bear. Naturally, this method of development has produced national groups of cheeses which have many common characteristics but differ in detail. The English cheeses form a typical group of this kind.

Emigration to America carried English practices across the Atlantic. The story of cheese-making in America has been so closely linked with the development of the American Cheddar process that the historical aspects of the industry in this country are considered under that head in Chapter VIII.

CHAPTER II

THE MILK IN ITS RELATION TO CHEESE

The opaque whitish liquid, secreted by the mammary glands of female mammals for the nourishment of their young, is known as milk. The milk of the cow is the kind commonly used for cheese-making in America.

6. Factors affecting the quality.—The process of cheese-making begins with drawing the milk from the udder. The care and treatment the milk receives, while being drawn, and its subsequent handling, have a decided influence on its qualities. The process of cheese-making is varied according to the qualities of the milk. There are five factors that influence the quality of the milk for cheese-making: (1) its chemical composition; (2) the flavor of feed eaten by the cow; (3) the absorption of flavors and odors from the atmosphere; (4) the health of the cow; (5) the bacteria present. The first factor is dependent on the breed and individuality of the cow. The other four factors are almost entirely within the control of man. Of these factors, number five is of the most importance, and is the one most frequently neglected.

7. Chemical composition.—The high, low and average composition of milk is approximately as follows:

TABLE I

8. Factors causing variation in composition.—The composition of cow's milk varies according to several factors. The composition of the milk of different breeds differs to such a degree that whole series of factories are found with lower or higher figures than these averages on account of dominant presence of particular kinds of cattle.

The following table shows the usual effect of breed on fat and total solids of milk:

TABLE II

The figures[1] in Tables I and II are compiled and averaged from a large number of analyses made at different agricultural experiment stations.

This variation not only affects the fat, but all constituents of the milk. While there is a difference in the composition of the milk from cows of different breeds, there is almost as wide variation in the composition of the milk from single cows[2] of the same breed. With the same cow the stage of lactation causes a wide variation in the composition of the milk.[3] As the period of lactation advances, the milk increases in percentage of fat and other solids.

9. Milk constituents.—From the standpoint of the cheese-maker, the significant constituents of milk are water, fat, casein, milk-sugar, albumin, ash and enzymes. These will be discussed separately.

10. Water.—The retention of the solids and the elimination of the water are among the chief considerations in cheese-making. Water forms 84 to 89 per cent of milk. Cheese-making calls for the reduction of this percentage to that typical of the particular variety of cheese desired with the least possible loss of milk solids. This final percentage varies from 30 to 70 per cent with the variety of cheese. The water has two uses in the cheese: (1) It imparts smoothness and mellowness to the body of the cheese; (2) it furnishes suitable conditions for the action of the ripening agents. To some extent the water may supplement or even replace fat in its effect on the texture of the cheese. If the cheese is properly made, the water present is in such combination as to give no suggestion of a wet or "leaky" product.

11. Fat.—Fat is present in the milk in the form of suspended small transparent globules (as an emulsion). These globules vary in size with the breed and individuality of the cow and in color from a very light yellow to a deep yellow shade as sought in butter. Milk with small fat globules is preferred for cheese-making, because these are not so easily lost in the process. Milk-fat is made up of several different compounds called glycerids,[4] which are formed by the union of an organic acid with glycerine as a base.

Fat is important in cheese-making for two reasons: (1) Its influence on the yield of cheese; (2) its effect on the quality of the cheese. Many of the details of cheese-making processes have been developed to prevent the loss of fat in manufacture. The yield of cheese is almost directly in proportion to the amount of fat in the milk; nevertheless, because the solids not fat do not increase exactly in proportion to the fat, the cheese yield is not exactly in proportion to the fat. The fat, however, is a good index of the cheese-producing power of the milk.

12. Casein.—Cheese-making is possible because of the peculiar properties of casein. This is the fundamental substance of cheese-making because it has the capacity to coagulate or curdle under the action of acid and rennet enzymes. Casein is an extremely complex organic compound.[5] Authorities disagree regarding its exact composition, but it contains varying amounts of carbon, oxygen, nitrogen, hydrogen, phosphorus and sulfur, and it usually is combined with some form of lime or calcium phosphate. It belongs to the general class of nitrogen-containing compounds called proteins. It is present in milk in the form of extremely minute gelatinous particles in suspension. Casein is insoluble in water and dilute acids. The acids, when added, cause a heavy, white, more or less flocculent precipitate. Rennet (Chapter III) causes the casein to coagulate (curdle), forming a jelly-like mass called curd, which is the basis of manufacture in most types of cheese. In the formation of this coagulum (curd), the fat is imprisoned and held. The casein compounds in the curd hold the moisture and give firmness and solidity of body to the cheese. Casein contains the protein materials in which important ripening changes take place. These changes render the casein more soluble, and are thought to be the source of certain characteristic cheese flavors.

13. Milk-sugar.—Milk-sugar (lactose) is present in solution in the watery part of the milk. It forms on the average about 5 per cent of cow's milk. Since it is in solution, cheese retains the aliquot part of the total represented by the water-content of the cheese, plus any part of the sugar which has entered into combination with the milk solids during the souring process. The larger part of the lactose passes off with the whey. Lactose[6] is attacked by the lactic-acid bacteria and by them is changed to lactic acid. Cheeses in which this souring process goes on quickly, soon contain a large enough percentage of acid to check the rotting of the cheese by decay organisms. Without this souring, most varieties of cheese will begin to spoil quickly. For each variety there is a proper balance between the souring, which interrupts the growth of many kinds of putrefactive bacteria, and the development of the forms which are essential to proper ripening.

14. Albumin.—This is a form of protein which is in solution in the milk. Albumin forms about 0.7 per cent of cow's milk. It is not coagulated by rennet. Most rennet cheeses, therefore, retain only that portion of the total albumin held in solution in the water retained, as in the case of milk-sugar. Albumin is coagulated by heat, forming a film or membrane upon the surface. There are certain kinds of cheese, such as Ricotte, made by the recovery of albumin by heating.

15. Ash.—The ash or mineral constituents make up about 0.7 per cent of cow's milk. This total includes very small amounts of a great many substances. The exact form of some of the substances is still unknown. Of these salts, the calcium or lime and phosphorus salts are most important in cheese-making. They are partially or completely precipitated by pasteurization. After such precipitation rennet fails to act[7] or acts very slowly; hence pasteurized milk cannot be used for making rennet cheese unless the lost salts are replaced, or the condition of the casein is changed by the addition of some substance, before curdling is attempted.

16. Enzymes.—Milk also contains enzymes. These are chemical ferments secreted by the udder. They have the power to produce changes in organic compounds without themselves undergoing any change. Minute amounts of several enzymes are found in milk as follows: Diastase, galactase, lipase, catalase, peroxidase and reductase. Just what part they play in cheese-making is not definitely known.

17. The flavor of feeds eaten by the cow.—Undesirable flavors in the milk are due many times to the use of feed with very pronounced flavors. The most common of these feeds are onions, garlic, turnips, cabbage, decayed ensilage, various weeds and the like. These undesirable flavors reach the milk because the substances are volatile and are able to pass through the tissues of the animal. While feed containing these flavors is being digested, these volatile substances are not only present in the milk, but in all the tissues of the animal. By the time the process of digestion is completed, the volatile flavors have largely passed away. Therefore, if the times of milking and feeding are properly regulated, a dairy-man may feed considerable quantities of strong-flavored products, such as turnip, cabbage and others, without any appreciable effect on the flavor of the milk. To accomplish this successfully, the cows should be fed immediately before or immediately after milking, preferably after milking. This allows time for the digestive process to take place and for the volatile substances to disappear. If, however, milking is performed three or four hours after feeding, these volatile substances are present in the milk and flavor it.[8]

In the case of those plants which grow wild in the pasture, and to which the cows have continued access, it is more difficult to prevent bad flavor in the milk. The cows may be allowed to graze for a short time only, and that immediately after milking, without affecting the flavor of the milk. This will make it necessary to supplement the pasture with dry feed, or to have another pasture where these undesirable plants do not grow.

Undesirable flavors are usually noticeable in the milk when the cows are turned out to pasture for the first time in the spring; and when they are pastured on rank fall feed, such as second growth clover.

