5S6 n5ri6 XaBiKvW.l; V^' Milner The use of milk as food THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES ■■■■bHHHa •WHO 'liOiijao*? •AN '•W130JAS •aui -soaa awoiAvs ; iNnovwiAvs i I issued July 14 1908. U. S. DEPARTMENT OF AGRICULTURE. FARMERS' BULLETIN 363. STATE NORMAL SCHOOL LOS ANGELES. CAUit)RNlA THE USE OF MILK AS FOOD. BY R. D. IMILNER, PTtl. B., Assistant in Nutritio?t Investigations. rSEPARED UNDER THE SUPERVISION OF THE OFFICE OF EXPERIMENT STATIONS, A. C. TRUE, Director. Corrected to December 15, 1910, WASHINGTON: GOVERNMENT PRINTING OFFICE. 1911. B5^ LETTER OE TRANSMITTAL U. S. Department of Agricultuke, Office of Experiment Stations, Washington^ D. C.yFebruary 12^ 1900. Sir: I have the honor to transmit herewith, and to recommend for publication as a Farmers' Bulletin, an article on the use of milk as food, prepared by R. D. Milner, of this Office, which summarizes the results of numerous experiments carried on in connection with the nutrition investigations of the Office of Experiment Stations, as well as data gathered from publications of the agricultural experi- ment stations and general sources. Perhaps no article of diet is more important than milk, and the present bulletin discusses such questions as composition, digestibility, care of milk in the home, use of milk in cookery, and its economy as compared with other foods. Some general information is also summarized regarding such milk products as butter, cheese, butter- milk, and whey. The present bulletin is a revision and extension of an earlier Farmers' Bulletin, No. 74, " Milk as Food," which it is designed to replace. Respectfully, A. C. True, Director. Hon. James Wilson, Secretary of Agriculture. 3G3 CONTENTS, Page. Introduction 7 Sources and kinds of milk 8 Composition and characteristics of milk 9 The protein compounds of milk 10 The fats and carbohydrates of milk 11 Mineral matters in milk 11 Bacteria in milk 11 Specific gravity and freezing point of milk 12 Variations in milk 13 Flavor 14 Dirt in milk 14 Milk as the possible carrier of disease 15 Preserving milk 15 Cold 16 Heat 16 Chemical preservatives 17 Condensed milk and milk powder 18 Graded and certified milk 19 Care of milk in the home 20 Digestibility of milk 20 Process of digestion 21 Proportion of nutrients digested 22 Relative value of cooked and raw milk 23 Milk for infants — Modified milk — Homogenized milk 25 Special infant foods , 26 Nutritive value of milk conipared with other foods 27 Nutritive value of skim milk 29 Cost of nutrients in whole milk and skim milk 33 The use cif milk in cooking 35 Milk products 36 Butter and cheese (^ Junket 38 Cottage cheese 38 Cream 39)' Buttermilk 40 Whey 40 Sour milk or clabber 40 Kephir, koumiss, and other fermented milk products 41 Summary 42 363 5 ILLUSTRATIONS. Page. Chart I. Composition and fuel value of milk, milk products, and other foods. . 28 II. Composition and fuel value per pound of milk and milk products 30 III. Pecuniary economy of milk and other foods 34 IV. Composition and fuel value of some milk products 37 303 6 THE USE OF MILK AS FOOD. INTRODUCTIOlf. According to the census of 1900, about two-tliirds of all the niilk produced for sale by the farmers of the United States was used to make butter and cheese, while the remaining third was consumed directly as milk and cream. The figures given showed that on the average each person in this country consumed about 20 gallons of milk a year, or that each family of five used about 1 quart a day. Accord- ing to estimates of the Bureau of xVnimal Industry of this Depart- ment, the average annual consumption in 1903 was about 30 gallons per person. This Department has previously published bulletins which discuss the various phases of the milk question as they appear to the farmer who produces it, the dealer who distributes it, and the manufacturers of butter and cheese. The purpose of the present article is to consider the question of milk as a food from the point of view of the consumer, to show why it is a valuable part of his diet, and under what conditions it is most valuable. The importance of such a consideration is indicated by the fact that milk and cream together furnish 16 per cent of the total food of the average American family. Questions pertaining to the care and management of dairy cows, the effect of different systems of feeding on the yield and quality of milk, the distribution and marketing of milk, the manufacture of butter, cheese, and other dairy products, and related questions have been studied at the agricultural experiment stations of the United States and elsewhere. The Dairy Division of the Bureau of Animal Industry of this Department has carried on extensive investigations of a great many problems of milk and its products, and to their work much of the data at present available may justly be attributed. The study of the digestibility and nutritive value of milk and its products has formed a part of the cooperative nutrition investigations carried on under the auspices of the Office of Experiment Stations, and many facts which are of interest and importance have been secured. In the following pages information drawn from this and similar sources has been summarized. 363 THE USE OF MILK AS FOOD. SOURCES AND KINDS OF MILK. In civilized countries where the climate allows, cows have been most generally bred for the piirj^ose of giving milk, probabh' not so much because their milk was more particularly desirable for human food than that of some other mammals as because, all things considered, they can be made to give the best results for a given amount of care and feed. Our preference for their milk is un- doubtedly the result of habit and acquired taste rather than of any intrinsic superiority, save, of course, as special breeding has de- veloped certain desirable characteristics. In some parts of the world other kinds of milk are used ; goat's milk is very common, especially in the rough, hilly districts of Europe; buffalo's milk is much used in India, and llama's milk in South America, while camel's milk is esteemed in desert countries, and mare's milk on the steppes of Russia and Central Asia. Sheep's milk is used in Europe and elsewhere for making certain kinds of cheese and in other ways, and the milk of reindeers is commonly used as food in the arctic regions. So much does cow's milk predominate in the western world, however, that unless otherwise specified the word milk almost always refers to that kind. Perhaps no food has been more often studied by chemists than milk and its products, and so a great deal of information is available regarding the composition and properties of these important food materials. The average composition of cow's and some other kinds of milk used for food is given in the table which follows: Average composition of milk of various kinds. Kind of milk. Water. Total solids. Protein. Fat. Carbo- hydrates (milk sugar). Mineral matters. Fuel value Casein. Albumin. Total. per pound. Per cent. 87.58 87.27 86.88 83.57 82. IG 86.13 87.13 86. 55 67.20 90. 58 90.12 Per ct. 12.6 12.8 13.1 16.4 Per ct. 0.80 2.88 2.87 4.17 4.26 Per cent. 1.21 .51 .89 .98 .46 Per ct. 2.01 3.39 3.76 5.15 ""'3.' 63" Per cent. 3.74 3.68 4.07 6.18 7.61 4.80 2.87 3.15 17.09 1.14 1.37 Per cent. 6.37 4.94 4.64 4.73 4.77 5.34 5.39 5.60 2. 82 6.87 6.19 Per cent. 0.30 .72 .85 .96 .84 .70 .74 .80 1.49 .36 .47 Calories. 310 Cow 310 Goat 315 410 Buffalo (Indian) ... Zebu '""'9.'9' 10.4 3.49 3.00 8.38 1.30 .79 .38 .90 1.51 .75 1.06 Ass 2i5 As the figures in the table make plain, milk of all sorts is a dilute food, as it contains a large percentage of water, the lowest proportion according to the figures cited being noted with reindeer milk and the highest wuth mare's milk. The three groups of protein, fat, and 363 THE USE OF MILK AS FOOD. 9 carbohydrates are represented by fair proportions, the quantities of protein and fat being especially noteworthy, as it is these constituents and the mineral matt^n- or ash which to a large degree give milk its peculiar value as a food for young mammals. COMPOSITION AND CHARACTERISTICS OF MILK. In the discussion of the composition and characteristics of milk which follows, reference is made almost exclusively to cow's milk. as this is the only kind which is especially important in diet in the United States, at least after infancy. From the large number of analyses of milk and milk products which have been reported by the agricultural experiment station chemists and other investigators the following data showing the average composition of these mate- rials have been compiled. The table also includes, for purposes of comparison, the average composition of a number of other common food materials. Average composition of milJc products and other food materials. Material. Vv'hole milk Skim milk Cream Buttermilk Whey Condensed milk, unsweetened Condensed milk, sweetened Butter Cheese, American Cheddar Cheese, cottivge Cheese, Swiss Milk powder (from skimmed milk) Kephir Koumiss Infant and invalid foods, farinaceous Infant and invalid foods containing milk and starches Infant and invalid foods, malted prepa- rations Beef, sirloin steak Eggs, as purchased Wheat flour, patent roller process Wheat bread, white Beans, baked Potatoes, as purchased Apples, as purchased Refuse. Per cent. 12.8 H.2 20.0 25.0 Water. Per cent. 87.0 90.6 74.0 91.0 93.0 71.3 26.0 13.0 33.5 53.0 31.4 3.0 89.6 90.7 9.4 4.3 4.2 64.0 65.5 12.0 35.3 68.9 62.6 63.3 Protein. Per cent. 3.3 3.4 2.5 3.0 1.0 7.4 8.2 1.0 26.0 19.6 27.6 34.0 3.1 2.2 9.4 9.6 12.0 16.5 11.9 11.4 9.2 . 6.9 1.8 .3 Fat. Per cent. 4.0 .3 18.5 .5 .3 8.5 9.6 83.0 36.5 23.2 34.9 3.1 2.0 2.1 .4 3.8 1.0 16.1 9.3 1.0 1.3 2.5 .1 .3 Carbohy- drates. Per cent. b-.O 5.1 4.6 4.8 6.0 11.1 54.3 1.5 2.1 1.3 51.9 o4.5 6 4.1 C79.9 «79.8 75.1 53.1 19.6 14.7 10.8 Per cent. 0.7 !5 .7 .7 1.7 1.9 3.8 3.5 2.1 4.8 8.0 .8 3.0 .9 .9 .5 1.1 2.1 " Including 2.1 per cent alcohol and 0.8 per cent lactic acid. ' Including 1.7 per cent alcohol and 0.0 per cent lactic acid. " Including 6.62 per cent soluble carbohydrates (sugars). ''Including 49.05 per cent soluble carbohydrates (sugars). « Including 4S.39 per cent soluble carbohydrates (sugars). Milk as it is drawn from the cow is familiar to all as an opaque, whitish liquid, which varies considerably in appearance and in flavor. It is commonly described as consisting of a thin, bluish-white, some- what transparent liquid, called the plasma, in which are floating yel- lowish globules of fat so numerous and so well distributed that the whole appears to be a white fluid. The chief bulk of milk is water, 71701°— Rull. 308—11 2 10 THE USE OF MILK AS FOOD. the amount of which may vary, even in ordinary unadulterated milk, from 90 per cent in a very poor product to 84 per cent in an unusually rich milk, the average being, as appears from the data in the table, about 87 per cent. The corresponding solid matter varies from 10 to 16 per cent. This solid matter, or " total nutrients," is made up of protein, fats, carbohydrates, and mineral matter. The proportion of these vary within certain limits ; but, roughly speaking, one-twentieth of the total solids are mineral substances, one-fourth protein, three- tenths fat, and four-tenths carbohydrates, as the above table shows. THE PROTEIN COMPOUNDS OF MILK. Protein compounds are the important nitrogenous ingredients which are indispensable in the formation of body tissues and fluids; they may also be burned in the body to furnish energy, their fuel value being about equal to that of the carbohydrates and less than half as great as that of the fats.** Protein is found in different, forms, such as the Avhite of egg (egg albumin), the lean of meat (myosin), the legumin of peas and beans, etc. In milk the principal protein com- pound is casein. In chemical composition the casein differs fi*om the other protein compounds of milk in that it contains both phosphorus and sulphur. Wlien acid is added to a solution of this casein, it is pre- cipitated in light, white flakes, such as are seen in sour milk. If, how- ever, an alkali such as limewater is also added to neutralize the acid, these curds do not form or are redissolved. Another kind of pre- cipitation or curdling of the casein occurs when rennet is added, as in cheese making. The ferment rennin, which is present in rennet (and also in human gastric juice), throws down the casein, but into a tougher and less flocculent or flaky mass than the acid, and ono which can not be redissolved by the addition of an alkali. Besides the casein there is a certain amount of albumin present in milk, some- times called lactalbumin, but the quantity is verj'^ much smaller than that of the casein, being on the average about one-seventh of the total protein. The proportion of albumin to casein varies in the milk of different kinds of animals and also in the milk of a single cow. There are other nitrogenous substances occurring in milk, but in such small quantities that they need not be considered here. The total protein of milk should not vary in any great degree, and will average not far from 3.3 per cent of the whole milk, or about 25 per cent of the total solids. <» The fuel value of any substance is artificially determined by burning it in a bomb calorimeter and measuring the beat given off. The unit of measure is a caloric, wliich represents the amount of heat necessary to raise the temperature of 1 poimd of water 4° F. One pound of protein or carbohydrates has a fuel value of 1,820 calories, and 1 ix)und of fats 4,040 calories. 