18. Absorption of odors.—Milk, especially when warm, possesses a remarkable ability to absorb and retain odors from the surrounding atmosphere.[9] For this reason, the milk should be handled only in places free from such odor. Some of the common sources of these undesirable odors are bad-smelling stables, strong-smelling feeds in the stable, dirty cows, aërating milk near hog-pens, barn-yards and swill barrels. The only way to prevent these undesirable flavors and odors is not to expose the milk to them. The safest policy is to remove the source of the odor.

19. Effect of condition of the cow.—Any factor which affects the cow is reflected in the composition and physiological character of the milk. (1) Colostrum. Milk secreted just before or just after parturition is different in physical properties and chemical composition from that secreted at any other time during the lactation period. This milk is known as colostrum. It is considered unfit for human food, either as milk or in products manufactured from the milk. Most states[10] consider colostrum adulterated milk, and prohibit the sale of the product for fifteen days preceding and for five days after parturition. (2) Disease. When disease is detected in the cow, the milk should at once be discarded as human food. Some diseases are common both to the cow and to man, such as tuberculosis, foot-and-mouth disease. If such diseases are present in the cow, the milk may act as a carrier to man. Digestive disorders of any sort in the cow are frequently accompanied by undesirable flavors in the milk. These are not thought to be due to the feed, but to the abnormal condition of the cow. When the normal condition is restored, these undesirable flavors disappear.

20. Bacteria in the milk.—Bacteria are microscopic unicellular plants, without chlorophyll. Besides bacteria, there are other forms of the lower orders of plants found in milk, such as yeasts and molds. While the bacteria are normally the more important, frequently yeasts and molds produce significant changes in milk and other dairy products. Bacteria are very widely distributed throughout nature. They are so small that they may easily float in the air or on particles of dust. Many groups of bacteria are so resistant to adverse conditions of growth that they may be present in a dormant or spore stage, and, therefore, not be easily recognized; when suitable environments for growth are again produced, development begins at once. They are found in all surface water, in the earth and upon all organic matter. There are a great many different groups of bacteria; some are beneficial, and some are harmful. As they are so small, it is difficult to differentiate between the beneficial and harmful kinds, except by the results produced, or by a careful study in an especially equipped laboratory. The bacteria multiply very rapidly. This is brought about by fission; that is, the cell-walls are drawn in at one place around the cell, and when the walls unite at the center, the cell is divided. There are then two bacteria. In some cases, division takes place in twenty to thirty minutes. Like other plants, they are very sensitive to food supply, to temperature and to moisture, as conditions of growth. Inasmuch as the bacteria are plant cells, they must absorb their food from materials in solution. They may live on solid substances, but the food elements must be rendered soluble before they can be used. Most bacteria prefer a neutral or slightly acid medium for growth, rather than an alkaline reaction. Ordinary milk makes a very favorable medium for the growth of bacteria, because it is an adequate and easily available food supply.

In milk, certain groups of bacteria are commonly present, but many others which happen to get into it live and multiply rapidly. A favorable temperature is very necessary for such organisms to multiply. There is a range of temperature, more or less wide, at which each group of bacteria grows and multiplies with the greatest rapidity. This range varies with the different groups, but most of them find temperatures between 75° F. and 95° F. the most favorable for growth. Excessive heat kills the bacteria. Low temperatures stop growth, but kill few if any bacteria. Temperatures of 50° F. and lower retard the growth of most forms of bacteria found commonly in milk. Many forms will slowly develop, however, below 50° and some growth will occur down to the freezing point. Milk held at 50° F. or lower will remain in good condition long enough to be handled without injury to quality until received in the cheese factory. In the place of seeds, some groups of bacteria form spores. The spores are exceedingly resistant to unfavorable conditions of growth, such as heat, cold, drying, food supply and even chemical agents. This property makes it difficult to destroy such bacteria.

21. Groups of bacteria in milk.—Milk when first drawn usually shows an amphoteric reaction; that is, it will give the acid and alkaline reactions with litmus paper. Under normal conditions, milk soon begins to undergo changes, due to the bacteria. Changes produced in this way are called "fermentations"; the agents causing them, "ferments." Normally the acid fermentation takes place first, and later other fermentations or changes begin, which, after a time, so decompose the milk that it will not be suitable for cheese-making or human consumption.

The following grouping of the organisms in milk is based on their effects on the milk itself[11]:

Each type of bacteria produces more or less specific changes in the milk. As a general rule, the predominance of one of these types is an aid in the interpretation of the quality of the product at the time of analysis, such as the age, the temperature at which it has been held, the conditions under which it was produced and, in some cases, the general source of the contamination. The reaction due to certain bacteria is utilized in the manufacture and handling of dairy products; other groups have deleterious effects. (See [Fig. 2.])

Fig. 2.—Effect of different fermentations of milk: U, Curd pitted with gas holes; G and O, gassy curds which float; K, smooth, solid desirable curd.

22. Acid fermentation of milk.—By far the most common and important fermentation taking place in milk is due to the action of the lactic acid-forming bacteria on the milk-sugar or lactose. The bacteria that bring about this fermentation may be divided into several groups on the basis of their morphology, proteolytic activity, gas production, temperature adaptation and production of substances other than lactic acid. The larger number of organisms producing lactic acid in milk also produce other organic acids in greater or less abundance. Inasmuch as lactic acid is the principal substance produced, they are called lactic acid organisms. This group contains different kinds of organisms which may be subdivided into small groups as follows:

23. Bacterium lactis-acidi group.—There are many strains or varieties in this group which are closely related in their activities. They are universally present in milk and are commonly the greatest causal agent in its souring. They are widely distributed in nature. At a temperature of 65° F. to 95° F., these bacteria grow and multiply very rapidly; at 70° F. (approximately 20° C.) these forms usually outgrow all others. The total amount of acid produced in milk by these organisms varies from 0.6 of one per cent to 1 per cent acid calculated as pure lactic acid. These forms coagulate milk to a smooth curd of uniform consistency. In addition to the lactic acid, there are produced traces of acetic, succinic, formic and proprionic acids, traces of certain alcohols, aldehydes and esters. Substances other than lactic acid are not produced by organisms of this group to such an extent as to impart undesirable flavors to the milk. The action of this group on the milk proteins is very slight. They produce no visible sign of peptonization. The B. lactis-acidi group of organisms are essential to the production of the initial acidity necessary in most types of cheese. The practical culture and utilization of them for this purpose under factory conditions are discussed in Chapter IV, entitled "Lactic Starters."

24. Colon-aërogenes group.—This group takes its name from a typical species, Bacterium coli communis, which is a normal inhabitant of the intestines of man and animals, and from Bacterium coli aerogenes, which is similar in many respects to B. coli communis. The initial presence of these bacteria in milk is indicative of fecal contamination or unclean conditions of production. These organisms, however, grow and develop in milk very rapidly at high temperatures of handling. The total acidity produced by these forms is less than that by the Bacterium lactis-acidi group. Of the acid produced, less than 30 per cent is lactic acid; the other acids are formic, acetic, proprionic and succinic. The large percentage of these acids, with comparatively large amounts of certain alcohols, aldehydes and esters, invariably impart undesirable flavors and odors to the milk. Members of this group uniformly ferment the lactose with the production of the gases, carbon dioxide and hydrogen. The milk is coagulated into a lumpy curd, containing gas pockets.

25. Acid peptonizing group.—These are often associated with colon organisms. The group includes those bacteria which coagulate milk with an acid curd and subsequently partly digest it. They grow and multiply rapidly at a temperature between 65° and 98° F. They impart undesirable flavors and odors to the milk, which appear to be due to the formation of acids other than lactic acid, and to action on the milk proteins.

26. Bacillus bulgaricus group.—These organisms grow best at a temperature of 105° to 115° F. They will develop at lower temperatures, but not so rapidly. They survive heating to 135° F. without loss of vigor, as occurs in Swiss cheese-making. They produce from 1 to 4 per cent of acid in milk, which is practically all lactic acid. They do not produce gas. They impart no undesirable flavors to the milk.

27. Acid cocci or weak acid-producers.—This group of organisms is not very well defined. It consists mostly of coccus forms, commonly found in the air and in the udder. Their presence in the milk may indicate direct udder contamination. These are regarded as of little importance, unless in very large number, and they have been only partially studied. They produce little or no lactic acid, and small amounts of acetic, proprionic, butyric and caproic acids. These forms rarely create enough acid to coagulate milk.