363 THE USE OF MILK AS FOOD. H THE FATS AND CARBOHYDBATES OF MILK. The fat of milk is commercially the most important of its con- stituents, since it is the source of butter and enters largely into the composition of cheese. It is found throughout the milk in globules, varying in size in the milk from different animals, which, being lighter than water, tend to rise to the top of the milk as it stands, forming the cream. Chemically speaking, the fat of milk, commonly called butter fat, consists of several different fats, the chief of which, called stearin, palmitin, and olein, are the same as those that make up the bulk of meat fat (tallow, lard, etc.), as well as of many vegetable fats. The amount and quality of fat in milk varies widely, for rea- sons which are mentioned later. The amount should not fall below 3 per cent, and except in unusually rich milk will not exceed 5 jjcr cent. It averages about 4 per cent of the milk, or about 31 per cent of the total of solids. The chief carbohydrate which occurs in milk is lactose or milk sugar. It is similar in chemical composition to cane sugar,^but is not as readily soluble, and this is one reason at least why it does not taste as sweet, though tests made with solutions show that cane sugar is markedly superior as regards this property. It may be obtained in crystals, but milk sugar is usually marketed as a fine white powder much like confectioners' sugar in appearance, and is largely used by pharmacists and j)hysicians as a vehicle for drugs in the manu- facture of powders, pills, tablets, etc. The amount of milk sugar in cow's milk varies from 4 to G per cent, the average being about 5 per cent of the total milk," or 38 per cent of the total solids. The milk sugar remains in the whey when casein or curd is removed in cheese making, and may be readil}^ recovered. The manufacture of milk sugar from whey is an important industry. MINERAL MATTERS IN MILK. The ash constituents in milk consist mainly of the phosphates and chlorids of soda, potash, and lime. Most of these are dissolved in the plasma, but tiny particles of the phosphates of lime remain un- dissolved, and are said to be a cause of the bluish tint found in milk. The mineral matters make up about seven-tenths of 1 per cent of the whole milk or 5 per cent of the solid matter. Along with these prin- cipal ingredients milk also contains minute quantities of other sub- stances, some of which will be referred to later. BACTERIA IN MILK. Besides the chemical compounds, milk also contains large numbers of minute organisms called bacteria. Few, if any, are normally 363 12 THE USE OF MILK AS FOOD. present in the milk within the udders of clean, healthy cows, but they are so abundant everywhere in the air, especially about the stable and barnyard, and cling in such numbers to the bodies of the cows, that they are almost always found in milk as soon as it leaves the udders or even just inside the teats. They reproduce very rapidly in a favorable medium, such as warm milk, so that the number present becomes very large unless measures are taken to hinder their increase. The amount in milk of a given age of course varies with the condi- tions; that from clean cows, with freshly washed udders, milked into well-scalded pails, in a clean place, free from air currents, by persons with clean hands and clothes, and quickly cooled and care- fully handled, mny contain very few, while milk from ill-kept animals untidily handled in a dirty place may contain enormous quantities. Since, as will be shown later, bacteria cause the spoiling of milk, and may be harmful in other ways, it is very evident that scrupulous cleanliness about everything which comes in contact with the milk is of first importance. A great many kinds of bacteria have been found in milk, each of which occasions a special set of changes as it develops. Perhaps the most important kinds are those which cause the ordinary souring of milk and are the first to produce any noticeable change in the taste and odor. In their growth they feed upon the milk sugar and con- vert it into lactic acid, which gives slightly soured milk its peculiar taste and odor. When enough of this lactic acid has formed it acts upon the casein, causing it to separate into loose, light flakes such as we see in ordinary sour " lobbered " milk. Other bacteria may also develop in sour milk which give it a strong, unpleasant odor or flavor. Some of the products of bacterial action on milk are desir- able — for instance, those which give to butter and cheese the character- istic flavors and odors. Still others occasionally apj^car in milk which color it very brightly or give it a slimy or ropy consistency. Milk sometimes sours more quickly than ordinarily when there is a thunderstorm. This is often said to be due to electricity in the air, but bacteriologists claim that it is due to the hot, damp atmosphere which usually precedes the storm, as the heat is especially favorable to the growth of bacteria. SPECIFIC GRAVITY AND FREEZING POINT OF MILK. Milk is slightly heavier than water, its specific gravity ranging from 1.029 to 1.034 at 60° F. This means that while a quart of water weighs 2 pounds 1.333 ounces, a quart of milk weighs from 1.029 to 1.034 times as much, or not far from 2 pounds 2.5 ounces. The spe- cific gravity depends upon the proportion of water and other sub- stances. Since the fat is lighter than water, the richer the milk is in 363 THE USE OF MILK AS FOOD! 13 butter fat the lower its specific gravity, provided, of course, that the other solids are not increased proportionally. It follows also that the removal of the fat increases the specific gravity, so that skim milk has a specific gravity of from 1.033 to 1.037. On the other hand, the addition of water to skimmed milk brings down the specific gravity. The freezing point of milk also varies with the composition, falling as the proportion of solids increases and rising if water is added. In average milk it is about 31° F. This factor is of importance in judg- ing of the purity of milk. VABIATIONS IN MILK. The milk which is ordinarily sold for household use is subject to considerable variatit^n in composition, and it is entirely possible for one man to pay nearly twice as much as his neighbor for an equiva- lent nutritive value in milk at the same price per quart but from dif- ferent dealers. It is owing to such natural variation in composition that milk is purchased at creameries on the basis of its fat content. (See p. 19.) This variation in nutritive value is in large part attrib- utable to differences in the breeds of cows kept, certain breeds pro- ducing milk which contains more fat than others. Among the best for producing cream are the Channel Islands breeds, which give a milk rich in large fat globules, that rise easily to the surface. For general purposes, however, breeds which yield milk containing fairly good proportions of all the ingredients of the solids are considered more desirable. The age of the animal also has considerable influ- ence, young cows producing richer milk than old ones of the same kind. In general a well-fed cow gives more and better milk than if poorly fed, but the relative proportions of fat, casein, and sugar in the milk are not so greatly influenced by the composition of the food as is the quantity of milk. The average cow of a given breed pos- sesses certain capabilities for producing milk, but does not reach her normal capacity of milk production unless she is well fed. AVlien once she has a sufficient and well-balanced ration, neither the compo- sition nor the amount of the milk yield seems to be greatly improved by either increasing the ration or changing the proportion of the nutrients it supplies. The milk flow of a given cow is usually largest soon after calving ; as the period of lactation progresses the milk flow gradually falls off, and, as a rule, the milk grows richer, i. e., the proportion of solids in- creases. The proportion of fat to the other solids in the milk of a given cow varies from day to day and from milking to milking. On account of all these variations in the milk of individual cows, dairymen who supply the milk trade usually find it best to mix the milk from 'all the cows in the herd immediately after it is drawn. 363 14 THE USE OF MILK AS FOOD. This is of considerable advantage to the consumer, as it makes the milk much more uniform from day to day. A cause of variation in milk is found in the adulteration not infre- quently practiced by unprincipled producers or dealers. The chief methods of adulteration aside from the use of chemical preservatives are the addition of water and the removal of a portion of the fat, both methods being often used together. These methods are not only fraudulent as regards money value, but also diminish the food value. The specific gravity of the milk is sometimes used as a test of its purity, but since removing part of its fat in the form of cream raises and adding water lowers the specific gravity, one form of adulteration may cover up the other, and thus render this test alone unreliable. A similar difficulty is found in testing milk by its freez- ing point, which varies with the proportion of the different soluble constituents. FLAVOR. The flavor of milk varies almost as much as its composition. It is in part due to the amount of fat present, but also to the number and kind of bacteria, or rather to the product of bacterial action. The flavor is also often influenced by the food of the cow ; this is especially noticeable when the cows have eaten some strong-tasting substance, such as turnips, and is due to the fact that the volatile oil or other flavor of the food passes directly through the animal tissues into the milk. Milk also absorbs flavors and odors from the air much more quickly than most foods. The " animal " or " cowy " taste and smell, which are often noticeable in new milk, are believed to be due in part to this absorption. It may also be due in part to stable dirt which accidentally falls into the milk. The cleaner the animal the less noticeable is this taste in milk. It may be considerably lessened by promptly cooling and aerating the milk after it is drawn. A pan of milk kept in a closed ice box with fish or high-flavored vegetables or fruit is very likely to acquire flavor and odor from them ; hence milk sliould be kept where the air is free from such taint. DIRT IN MILK. There is frequently more or less dirt in freshly drawn milk, most of it fine particles of litter and manure which fall into the pail from the body of the cow. Milk should always be strained directly the milking is over, or, better still, it should be drawn into pails covered with straining cloths; but even with these precautions some dirt may be present. Of course, the amount varies Avith the condition in Avhich the cow and her surroundings are kept ; under ideal dairy conditions only very small quantities are found, while milk from untidy estab- 363 THE USE OF MILK AS FOOD. 15 lisliments may contain enough in a quart to form a noticeable sedi- ment. Milk with enough dirt to be visible indicates a badly kept dairy and should not be tolerated, and consumers should always insist upon having clean milk. It should be remembered that the visible dirt does not tell the whole story ; some of the manure that falls into milk is dissolved, and is no longer noticeable to the eye. If milk is measured out from a large can at the door of each cus- tomer dust and dirt from the street as well as bacteria may get into it. The custom of delivering it in sealed bottles is much neater and has the further advantage of giving the buyers more uniform amounts of cream than where some get the top and some the bottom of the milk taken from a large can. MILK AS THE POSSIBLE CARRIEB, OE DISEASE. Disease germs may get into milk either directly from a diseased cow or indirectly from an infected person, from polluted water, or in some similar way. The most dreaded disease w4iich may come directly from the