28. Peptonizing organisms.—This group includes all bacteria which have a peptonizing effect on the milk. It includes the acid peptonizing organisms, although they are of primary importance in the acid type of bacteria, because the acid-producing power is greater than the peptonizing power. Some of the specific organisms in this class are Bacillus subtilis, Bacterium prodigiosus and Bacterium liquefaciens. These are commonly found in soil water and in fecal material. The presence of these organisms denotes contamination from such sources.

29. Inert types.—As the name indicates, these are organisms not known to have an appreciable effect on milk. The ordinary tests fail to connect them with important processes; hence they appear to feed upon, but not to affect the milk in any serious way. Milk ordinarily contains more or less of these organisms, but no particular significance is attached to their presence.

30. Alkali-producing bacteria.—This group of organisms has only recently been studied in relation to its action on milk. Investigators still disagree as to the usual percentage in the normal milk flora. Their presence in milk has been considered to be relatively unimportant.

31. Butyric fermenting types.—Organisms causing butyric fermentation may be present in the milk, but seldom become active, because they are commonly anaërobic and so will not develop in milk kept under ordinary conditions, and the rapid growth of the lactic acid-forming bacteria prevents their growth. These organisms act on the milk-fat, decomposing it. Butyric acid fermentations are more common in old butter and cheese. In these, the fermentation causes a rancid flavor.

32. Molds and yeasts.—The cattle feed and the air of the barn always contain considerable numbers of yeasts and mold spores. Yeasts have been found by Hastings[12] to cause an objectionable fermentation in Wisconsin cheese. No further study of this group as factors in cheese-handling has been reported. Mold spores, especially those of the blue or green molds (Penicillum sp.) and the black molds (Mucors), are always abundant in milk. These spores are carried into all cheeses made from unpasteurized milk, in numbers sufficient to cover the cheeses with mold if they are permitted to grow. Pasteurization[13] kills most of them. The border-line series commonly referred to as the streptothrix-actinomyces group are also very abundant in all forage and are carried in large numbers into all milk and its products.

33. Bacterial contamination of milk.—When drawn from the cow, milk is seldom if ever sterile. Organisms usually work their way from the tip of the teat into the udder and multiply there. The fore milk usually contains more organisms than does that drawn later. Most of the bacterial contamination of the milk is due to the handling after it is drawn from the cow.

34. Germicidal effect of milk.—Authorities agree that when a bacterial examination of the milk is made, hour by hour, beginning as soon as it is drawn from the cow, there is no increase in the number of organisms for a period of several hours at first, but an actual reduction not infrequently takes place. This is called the "germicidal"[14] property of milk. The lower the temperature of the milk, the longer and less pronounced is the germicidal action; the higher the temperature, the shorter and more pronounced is this action.

This is explained as either: (1) a period of selection within which types of bacteria entering by accident and unadapted for growth die off; or (2) an actual weak antiseptic power in the milk-serum itself; or (3) the forming of clusters by the bacteria and so reducing the count.

In working on a small scale or on an experimental basis, this property at times introduces a factor of difficulty or error which is not to be lost sight of in the selection of the milk for such purposes.

35. Sources and control of bacteria in milk.—Most of the bacterial infection of milk is due to lack of care in handling. Some of the common sources[15] of contamination are: the air in the stable; the cow's body; the milker; the utensils; the method of handling the milk after it is drawn from the cow; unclean cheese factory conditions.

Since bacteria cause various kinds of fermentation, not only in the milk but in the products manufactured from it, the question of their control is of prime importance. There are two ways in which the bacterial growth in milk used for cheese-making may be controlled: (1) prevention of infection; (2) the retardation of their development when present. The former is accomplished by strict cleanliness, the latter by adequate cooling.

36. The cow.—The body of the cow may be a source of bacterial contamination. Bacteria adhere to the hair of the animal, and to the scales of the skin, and during the process of milking these are very liable to fall into the milk. To prevent this, the cow should be curried to remove all loose material and hair. Just before milking, the udder and flank should be wiped with a damp cloth; this removes some of the material, and causes the remainder to adhere to the cow.

37. Stable air.—If the air of the stable is not clean, it will be a source of contamination. Particles of dust floating in the air carry more or less bacteria, and these fall into the milk during the process of milking. To keep the stable air free from dust at milking time, all operations which stir up dust, such as feeding, brushing the cows, cleaning the floor, should be practiced after milking or long enough before so that the dust will have settled. It is a good plan to close the doors and to sprinkle the floor just before milking.

38. The milker himself may be a source of contamination. He should be clean and wear clean clothing. The hands should not be wet with milk during milking.

Fig. 3.—Types of small-top milk pails.

39. Utensils.—The utensils are an important source of bacterial contamination. The bacteria lodge in the seams and corners unless these are well-flushed with solder. From these seams they are not easily removed. When fresh warm milk is placed into such utensils, the bacteria begin to grow and multiply. All utensils with which milk comes in contact should first be rinsed with cold water and then thoroughly washed and finally scalded with boiling water, and drained or blown absolutely dry. They should then be placed in an atmosphere free from dust until wanted for use again. If an aërator is used, this should be operated in pure air, free from odors and dust. One of the greatest sources of bacterial contamination of cheese milk is the use of the milk-cans to return whey to the farms for pig feed. Frequently, sour whey is left in the cans until ready to feed. These cans are then not properly washed and scalded. The practice of pasteurizing the whey at the cheese factory is a great help in preventing this source of infection and the spreading of disease.

The use of a small-top milk pail[16] is to be especially recommended in preventing bacterial contamination. Because of the small opening, bacteria cannot easily fall into the milk in as large numbers as when the whole top of the pail is open. (See [Fig. 3.])

If a milking machine[17] is used, great care must be exercised to see that all parts that come in contact with the milk are cleaned after each milking, and then put in a clean place until ready to use again.

40. The factory.—Another source of contamination is the cheese factory itself. The cheese-maker should keep his factory in the cleanest condition possible, not only because of the effect on the milk itself, but as a stimulus for the producers to follow his example. All doors and windows in the factory should be screened to keep out flies.

41. The control of bacteria.—If, in spite of preventive measures, bacteria get into the milk, their growth can be retarded by controlling the temperature. If the temperature of the milk, as soon as drawn, can be reduced below that at which the bacteria grow and multiply rapidly, it will retard their development. In general, all milk should be cooled to 50° F. or below. In cooling the milk, it should not be exposed to dust or odors. One of the best methods of cooling is to set the can containing the milk into a tub of cold running water, and then stir. If running water is not available, cold well-water[18] may be used, but the water should be changed several times. If the milk is not stirred during the cooling process, it will not cool so rapidly, because the layer of milk next the can will become cold and act as an insulator to the remainder in the center of the can.

One way to destroy many of the bacteria in milk is by pasteurization. This consists in heating the milk to such a degree that the bacteria are killed, and then quickly cooling it. After pasteurization, the milk is so changed that some kinds of cheese cannot be made successfully.

42. Fermentation test.—When a cheese-maker is having trouble with gas in his cheese, or bad flavors, he can generally locate the source of difficulty. This can be done by making a small amount of cheese from each patron's milk, called a fermentation test.[19] Pint or quart fruit jars or milk bottles make suitable containers. They should be thoroughly washed and scalded, to be sure they are clean and sterile, and then covered to prevent contamination. As the milk is delivered to the factory, a sample is taken of each patron's milk. The best way to secure the sample is to dip the sterile jar in the can of milk as delivered and fill two-thirds full of milk.

The jars are then set in water at 110° F. to bring the temperature of the milk to 98° F. The jar should be kept covered. A sink or wash-tub makes a convenient place in which to keep the jars. When the temperature of the milk is 98° F., ten drops of rennet extract or pepsin is added to each jar. A uniform temperature of 98° F. should be maintained in the jars. This will necessitate the addition of warm water occasionally to the water surrounding the jars. When the milk is coagulated, the curd is broken up with a sterile knife. Precaution should

Fig. 4.—A gang sediment tester, one tester removed. be taken to sterilize the knife after using it in one jar before putting it into another. The best way to do this is to hold the knife for a minute in a pail of boiling water, after taking it out of each jar. The same precaution should be observed with the thermometer. Unless care is taken, contamination is liable to be carried from one jar to the other. After cutting, the whey is poured off. The temperature should be kept at 98° F. so that the organisms will have a suitable temperature for growth. The whey should be poured from the jars occasionally, usually about every half hour.