cows is tuberculosis. It is certain that the tubercle bacillus, the germ which causes the disease, does sometimes exist in milk from tuberculous animals ; whether or not persons who drink such milk may become infected by it is extremely hard to prove. There are many other possible sources of contagion, and the disease develops so slowly that by the time it is recognized it is usually too late to trace the cause. Nevertheless, there is quite enough evidence that the disease may be carried in this way to make the use of milk from tuberculous, cows too dangerous to be tolerated, even when the animals are only slightly diseased. Among the contagious diseases to which the cows are not liable but which may be spread by milk the most common are perhaps scarlet fever, typhoid fever, and diphtheria. Statistics show that milk is very often the cause of an epidemic of such contagious disease, and not infrequently a serious epidemic may be traced to the milk from a single farm. The bacteria causing these diseases frequently enter the milk from contaminated water used in washing milk utensils, etc., or from persons who have been exposed to disease and who handle the milk. For this reason no water which is not above suspicion should be used about the dairy (or anywhere else) for either drinking or washing, and no person who has been exposed in any way to such diseases should be allowed about the cows, the milk, or the milk utensils. PRESERVING MILK. If milk could be obtained and kept free from bacteria it would probably remain sweet almost indefinitely. Hoav near to this ideal it is possible to come may be seen from the fact that milk from several 363 16 THE USE OF MILK AS FOOD. American dairies exhibited at the Pai'is Exposition in 1900 was sweet when over two weeks old, no method of preservation being followed except cleanliness and keeping it at a temperature of 10° to 42° F. According to more recent data, published by the Bureau of Animal Industry of this Department, clean milk may be kept five to seven weeks." The conditions which make such milk possible are still un- common, but they are becoming and will continue to become more common as consumers realize that it is worth the extra cost which the necessary carefulness entails. At present, however, it is often neces- sary to find some means of checking the growth of the bacteria. Besides the various methods of drying milk, which will be discussed later (see p. 18), there are three means of accomplishing this: (1) By cold, (2) by heat, and (3) by the use of chemical preservatives. COLD. Low temperature, even when extreme, does not of itself kill bac- teria, but any temperature below 50° F. makes them more or less in- active for the time being. Occasionally milk is preserved by freezing, but this is undesirable because the character of the ingredients is slightly altered by the process. The changes begin at about 35° F., and milk should not ordinarily be kept at a lower temperature than this. Down to that point, however, the cooler the milk the better it keeps. As has been suggested, milk should be cooled as soon as it is drawn, in order that bacteria, whose growth is very rapid, may not make too much headway in the warm liquid. HEAT. Slight warmth is very favorable to the growth of bacteria, but great heat is fatal to them. Unfortunately, heat sufficient to destroy all the varieties also causes changes in the chemical composition and flavor of the milk, as seen in boiled milk; otherwise cooking milk would be a very simple and satisfactory way of preserving it. "When heat is used, it is well to apply it in such a way as to do the most damage to the bacteria with the least damage to the other materials. The two most common methods are pasteurization and sterilization. These terms are often used interchangeably, but they really refer to distinct' processes. In pasteurizing milk the aim is to destroj'^ as many bacteria as possible without producing any of the changes in the chemical con- stituents which take place when the milk is heated and which begin at about 1G7° F. Experience has shown that the temperature of milk during pasteurization should not exceed 185° F. nor fall below « U. S. Dept. Agr., Bur. Anim. Indus. Circ. 117. 363 THE USE OF MILK AS FOOD. IT 140° F. To pasteurize milk it may be placed in air-tight bottles or bottles which are stoppered with sterilized cotton, and immersed to the neck in hot water and heated, for instance, at 149'' F. for a half hour or at 167° F. for lifteen minutes, and then quickly cooled to 50° F. or less. This rapid cooling lessens the " cooked " taste so objectionable to many persons. Sterilized milk, on the other hand, is milk subjected to heat suf- ficient to destroy all the organisms in it. It is not always accom- j)lished by simply boiling the milk unless the boiling is repeated on two or three successive days. Higher temperatures than the boiling jDoint are necessary to assure sterilization or the complete destruction of all organisms at one application of heat of fifteen to thirty min- utes' duration. For many practical purposes, however, simply rais- ing the milk to the boiling point is sufficient. Much of the so-called " sterilized milk '' has been thus treated and is by no means free from living organisms. Sterilized milk will remain sweet in a warm place longer than pasteurized, but the greater heat produces more of the undesirable chemical changes. Both pasteurized and sterilized milk should be kept in clean, air-tight bottles or new bacteria will enter and begin their destructive work. Pasteurized milk can now be obtained in many large milk depots; but under ordinary conditions housekeepers usually prefer to buy fresh milk and do their own heating when necessary. Pasteurized cream is frequently sold, but is considered less desirable than fresh. It seems thinner and does not "whip" well (see p. 39). These dif- ferences do not necessarily mean that the pasteurized cream contains less fat than the fresh, but that the heat has somewhat changed the character of the fat globules. CHEMICAL PEESEBVATIVES. These are substances put into milk to kill or hinder the growth of bacteria by chemical means, a practice which is forbidden by the pure-food legislation which has been enacted in many States. The most common are borax, boric acid, salicylic acid, formalin, and salt- peter. There are several decided objections to their use, the most important being that they may seriously injure the health and that they help to conceal dirt with the accompanying bacteria. Although an occasional small dose may not be harmful, the consensus of opin- ion is that, taken continuously, such preservatives retard digestion and have an irritating efTect on the intestines, often causing serious disturbances. They are of course most dangerous to the delicate organs of infants and invalids, who are also the most dependent on milk for nourishment^ ^loreover, while they kill many micro- (jrganisms, they do not kill all ; some of the most dangerous, such as 71701°— Dull. CfiC— 11 3 18 THE USE OF MILK AS FOOD. the tubercle bacilli, have been known to resist considerable quantities. Finally, they are so often emJDloyed to make old milk pass for fresh, or dirty milk for clean, that their use in ordinary milk suggests a dishonest or a dirty milkman. CONDENSED MILK AND MILK POWDER. Wlien milk is heated the water in it evaporates readily, and if the heating is continued long enough, as, for instance, in making the well- known pudding of rice and milk, cooked slowlj^ in the oven for a long time, the milk becomes thick and creamy. In the case of the pudding the thickening is due in part, of course, to the effect on the starch of the rice, but also in considerable measure to the elimination of water from the milk. This fact is taken advantage of in the manufacture of evaporated or condensed milk. On a commercial scale the process is commonly carried on in vacuum pans, as under such conditions a lower degree of heat may be employed and the danger of scorching is lessened. Sugar is very commonly added to condensed milk, though unsweetened condensed milk is also a common commercial product. Such unsweetened milk evaporated to about the consist- ency of cream was formerly sold under the name evaporated cream. Condensed milk is ordinarily marketed in cans of varying size and its manufacture is an enterprise of very great importance. Its low water content and large percentage of sugar do not favor bacterial growth, and so it will keep for a reasonable time after the cans are opened. Condensed milk is used in many ways like fresh milk or cream. "WTien properly diluted with water it may be used in cookery like ordinary milk and is very satisfactory for the purpose, especially on shipboard, in the Tropics, or elsewhere where fresh milk can not be readily obtained. The small bulk is an additional- advantage in shipping. Condensed milk is used to a large extent for infant feed- ing, and has the advantage of being sterile, but it is not considered so satisfactory as fresh milk or as pasteurized milk. Investigations have shown that with children and adults condensed milk has prac- tically the same coefficients of digestibility as fresh milk. Within recent years special devices have been put on the market for evaporating milk to a fine flour-like powder, which is easy to keep, nutritious, and, of course, convenient for shipment. When water is added the resulting mixture closely resembles ordinary milk in ap- pearance and may be used in cookery and in other ways. The flavor depends to a great extent upon the method of manufacture, in some cases being almost the same as that of fresh milk, while in other cases a slightly strong or scorched taste is present, but is not pronounced enough, it is said, to be noticeable in cooked products in which it is used. 363 THE USE OF MILK AS FOOD. 19 The relative amount of nutrients in milk, condensed milk, pow- dered, dried, skim milk, and other milk products, is shown in the table on page 9 and graphically in the charts on pages 30 and 37. GRADED AND CERTIFIED MILK. There is so much difference in the composition of milk that many large butter and cheese factories now test all the milk which they buy and pay for it according to the amount of fat which it contains. The importance of such an arrangement for the retail millc trade is evident. The terms " standardized " milk or " blended " milk are applied to milk which has been so modified as to contain a definite amount of one or more of its constituents. The most important and at the same time the most variable constituent is fat. To standardize milk as regards fat it is only necessary to add or ren^ove a certain amount of this constituent or to add or remove a certain amount of skimmed milk. Directions for such standardizing or blending have been sum- marized in an earlier publication " of this Department. Most States and cities now have a legal standard for milk, representing a fair amount of fat and other solids. If all milk sold could be tested by such a standard, and the price regulated according as the milk surpassed or fell below it, both pro- ducer and consumer would be better satisfied, the producer because he would get credit for good milk, and the consumer because he would know what he was really buying. Such graded milk is sold in some European cities, and to a less extent in this country. The principle has been more commonly applied to cream, of which different quali- ties are sold at prices varying with the amount of fat. In some of oUr large cities what is known as " certified " milk may be obtained. The dealers' certificate is usually in the form of a label pasted over the top of the bottle. Under local pure-food laws the use of this label is permitted only to those establishments which are periodically inspected by public officials as to their sanitarj^ condi- tions, and which market milk conforming in composition to a fixed standard. Such milk justly commands a higher price than that of which the quality is not thus guaranteed. The term " sanitary " milk is quite commonly used, being applied somewhat indefinitely to milk produced and handled under condi- tions considered necessary to secure a pure, wholesome jiroduct. It is often applied by dealers, for purposes of advertising, to milk pro- duced under decidedly insanitary conditions. The term " hygienic " milk is similarly abused. " U. S. Dept. Agr., Farmers' Bui. 2S1. 