As the fermentation takes place, different odors will be noticed in different jars. In ten to twelve hours the jar should be finally examined for odors and the curd taken out and cut to examine it for gas pockets. By this means, bad flavors and gas in the cheese can be traced to their sources.

43. The sediment test.—The presence of solid material or dirt in the milk is always accompanied by bacterial contamination. By means of the sediment test, the amount of solid material can be determined. The test consists of filtering the milk through a layer of cotton; the foreign material is left on the cotton filter. Various devices for filtering the milk have been

Fig. 5.—A single sediment tester. manufactured. (Figs. 4 and 5.) In order to be able to compare the filters from the different dairy-men's milk, the same amount of each patron's milk is filtered, usually about a pint. These tests are usually made once or twice a month at the factory and the filters placed on a card where the dairy-men can see them. Much improvement in the quality of the milk has been accomplished by the use of the sediment test. The purpose of this test may be and often is defeated by the use of efficient strainers. Milk produced in an unclean way may be rendered nearly free from sediment if carefully strained. It must be remembered that the strainer takes out only the undissolved substances and that bacteria and soluble materials which constitute a very large part of the filth pass through with the milk.

CHAPTER III

COAGULATING MATERIALS

At the present time, two substances are used to coagulate milk for cheese-making,—rennet extract and commercial pepsin.[20] Many substances will coagulate milk, such as acids and other chemicals. Enzymes in certain plants will also coagulate it.

The curing or ripening of the cheese seems to depend on the physical and chemical properties of the curd, on the activity of certain organisms and on enzymes produced by them or in the milk. Rennet extract and pepsin are the only known substances which will produce curd of such character as will permit the desired ripening changes to take place. Until recently, rennet extract was principally used to coagulate the milk, but because of the scarcity, pepsin is now being substituted.

44. Ferments.—Many of the common changes taking place in milk are due to fermentations. The souring of milk is one of the most familiar cases of fermentation. The important change taking place is the formation of lactic acid from the milk-sugar. The change is brought about by certain living organisms, namely, the lactic acid-forming bacteria. Another familiar case of fermentation is the coagulation of milk by rennet extract or pepsin. In this case, the change is produced by a chemical substance, not a living organism. Fermentation may be defined as a chemical change of an organic compound through the action of living organisms or of chemical agents.

There are two general classes of ferments: (1) living organisms, or organized ferments; (2) chemical, or unorganized ferments. Organized ferments are living microorganisms, capable, as a result of their growth, of causing the changes. Unorganized ferments are chemical substances or ferments without life, capable of causing marked changes in many complex organic compounds, while the enzymes themselves undergo little or no change. These unorganized ferments are such as rennin, pepsin, trypsin, ptyalin. The rennet and pepsin must, therefore, be very thoroughly mixed into the milk to insure complete and uniform results, because they act by contact, and theoretically, if they could be recovered, might be used over and over again. Practically, the amount used is so small a percentage that recovery would be impractical even if possible.

45. Nature of rennet.—Two enzymes or ferments are found in rennet extract, rennin and pepsin. They are prepared from the secreting areas of living membranes of the stomachs of mammalian young. For rennet-making, these stomachs are most valuable if taken before the young have received any other feed than milk. Rennin at this stage appears to predominate over pepsin which is already secreted to some extent. With the inclusion of other feed, the secretion of pepsin comes to predominate. Rennin has never been separated entirely from pepsin. Both of these enzymes are secreted by digestive glands in the same area, perhaps even by the same glands. They are so closely related that many workers have regarded them as identical. In practical work the effectiveness of rennet preparations has been greatest when stomachs which have digested feed other than milk are excluded. The differences, therefore, however difficult to define, appear to be important in the commercial preparation of rennet.

It was the practice until a few years ago for each cheese-maker to prepare his own rennet extract. Each patron was supposed to supply so many rennets. Now commercial rennet extract and pepsin are on the market; however, some Swiss cheese-makers prefer to make their own rennet extract. For sheep's and goat's milk cheese, some makers hold that rennet made from kid or lamb stomachs is best for handling the milk of the respective species. The objection to the cheese-maker preparing his own rennet extract is that it varies in strength from batch to batch and is liable to spoil quickly. Taints and bad odors and flavors develop in it and so taint the cheese.

46. Preparation of rennet extract.—This extract may be manufactured commercially from digestive stomachs of calves, pigs or sheep. An animal is given a full meal just before slaughtering; this stimulates a large flow of the digestive juices, containing the desired enzymes.

The stomach is taken from the animal, cleaned, commonly inflated and dried. It may be held in the dry condition until needed for use. Such stomachs are usually spoken of as "rennets" in the trade. Such old rennets may be seen to-day hanging from the rafters of some of the older cheese factories. When wanted for use, rennets are placed in oak barrels and covered with water. Before placing them in the barrel, they are cut open so that the water may have easy access. Salt is usually added to the water at the rate of 3 to 5 per cent. They are stirred and pounded in this solution from five to seven days. At the end of this time, they are wrung through a clothes-wringer to remove the liquid. The rennets are put back into a fresh solution of salt and water, the object being to obtain all the digestive juices possible. They are usually soaked from four to six weeks. At the end of this time, most of the digestive juices will have been removed. The liquid portion is passed through a filter made of straw, charcoal and sand. When clean, an excess of salt is added to preserve it.

Such extracts cannot be sterilized by heat because the necessary temperature would destroy the enzyme. Effective disinfectants cannot be used in food products. The extract, therefore, should be kept cool to retard bacterial growth. The extract is kept in wooden barrels, stone jugs or yellow glass bottles to protect it from light, which is able to destroy its activity. Rennet extract should be clear, with a clean salty taste and a distinct rennet flavor. There should be no cloudy appearance and no muddy sediment in properly preserved rennet. Rennet extract is on the market in the form of a liquid and a powder, the former being much more common. The commercial forms of rennet have the advantage in the skill used in their preparation and standardization. The combined product from large numbers of stomachs may not be as effective a preparation as the most skillfully produced sample from the very choicest single stomach, but it gives a uniformity of result which improves the average product greatly.

47. Pepsin.—Pepsin is on the market in several commercial forms, as a liquid, scale pepsin and in a granular form known as spongy pepsin. Some commercial concerns put out a preparation which is a mixture of rennet extract and commercial pepsin.

48. Chemistry of curdling.—The chemistry of casein[21] and of curd formation under the influence of acid and rennet extract and pepsin has been the subject of many years' research. While many points remain unsettled, the general considerations together with a large mass of accepted facts may be presented and some of the unsolved problems pointed out as left for future researches.

Casein is a white amorphous powder, practically insoluble in water. It is an acid and as such readily dissolves in solutions of the hydroxides or the carbonates of alkalies and alkaline earths by forming soluble salts.

Pure casein salt solutions and fresh milk do not coagulate on boiling, but in the presence of free acid coagulation may take place below the boiling temperature. The coagulum formed in the case of milk includes fat and calcium phosphate. The slight pellicle which coats over milk when it is warmed is of the same composition.

49. Use of acid.—A commonly accepted explanation of the precipitation of casein by acids is that the casein is held in solution by chemical union with a base (lime in the case of milk); that added acid removes the base, allowing the insoluble casein to precipitate; and that excess of acid unites with casein, forming a compound which is more or less readily soluble.

50. Robertson's theory.—According to Robertson's conception, in a soluble solution of a protein or its salt, the molecules of the protein unite with each other to a certain extent, in this way forming polymers. The reaction is reversible, and the point of equilibrium between the compound and its polymeric modification varies under the influence of whatever condition affects the concentration of the protein ions. Addition of water, or of acid, alkali or salt, or the application of heat has such an effect, and consequently alters the relative number of heavier molecule-complexes. Robertson's experiments give evidence that one of the effects of increase of temperature on a solution of casein is a shifting of the equilibrium in the direction of the higher complexes. He explains coagulation as being a result of these molecular aggregates becoming so large as to assume the properties of matter in mass and to become practically an unstable suspension and then a precipitate. The acid curd then is casein or some combination of casein with the precipitant acid.