3G3 20 THE USE OF MILK AS FOOD. When milk is passed through a centrifugal separator much of the solid impurities remains in the separator slime. A mixture of the skim milk thus purified and the cream which was separated from it is often referred to as " clarified milk." CARE OF MILK IN THE HOME. No matter liow well milk has been handled up to the time it is de- livered to the consumer, it can not be expected to keep well if it is carelessly treated thereafter. It should be poured into pitcher, pan, or other vessel — freshly scalded to remove any bacteria or mold spores — and kept in a cool, clean place free from dirt, flies, etc. New milk should never be mixed with old unless it is to be used at once, as the bacteria in the old milk w^ill of course be added also and the mixed milk will not keep as well as the fresh milk alone. Bacteria are thick- est where there is dirt arid decay, and milk should therefore be stored only in clean, sweet places. It is safer to keep it covered, to exclude not only dirt and bacteria but also the flavors and odors which it so easily absorbs. If kept at a temperature of 50° F. or less, good milk should remain sweet for twelve hours, at least, after it reaches the consumer, and ordinarily for twenty-four hours or more. Sometimes in very hot weather housekeepers complain that in spite of all pre- cautions it sours quickly, even in the ice box. This is often due to the fact that the air of the ice box, although it seems cold in contrast with the heat outside, is really not cold enough to check the growth of the bacteria ; if a thermometer placed inside registers more than 50° F. the fault is almost surely in the temperature of the ice box and not in the milk. In large cities, where most of the milk comes by morning trains from a considerable distance, it is often impossible to deliver fresh morning's milk in time for breakfast, and that milked the morning before must be given to patrons who insist on an early delivery. They would get their milk from twelve to eighteen hours fresher if they would take it in the afternoon instead. DIGESTIBILITY OF MILK. The amount of nourishment which any kind of food furnishes to the body depends, in the first place, on the food ingredients, the pro- tein, fats, and carbohydrates and mineral matters which it contains, and in this respect milk is a well-balanced, nutritious food. But there is another consideration which is brouglit out l\y the much- quoted phrase, " We live not upon what we eat but upon what we digest." The nutritive value of food depends not only upon the ma- terials it contains but also upon the amounts of those nutrients which 3G3 THE USE OF MILK AS FOOD. 21 the body can actually utilize — upon its digestibility. As commonly used, the word digestibility refers mainly to the ease and comfort of digestion. This is an important consideration, but does not tell the whole story. As here used the words digestible and digestibility refer to the amount or proportions of the nutritive ingredients of a food material which may by the average healthy digestive organs be made available for the uses of the body. Whether milk is to be classed as a digestible or indigestible food will therefore depend upon the proportions of protein, fats, carbohydrates, and mineral matters which the digestive organs of the average normal person are found to transform into material available for the use of the body. PROCESS OF DIGESTION. No one element of the various digestive juices which act upon the food as it passes through the alimentary canal acts upon ail classes of nutrients. All are digested separately, though sometimes simulta- neously. Since milk contains considerable quantities of each class of nutrients, its digestion w^ill be better understood if we follow the changes in each nutrient separately. The protein compounds of milk are usually considered to give the most trouble in its digestion, and this is mainly due to the casein. Milk is commonly classed as a liquid food, but this is true only out- side of the body; as soon as it reaches the stomach the rennin of the gastric juice precipitates the casein into a curd from which the other ingTedients are separated, much as the whey separates from the curd in cheese making. Sometimes the curd tends to become tough and leathery and consequently difficult for the digestive juices to work upon. This is especially marked when for any reason there is considerable calcium phosphate present, and may be partially prevented by adding limewater to the milk. The acid and the pepsin of the gastric juice together work upon the curd and render a small part of it more soluble, but the bulk of the casein is digested in the small intestine by the trypsin of the pancreatic juice. Experiments <* have shown that casein which has not been curdled b}'^ rennin is more completely digested by trypsin than the curdled. Apparently, then, the formation of the curd, especially when tough and leathery, means extra work for the digestive organs. The albumin of the milk is for the most part easily digested by either the pepsin or the trypsin. The digestion of fats depends mainly upon getting the globules into such fine size that they may be easily passed through the walls of the intestines. Separating the fat into such tiny globules is called "Arch. Physiol. [Pfliiger] (1902), p. G05. 363 22 THE USE OF MILK AS FOOD. emulsification. Another change, saponification, also helps in the absorption of fat. The alkaline pancreatic juice unites with the fatty acids of the fat to form a soap, while the glycerin is set free, much as in ordinary soap making. Both the soap and the glycerin are more easily absorbed than the original fat. The fat globules of milk are smaller than those from most other foods and are more easily emulsified; for this reason milk fat is commonly thought to be the most easily and thoroughly digested of the fats in the ordinary diet. As has been said, the size of the globules varies in milk from dif- ferent animals. There is a theory that the fat of milk containing the larger globules — that is, milk on which cream rises freely (see p. 13) — is less easily digestible than that in which smaller globules remain longer in suspension. Ordinarily the digestion of carbohydrates is begun by the saliva in the mouth, and ceases in the stomach after the food is swallowed, owing to the acid reaction of the gastric juice. The results of recent investigations make it seem certain that usually the digestion of carbohydrates continues in the stomach for a much longer period than was formerly supposed. Milk is so quickly swallowed that the saliva would have little chance to act upon the milk sugar in the mouth. Just how far this digestion would continue in the stomach