51. Rennet curd.—Rennet extract and pepsin coagulation differs from coagulation by acids, and cannot be looked on as a simple removal of the base from a caseinate. The presence of soluble calcium salts (or other alkaline earth salts) seems to be essential, and the precipitate formed is not casein or a casein salt, but a salt of a slightly different nucleoalbumin called "paracasein." Many writers, following Halliburton, call this modification produced by rennin the "casein" and that from which it is derived, "caseinogen." Foster and a few others have used the term "tyrein" for the rennet clot.

A number of investigations have been made on the conditions essential or favorable to formation of the coagulum, especially with regard to the effects of the degree of acidity and of conditions affecting the amount of calcium present, either as free soluble salt or bound to the casein. Soluble salts of calcium, barium and strontium favor or hasten coagulation, while salts of ammonium, sodium and potassium retard or prevent coagulation.

The bulk of the coagulum from milk is a calcium paracaseinate, but it carries down with it calcium phosphate and fat, both of which bodies have been helped to remain in their state of suspension in milk by the presence of the casein salt. Lindet (1912) has concluded that about one-half of the phosphorus contained in the rennet curd is in the form of phosphate of lime (probably tricalcic), the other half being organically combined phosphoric acid.

52. Hammarsten's theory.—According to Hammarsten (1877, 1896), whose view has been commonly held, the distinctive effect of the ferment is not precipitation but the transformation of casein into paracasein. This is evidenced by the fact that if rennet be allowed to act on solutions free from lime salts no precipitate occurs; but there is an invisible alteration of the casein, for now, even if the ferment be destroyed by boiling the solution, addition of lime salts will cause immediate coagulation. (See also Spiro, 1906.) Hence the process of rennet coagulation is a two-phase process; the first phase is the transformation of casein by rennin, the second is the visible coagulation caused by lime salts.

Furthermore, if the purest casein and the purest rennin were used, Hammarsten always found after coagulation that the filtrate contained very small amounts of a protein. This protein he designated as the "whey protein."

In accordance with these observations, Hammarsten (1911) explains the rennin action "as a cleavage process, in which the chief mass of the casein, sometimes more than 90 per cent, is split off as paracasein, a body closely related to casein, and in the presence of sufficient amounts of lime salts the paracasein-lime precipitates out while the proteose-like substance (whey-protein) remains in solution."

By continued action of rennin on paracasein, a further transformation has been found in several cases (Petry, 1906; Van Herwerden, 1907; Van Dam, 1909), but perhaps due to a contamination of the rennin with pepsin, or to the identity of these two enzymes. The action which forms paracasein and whey-protein takes place in a short time (Hammarsten, 1896; Schmidt-Nielson, 1906). The composition and solubilities of paracasein have received considerable attention. (See Loevenhart, 1904; Kikkoji, 1909; Van Slyke and Bosworth, 1912.) It is more readily digested by pepsin-hydrochloric acid than is casein (Hosl, 1910).

53. Duclaux theory.—Duclaux (1884) and Loevenhart (1904) and others do not accept Hammarsten's theory; but to most workers it seems probable, at least, that the action of the rennin is to cause a cleavage of casein with formation of paracasein. However, the chemical and physical differences observed between casein and paracasein appear to be so slight that Loevenhart and some others think that they are only physical, perhaps differences in the size of the colloid or solution aggregates. Loevenhart conceives of a large part of the work of the rennet (or of the acid, in acid and heat coagulation) as being a freeing of the calcium to make it available for precipitation. Some think that the aggregates of paracasein are larger than those of casein, but there is more evidence of their being smaller, which idea corresponds with the findings of Bosworth, though he looks on the change as a true cleavage.

54. Bang's theory.—Another description of the precipitation is given by Bang (1911), who studied the progress of the coagulation process by means of interruptions at definite intervals. His observations confirm the idea that rennin causes the formation of paracasein, and that the calcium salt serves only for the precipitation of the paracasein; the rennin has to do also with the mobilizing of lime salts. According to Bang, before coagulation occurs, paracaseins with constantly greater affinity for calcium phosphate are produced. These take up increasing amounts of calcium phosphate, until finally the combination formed can no longer remain in solution.

55. Bosworth's theory.—By a very recent work of L. L. Van Slyke and A. W. Bosworth (Van Slyke and Bosworth, 1912, 1913; and Bosworth and Van Slyke, 1913), in which ash-free casein and paracasein were compared as to their elementary composition, and as to the salts they form with bases, and the properties of these salts, it is indicated that the two compounds are alike in percentage composition and in combining equivalent, the paracasein molecule being one-half of the casein molecule. Moreover, Bosworth (1913) has shown that, if the rennin cleavage be carried out under conditions which avoid autohydrolysis, no other protein is formed; also that, if the calcium caseinate present be one containing four equivalents of calcium, the paracaseinate does not precipitate, save in the presence of a soluble calcium salt, while, if the calcium caseinate be one of two equivalents of base, rennin does cause immediate coagulation. Bosworth concludes that the rennin action is a cleavage (probably hydrolytic) of a molecule of caseinate into two molecules of paracaseinate, the coagulation being a secondary effect due to a change in solubilities, dicalcium paracaseinate being soluble in pure water but not in water containing more than a trace of calcium salt, and the monocalcium caseinate being insoluble in water. The alkali paracaseinates, as well as caseinates, are soluble. This explanation seems to promise to harmonize the observations with regard to acidity and the effects of the presence of soluble salts. This theory represents, therefore, many years of continuous work at the New York Experiment Station centered primarily on American Cheddar cheese. Disputed points remain for further study but these workers have contributed much toward a clear description of the chemical constitution of casein as affected by rennet action and bacterial activity.

The investigations of these authors and of Hart with regard to the changes which the paracasein, the calcium and the phosphorus undergo during the ripening of cheese (Van Slyke and Hart, 1902, 1905; Van Slyke and Bosworth, 1907, 1913; Bosworth, 1907) contributed to this interpretation.

Bang, Ivar, Ueber die chemische Vorgang bei der Milchgerinnung durch Lab, Skand. Arch. Physiol. 25, pages 105-144; through Jahresb. u. d. Fortsch. d. Thierchem. 41, pages 221-222, 1911.

Bosworth, A. W., The action of rennin on casein, N. Y. Exp. Sta. Tech. Bul. 31, 1913.

Bosworth, A. W., Chemical studies of Camembert cheese, N. Y. Exp. Sta. Tech. Bul. 5, 1907.

Bosworth, A. W., and L. L. Van Slyke, Preparation and composition of basic calcium caseinate and paracaseinate, Jour. Biol. Chem. Vol. 14, pages 207-210, 1913.

Duclaux, Émile, Action de la présure sur le lait, Compt. Rend. Acad. Sci. 98, pages 526-528, 1884.

Hammarsten, Olof, Zur Kenntnis des Caseins und der Wirkung des Labfermentes, Nova. Acta Regiae Soc. Sci. Upsaliensis in Memoriam Quattuor Saec. ab Univ., Upsaliensi Peractorum, 1877.

Hammarsten, Olof, Ueber das Verhalten des Paracaseins zu dem Labenzyme, Zeit. physiol. Chem. 22, pages 103-126, 1896.

Hammarsten, Olof, A text book of physiological chemistry, from the author's 7th German edition, 1911.

Hosl, J., Unterschiede in der tryptischen und peptischen Spaltung des Caseins, Paracaseins und des Paracaseinkalkes aus Kuh- und Ziegenmilch, Inaug. Diss. Bern., 31 pp., 1910.

Kikkoji, T., Beitrage zur Kenntniss des Caseins und Paracaseins, Zeit. physiol. Chem. No. 61, pages 130-146, 1909.

Lindet, L., Solubilité des albuminoides du lait dans les éléments du sérum; rétrogradation de leur solubilité sous l'influence du chlorure, Bul. Soc. Chim. (ser. 4) 13, pages 929-935.

Lindet, L., Sur les éléments mineraux contenus dans la caseine du lait, Rep. Eighth Internat. Congr. of Applied Chem. 19, 199-207, 1912.

Loevenhart, A. S., Ueber die Gerinnung der Milch, Zeit. physiol. Chem. 41, pages 177-205, 1904.

Petry, Eugen, Ueber die Einwirkung des Labferments auf Kasein, Beitrage z. Chem. Physiol. u. Path. 8, pages 339-364, 1906.

Robertson, T. Brailsford, On the influence of temperature upon the solubility of casein in alkaline solutions, Jour. Biol. Chem. 5, pages 147-154, 1908.

Schmidt-Nielson, Sigval, Zur Kenntnis des Kaseins und der Labgerinnung, Upsala läkaref. Förh. (N. F.) No. 11, Suppl.

Hammarsten Festschrift No. XV, 1-26; through Jahresb. u. d. Fortschr. d. Thierchem. No. 36, pages 255-256, 1906.

Spiro, K., Beeinflussung und Natur des Labungsvorganges, Beitrage z. Chem. Physiol. u. Path. 8, pages 365-369, 1906.

Van Dam, W., Ueber die Wirkung des Labs Auf. Paracaseinkalks, Zeit. physiol. Chem. No. 61, pages 147-163, 1909.

Van Herwerden, M., Beitrag zur Kenntnis der Labwirkung auf Casein, Zeit. physiol. Chem. 52, pages 184-206, 1907.

Van Slyke, L. L., and A. W. Bosworth, I. Some of the first chemical changes in Cheddar cheese. II. The acidity of the water extract of Cheddar cheese, N. Y. Exp. Sta. Tech. Bul. 4, 1907.

Van Slyke, L. L., and A. W. Bosworth, Composition and properties of some casein and paracasein compounds and their relations to cheese, N. Y. Exp. Sta. Tech. Bul. 26, 1912.

Van Slyke, L. L., and A. W. Bosworth, Method of preparing ash-free casein and paracasein, Jour. Biol. Chem. Vol. 14, pages 203-206, 1913.

Van Slyke, L. L., and A. W. Bosworth, Preparation and composition of unsaturated or acid caseinates and paracaseinates, Ibid. Vol. 14, pages 211-225, 1913.

Van Slyke, L. L., and A. W. Bosworth, Valency of molecules and molecular weights of casein and paracasein, Ibid. Vol. 14, pages 227-230, 1913.

Van Slyke, L. L., and A. W. Bosworth, Composition and properties of the brine-soluble compounds in cheese, Jour. Biol. Chem. 14, pages 231-236, 1913.

Van Slyke, L. L., and E. B. Hart, A study of some of the salts formed by casein and paracasein with acids; their relations to American Cheddar cheese, N. Y. Exp. Sta. Bul. 214, 1902.

Van Slyke, L. L., and E. B. Hart, Casein and paracasein in some of their relations to bases and acids, American Chem. Jour. 33, pages 461-996, 1905.

Van Slyke, L. L., and E. B. Hart, Some of the relations of casein and paracasein to bases and acids, and their application to Cheddar cheese, N. Y. Exp. Sta. Bul. 261, 1905.

CHAPTER IV

LACTIC STARTERS

Acidity in cheese-making arises almost exclusively from the lactic acid produced from the fermentation of milk-sugar (lactose) by bacteria. Hydrochloric acid is used in the Wisconsin[22] process of making pasteurized milk cheese and sometimes for making skimmed-milk curd for baking purposes. It is regularly used in precipitating casein not for food but for manufacturing purposes.

56. Acidifying organisms.—Many species of bacteria have been shown to possess the power to produce lactic acid by fermenting lactose. In practice, however, the cheese-maker seeks to control this fermentation by the actual introduction of the desired organisms and by the production of conditions which will insure this dominance through natural selection. For this purpose the initial souring for most types of cheeses is produced by some variety of the species originally described by Esten[23] and commonly referred to as Bacterium lactis-acidi, but variously named as B. acidi-lactici, Streptococcus lacticus, B. guntheri by different authors. Organisms of this series dominate all other species in milk which is incubated at 70° F. They produce a smooth solid mass without a sign of gas holes and without the separation of whey from the curd, and develop in milk a maximum acidity of about 0.90 of one per cent when titrated as lactic acid. (For titration see [Chapter V.]) This species is usually present in small numbers in fresh milk. There are many varieties or strains of the species with differing rates of activity and measurable differences in acid produced but with approximately the same qualitative characters. Most commercial starters for cheese- and butter-making belong to this group of species, although special mixtures with other organisms are prepared for special purposes. In addition to this group, most varieties of cheese contain some members of the colon-aërogenes group. When the milk is in proper condition, the activity of this group should be held in check by the early and rapid development of acid. Free development of members of this group usually shows itself in the presence of gas holes in the curd.

57. Starter.—The practice of using pure cultures in cheese-making has brought about the development of factory methods of producing day by day cultures of the organisms desired, in quantities sufficient to inoculate the total quantity of milk used in manufacture. For this purpose milk is mostly used and the product is known as "starter." For cheese-making purposes, a starter is a substance used in the manufacture of dairy products having a predominance of lactic acid-forming microorganisms in an active state. There are two general classes of starter: (1) Natural starter; (2) commercial starter.

58. Natural starter.—Milk, or other similar substance, which has become sour or in which large numbers of lactic acid-forming organisms are present, is called a natural starter when used in the manufacture of dairy products. To secure clean-flavored milk, the cheese-maker usually selects the milk of some producer who usually brings good milk and allows it to sour naturally for use the next day. There is often a variation from day to day in the milk delivered by the same producer, so that the cheese-maker is not certain of a uniform quality in his fundamental material. While the lactic acid-forming organisms are developing, other organisms may also be present in numbers sufficient to produce bad flavors. If a starter has any objectionable flavor, it should not be used. Natural starters very commonly develop objectionable flavors which at first are very difficult to recognize. When natural starters with objectionable but not easily recognizable odors are used, the effect may be seen on the cheese. Milk, sour whey and buttermilk are materials commonly used as natural starter. A common difficulty in skimmed-milk cheese is caused by the use of buttermilk as a starter.

59. Commercial starter or pure cultures.—The alternative practice consists in the introduction of pure cultures of known strains of lactic bacteria into special milk to make the starter. Since these cultures must be prepared by a bacteriologist, commercial laboratories have developed a large business in their production. Many such commercial brands are manufactured under trade-marked names. Some of these cultures represent races of lactic bacteria cultivated and cared for efficiently, hence uniformly valuable over long periods of time. Others carelessly produced are worthless, or even a peril to the user.

These organisms are usually shipped in small quantities in bottles of liquid or powder, or in capsules of uniform size. The contents may be either the culture medium upon which the organisms grew or inert substance designed merely to hold the bacteria in inactive form. In either solid or liquid form, the producer of the culture should guarantee its activity up to a plainly stated date.

It is the problem[24] of the cheese-maker or butter-maker to increase this small amount of lactic acid-forming organisms to such numbers and in such active condition that it may be used in the factory; while being built up, these organisms must be kept pure. The usual practice is to allow them to develop in some material, usually whole milk or skimmed-milk; dissolved milk powder may be used in the place of milk.

60. Manufacturer's directions.—The manufacturer usually sends directions with his starter preparation, telling how it should be used to secure the best result. These directions apply to average conditions and must be varied to suit the individual instances so that a good starter will be the result. The directions usually state the amount of milk necessary for the first inoculation. It is usually a small amount, one or two quarts. After the specific amount has been selected, this milk should be pasteurized.

61. Selecting milk.—The milk for use in starter-making should be selected with very much care. Only clean-flavored sweet milk, free from undesirable micro-organisms, should be used in the preparation of starter. The milk is ordinarily chosen from a producer whose milk is usually in good condition. The quality of the milk can be determined by the use of the fermentation test. (See [Chapter II.]) It is better to choose only the morning's milk for the making of starter, because the bacteria have not had so much opportunity to develop. In no case should the mixed milk be used in the preparation of starter, as this eliminates all opportunity for selection. The flavor of the starter will be the same as that of the milk from which it is made.

62. Pasteurization is the process of heating to a high temperature for a given length of time and quickly cooling. It kills most of the micro-organisms in the milk. In other words, it makes a clean seed-bed for the pure culture. The temperatures of pasteurization recommended for starter-making differ with the authority. A temperature of 180° F. for thirty minutes or longer seems to be very satisfactory, since under these conditions nearly all the micro-organisms in the milk are killed.

63. Containers.—Various kinds of containers may be used for starter-making. One-quart glass fruit jars or milk bottles make very satisfactory containers, because the condition of the starter may be seen at any time. They are also easily cleaned. They have the disadvantage, however, of being easily broken, if the temperature is suddenly changed, or if severely jarred. Tin containers may also be used. Such containers are not easily broken, but they are harder to clean and must be opened to examine the contents; hence the liability of contamination is very much greater.

This small amount of milk may be pasteurized by placing the container in water heated to the desired temperature. A very satisfactory arrangement is to cut of a barrel, and place a steam pipe in it. The barrel can then be filled partly full of water and heated by steam. The bottles of milk to be pasteurized are hung in the water in the barrel. Two or three more bottles should be prepared than it is expected will be used as some of the bottles are liable to be broken while cooling or heating. The bottles should be filled about two-thirds full. This leaves room enough to add the mother starter and later to break up the starter to examine it. It is desirable not to have the milk or starter touch the cover since contaminations are more likely. It is a good plan when pasteurizing to have one bottle as a check. This may be filled with water and left open and the thermometer placed in it. A uniform temperature may be obtained by shaking the bottles.

64. Adding cultures.—After being pasteurized, the milk should be cooled to a temperature of 80° F. This is a suitable temperature for the development of the lactic acid-forming organisms. The commercial or pure culture should now be added to the milk at the rate specified in the directions. Care should be exercised in opening bottles not to put the covers in an unclean place. A sterile dipper is a good place to put them. After the culture has been added to the milk, it should be mixed thoroughly by shaking the bottle. This should be repeated every fifteen or twenty minutes for four or five times. This is done to make certain that the culture is thoroughly mixed with the milk. The milk should be placed in a room or incubator as near 80° F. as possible, in order to have a uniform temperature for the growth of the organisms. The bacteria in the pure culture are more or less dormant so that a somewhat higher temperature than the ordinary is necessary to stimulate their activity. This milk should be coagulated in eighteen to twenty-four hours, depending largely on the uniformity of the temperature during incubation.

65. Cleanliness.—To produce a good starter, great care should be exercised that all utensils coming in contact with the milk are sterile. After the milk is in the container in which the starter is made, it should be kept covered as continuously as possible. Thermometers should not be put into it to ascertain the temperature. When examining the starter, do not dip into it, but pour out, as this prevents contamination. The cover, when removed from the container, should be put in a sterile place in such way that the dirt will not stick to it and later get into the starter.

66. "Mother" starter or startoline.—The thickened sour milk obtained by inoculating the sweet pasteurized milk with pure culture of lactic acid-forming bacteria is known as "mother starter" or "startoline."

67. Examining starter.—This starter should be examined carefully as to physical properties, odor and taste. The coagulation should be smooth, free from whey and gassy pockets or bubbles. Sometimes the first few inoculations from a new culture will show signs of gas, but, usually, this will quickly disappear, and not injure the starter. It should have a clean sour cream odor and clean, mild, acid flavor. After breaking up it should be thick and creamy, entirely free from lumps. This starter may have an objectionable flavor, due to the media in which the organisms were growing when shipped. In such cases it is necessary to carry the starter one or two propagations to overcome the flavor, to enliven the micro-organisms and to secure the quantity desired.

68. Second day's propagation.—For the second day, the milk for the starter is selected as on the first day. It is pasteurized, and this time cooled to 70° F. The milk is cooled a trifle colder the second day than the first, because the organisms have become more active and hence do not require as high a temperature to grow. Instead of inoculating with powder, as was done the first day, the mother starter prepared the first day is used. This requires only a very small amount, perhaps a tablespoonful to a quart bottle. It should be thoroughly mixed with the milk. This starter may have the

Fig. 6.—An improved starter-can. flavor of the media used in the laboratory culture, therefore may need to be carried one or two days more to eliminate it. After the flavor has become normal, the mother starter is ready for commercial use.

69. Preparation of larger amount of starter.—The first thing to determine is the quantity of starter required. As much milk should be carefully chosen as the amount of starter desired. This milk should then be pasteurized. An improved starter-can ([Fig. 6]) may be used in the pasteurization of the milk and the making of starter, or a milk-can ([Fig. 7]) placed in a tub of water in which there is a steam pipe. The former requires mechanical power to operate the agitator, but the latter can be used where mechanical power is not available. In the latter the milk and starter are stirred by hand. This is the kind of apparatus more

Fig. 7.—A simple device for the preparation of starter. often found in cheese factories.

If possible, this milk should be pasteurized to 180° F. for thirty minutes; this kills most of the bacteria and spores. The milk should be cooled to 60°-65° F., the temperature of incubation. This temperature may be varied with conditions, so that the starter will be ready for use at the desired time. The higher the temperature, the less time is required to ripen the starter.

70. Amount of mother starter to use.—The mother starter prepared the day before is now used to inoculate the starter milk. The amount to use will depend on:

1. Temperature of milk when mother starter is added;

2. Average temperature at which the milk will be kept during the ripening period;

3. Time allowed for starter to ripen before it is to be used;

4. Vigor and acidity of the mother starter added. There is a very wide range as to the amount of mother starter required, from 0.5 of one per cent to 10 per cent being used under different conditions.

Some operators prefer to add the mother starter while the milk is at a temperature of about 90° F., before it has been cooled to the incubating temperature. This reduces the amount of mother starter necessary.

If an even incubating temperature can be maintained, it will require less mother starter than if the temperature goes down.

If the ripening period is short, it will require a larger amount of mother starter, than if the ripening period is longer. If the starter has a low acidity or weak body indicating that organisms are of low vitality, it will require more mother starter.

71. Qualities.—The starter, when ready to use, may or may not be coagulated; a good idea of the quality of the starter may be gained by the condition of the coagulation. The coagulation should be jelly- or custard-like, close and smooth, entirely free from gas pockets and should not be wheyed off.

When broken up, the starter should be of a smooth creamy texture and entirely free from lumpiness or wateriness. It should have a slightly pronounced acid aroma. The starter should be free from taints and all undesirable flavors; the flavor should be a clean, mild acid taste.

72. How to carry the mother starter.—Some mother starter must be carried from day to day to inoculate the large starter. This may be carried or made in several ways:

1. Independently: By this method a mother starter is made and carried entirely separately from the large starter. It requires more time and work, but is by far the best method. With a good mother starter, there is not so much danger of the larger starter becoming poor in quality.

2. Mother starter may be made by dipping pasteurized milk from that prepared for the large starter with sterile jars and then inoculating these jars separately. By this method, if the milk selected for the large starter is poor, the mother starter for the next day will be the same. It is very difficult by this method to carry a uniform, high quality mother starter.

There is danger that the container used for the mother starter may not be sterile, and there is also danger of contamination in transferring the milk.

3. Another practice is to hold over some of the large starter used to-day for mother starter. This is by far the easiest method. By this practice, there is no certainty of the quality of the starter, because there is little or no control of the mother starter. If the large starter is for some reason not good, there is no separate reserve of mother starter on which to rely.

73. Starter score-cards.—The use of a score-card tends to analyze the observations in such a way as to emphasize all the characteristics desired in the starter. Such an analysis seeks to minimize the personal factor and produce a standardization of the quality. The score-card finally reduces the qualities of the starter to a numerical basis for ease of comparison. Many score-cards have been proposed but the one preferred by the authors is that used by the Dairy Department of the New York State College of Agriculture, which is as follows:

The qualities mentioned in this score-card can be quickly and easily determined by examining and tasting the starter and by making an acid test of a sample. The acid test is conducted as with milk (see [Chapter II]) except the starter must be rinsed out of the pipette with pure water. Some starter score-cards call for a bacterial examination and counting of the starter organisms. This takes a considerable period of time and is not entirely necessary. The physical properties and acid test are closely correlated with the presence of the desired organisms.

74. Use of starter.—If all milk could be clean and sweet and the only fermentation from it were the clean acid type, a starter would be useless. Such milk is hard to obtain; therefore, a starter is used to overcome the bad fermentation. This improves the flavor, body and texture of the cheese. The common contaminations which the starter will tend to correct are:

1. Gas-producing bacteria.
2. Yeasts.
3. Bad flavors or taints.

The length of time a starter may be carried depends on the accuracy and carefulness of the maker. This calls for scrupulous attention to the temperature, the selection of milk and keeping out contaminations. The maker must remember that a starter is not merely milk, but milk full of a multitude of tiny plants, very sensitive to food, temperature, clean surroundings and the quantity of their own acid.

75. The amount of starter to use depends on the amount of acid desired in the milk for any particular kind of cheese. The great abuse of starter is the practice of using too much. It is better and safer to add starter a little at a time and several times than to add too much at once. When starter is added to milk for cheese-making, it should be strained to remove any lumps; otherwise an uneven color is likely to result.

76. Starter lot-card.—For certain dairy operations, a permanent record is desired. This is especially true in the making of starter and certain varieties of cheese. A lot-card not only serves as a record but also points out the succeeding steps of the operation. This latter is especially useful for beginners and students. Page 53 shows a desirable lot-card to be used when making starter. Each operation has been referred to the page in the text where it is discussed. This makes this particular lot-card an index to the whole process of starter-making as here treated.

CHAPTER V

CURD-MAKING

Aside from the purely sour-milk cheeses, the coagulum or curd resulting from rennet action is the basis of cheese-making. The finished cheese, whatever its final condition, is primarily dependent on a particular chemical composition and fairly definite physical characters in the freshly made curd mass. These characters are determined by a series of factors under control of the cheese-maker. Assuming the milk to be normal in character, success depends on the use of a proper combination of these factors. The possible variations in each factor together with their number makes an almost infinite series of such combinations possible. The essential steps in the process are, therefore, presented as underlying all cheese-making. The special adaptations of each factor are considered in the discussion of the varieties group by group.

These factors follow:

A. The coagulation group:

1. Fat-content of the milk.

2. The acidity of the milk.

3. The temperature of renneting.

4. The effective quantity of rennet.

5. Curdling period or the time allowed for rennet action.

B. The handling group:

6. Cutting or breaking the curd.

7. Heating (cooking) or not heating.

8. Draining (including pressing, grinding and putting into hoops or forms).

77. The composition of the milk.—The fat percentage in the milk in the cheese-vat should be known to the cheese-maker and be strictly under his control. The fat tester and the separator make this clearly possible. He can go further. Milk from particular herds whose quality is a matter of record from the routine test of each patron's milk may be selected and brought together for the manufacturer of cheese of special quality. Control of casein or lactose, on the contrary, is not nearly so practicable. The purchase of milk on the fat test has become so well established in most dairy territories, as to insure the presence and constant use of the tester. A fat test of the mixed product in the cheese-vat in connection with established tables thus insures an accurate knowledge of the materials which go into each day's cheese. For some varieties of cheese, whole milk should always be used. For other varieties, the addition or removal of fat is regularly recognized as part of the making process. The presence of added fat or the removal of fat affects the texture of the product and the details of the process of making.

78. Cheese color.—An alkaline solution of annatto is usually used as a cheese color. This colors both casein and fat in contrast to butter color which is an oil solution of the dye and mixes only with the fat. Cheese color is added to the milk in making some varieties of cheese, and not for others. When lactic starter is used, the color should be added after the starter and just before the addition of the rennet. The amount is determined by the color desired in the cheese. The usual amount varies from one to four ounces to each thousand pounds of milk. Before adding, the color should be diluted in either milk or water, preferably water. It should then be mixed thoroughly with the milk.

79. The acidity factor.—Milk as drawn shows a measurable acidity when titrated to phenolphthalein with normal sodium hydroxide. This figure varies with the composition of milk. Casein itself gives a weakly acid reaction with this indicator. Calculated as lactic acid, this initial acidity varies within fairly wide limits, records being found from 0.12 to 0.21 of one per cent or even more widely apart. Commonly, however, such titration shows 0.14 to 0.17 per cent. Some forms of cheese (Limburger, Swiss, Brie) are made from absolutely fresh milk. Acidity from bacterial activity is important as a factor in the making of most types of cheese and probably in the ripening of all types.

Increasing the acidity of the milk hastens rennet action and within limits produces increased firmness of the curd. If carried too high, acidity causes a grainy or sandy curd. Normally fresh milk is sufficiently acid in reaction when tested to phenolphthalein to permit rennet to act, but the rate of action increases rapidly with the development of acid. Increase of acidity may be accomplished: (a) by the addition of acid as has been done by Sammis[25] and Bruhn in pasteurized milk for Cheddar cheese; or (b) by the development of acid through the activity of lactic organisms, which is the usual way. For renneting, the acidity necessary for particular cheeses runs from that of absolutely fresh milk still warm (as in French Brie, Limburger, Swiss, Gorgonzola) through series calling for increase of acidity, hundredth by hundredth per cent calculated as lactic acid. This ranges from 0.17 to 0.20 per cent as is variously used in American factory Cheddar to about 0.25 to 0.28 per cent as obtained by adding acid in Sammis' method. This method is discussed under the heading "Cheddar Cheese from Pasteurized Milk" (p. 229) since it requires special apparatus and has not thus far been used with other types of cheese. For the development of acidity by the action of bacteria, lactic starter is almost universally used. This may be added in very small quantities and the acidity secured by closely watching its development or by adding starter in amount sufficient to obtain the required acidity at once. In either case, the cheese-maker needs to know the rate of action of the culture to insure the proper control of the process. The amount of acid already present when the rennet is added affects not only the texture of the curd as first found, but within limits indicates also the rate at which further acidity may be expected to develop.

A series of experiments in making Roquefort were tabulated to show the rate of acidification from various initial points. In the graphs ([Fig. 8]) the curves for acid development are parallel after the determination reaches 0.30 per cent. These experiments were made at a temperature 80° to 84° F. Milk at the lowest acidities tried developed titratable acid very slowly. A period of several hours was required to produce sufficient acid to affect the curd texture. When the acid reached 0.25 per cent by titration, the further rise was rapid and all the lines became almost straight and parallel after the titration reached 0.30 per cent. If this rapid souring occurred after the completion of the cheese-making process, the texture of the experimental cheese was not measurably affected. In those cases, however, in which 0.30 per cent was reached before the cheese reached its final form in the hoop, the texture of the ripened cheese was entirely different from that desired for this variety under experiment. These curves apply directly to but one cheese process in which a particular combination of acidity, rennet and time is used to obtain a very delicately balanced result. In other varieties it is equally important to obtain exactly the adjustment of these factors which will bring the desired result.

Fig. 8.—The acidification of Roquefort cheese.

80. Acidity of milk when received.—If proper care has been taken, milk should be delivered to the factory fresh, clean and without the development of acid. If the milk has not been handled properly, the early stages of souring or some other unfavorable fermentation will have developed. Such milk may develop too much acid, or gas, or any one of several objectionable flavors during the making and ripening of the cheese. Some cheese-makers become very expert in detecting the first traces of objectionable qualities, but most makers are dependent on standardized tests to determine whether milk shall be accepted or rejected, and when accepted to determine the rate at which it may be expected to respond during the cheese-making process.

Various tests have been devised to determine the amount of acid present in milk. There are two tests commonly used in cheese-factories. One is known as the "acid test" and the other the "rennet test."

81. The acid test[26] is made by titrating a known amount of milk ([Fig. 9]) against an alkali solution of known strength, using phenolphthalein as an indicator. The object of the indicator is to tell the condition of the milk, whether it is acid, alkaline or neutral. The indicator does not change in an acid solution but turns pink when the solution is or becomes alkaline. To make the test, a known quantity of the material to be tested is placed in a

Fig. 9.—An acid tester. white cup, and to this several drops of indicator are added. As an indicator, a 1 per cent solution of phenolphthalein in 95 per cent alcohol is commonly used. As an alkali solution, sodium hydroxide (NaOH) is used in the standardized strength usually either tenth (N/10) normal or twentieth (N/20) normal. This solution should be obtained in some one of the standardized forms commercially prepared. The alkali is added, drop by drop, from a graduated burette until a faint pink color appears. This shows that the acid in the milk has been neutralized by the alkali. The amount of alkali that has been used can be determined from the burette. Knowing the amount of milk and alkali solution used, it is easy to calculate the amount of acid in the substance tested. The results are usually expressed either as percentages of lactic acid or preferably as cubic centimeters of normal alkali required to neutralize 100 or 1000 c.c. of milk. This kind of test is on the market under different names, such as Mann's, Publow's, Farrington's and Marschall's.

82. Rennet tests.—Several rennet tests have been devised, but the one most widely used is the Marschall ([Fig. 10]). This consists of a 1 c.c. pipette to measure the rennet extract, a small bottle in which to dilute the extract, a special cup to hold the milk and a spatula to mix the milk with the rennet extract.

Fig. 10.—Marschall rennet test. This cup has on the inside from top to bottom a scale graduated from 0 at the top to 10 at the bottom. There is a hole in the bottom to allow the milk to run out.