before being checked by the gastric juice it is impossible to say, but there is every reason to suppose that under usual and normal con- ditions the bulk of the milk sugar is digested by one of the ferments of the pancreatic juice after it passes from the stomach into the intestines. Eecent French studies" indicate that the major portion of milk and milk products leave the stomach within an hour and a half. It is commonly supposed that the lactic acid of sour milk is changed to simpler bodies in the digestive tract and assimilated, and its presence may even be beneficial in checking the growth of the putrefactive bacteria that cause intestinal disorders. (See p. 41.) PROPORTION OF NUTRIENTS DIGESTED. In connection with the nutrition investigations of the Office of Experiment Stations many experiments have been made to determine how thoroughly the protein, fats, and carbohydrates of milk are digested and assimilated, and much similar work has been reported by other investigators. The results obtained vary within rather wide limits, owing either to individual peculiarities of the subject or to conditions under which the milk was taken. In persons who digest milk well the average coefficients of digesti- bility may run as high as 98 per cent for protein, 99 per cent for fat, and 99 per cent for carbohydrates. The average values for animal <^Rev. G6n. Lait, 6 (1907), p. 441. 363 THE USE OF MILK AS FOOD. 23 food are 97 per cent protein, 95 per cent fat, and 98 per cent carbo- hydrates. In general it seems fair to say that milk is on an average as well or even more thoroughly digested than other animal foods. When milk is the only food eaten by healthy adults considerably less of the nutrients supplied are assimilated than is the case when it forms a part of a mixed diet, doubtless owing to the fact that the casein is apt to coagulate in the stomach into large lumps which resist the action of the digestive juices. It has been shown by experi- ments that when finer coagulation is secured the thoroughness of the digestion is increased. Taking other food with the milk hinders the formation of the lumps of casein in the stomach and so increases the thoroughness of digestion of milk. Of course, very young children digest mothers' milk alone better than any other food, but this is because of the peculiarities of their digestive organs, to which such milk is thoroughly adapted by nature, both in composition and in physical properties. If other milk is substituted for mother's milk it must be modified to secure best results. For adults in poor health milk is very commonly a most important food, and many individuals whose digestive organs are not in good condition can derive more benefit from a milk diet than from any other single food, RELATIVE VALUE OF COOKED AND RAW MILK. As was stated in an earlier section (p. 16), the heat of cooking pro- duces certain chemical changes which alter the flavor and appearance of milk. In how far these changes affect its digestibility is a point on which authorities differ greatly, for although many experiments have been made to determine the facts of the case, the results are very conflicting. The principal changes, apart from the destruction of more or less of the bacteria, are as follows : Part of the protein is coagulated, it is believed, and the protein undergoes cleavage to some extent — that is, the molecule is split up into simpler forms, one of which is a volatile sulphur compound which gives freshly boiled milk its peculiar odor. If the milk is cooked in an open vessel the forma- tion of more or less of a film or " skin " accompanies the other changes, though it is interesting to note that the formation of this skin and some of the disagreeable features which accompany it may be more or less prevented by stirring the milk while it is heating. The character of the fat globules appears to be somewhat altered by cooking, although the nature of the changes is not yet thoroughly understood; some of the other compounds which occur in small quantities also undergo cleavage and give place to new ones. The carbohydrates undergo practically no change unless the heat is con- tinued long enough to caramelize some of the milk sugar. The longer or harder the cooking, the more noticeable are the changes. 363 24 THE USE OF MILK AS FOOD. The skin formed by heating has been commonly said to consist largel}^ of coagulated albumin, but later investigators having found casein and lime salts in it have suggested that the heat, possibly by driving off carbon dioxid, liberates casein from lime salts, by which it is supposed to be held in solution, which then rises to the surface and is more or less dried by rapid surface evaporation. If the skin is removed it will form again, and apparently will continue to do so indefinite!}'. That the amount of material which may be thus re- moved is not great is indicated by tests ° in which the skin from 100 cubic centimeters of milk boiled for a quarter of an hour did not contain more than 0.27 gram of protein. The question has been studied more recently by Jamison and Hertz,^ whose experiments showed that the formation of skin or film on milk is not a peculiar propertj?^ of lactalbumin or of casein, since a similar film may be produced bj^ warming any protein solution which contains emulsified fat. Drying was found to be an essential con- dition for such skin formation. The character of the films formed on milk and other protein solutions was studied, and the conclusion drawn that the film is probably composed of dried protein with fat entangled in it. This coagulation of the protein compounds roughly resembles the hardening of the white of eggs in boiling, which is commonly said to render the egg albumin less easy of digestion. Doubtless for this reason it has been supposed that boiled milk is less easily digested than raw milk. Boiled milk coagulates less rapidly than raw milk, but under the influence of rennin the casein of the former forms about as tough a curd as that of the latter. In some processes of cheese making milk is heated before it is curdled. It may be that cooked milk fat is less easily emulsified than raw, but not enough is known to say definitely, and the difference, if it exists, is probably slight. Many physicians consider that infants fed for a long time on boiled or sterilized milk show a greater tendency to scurvy, rickets, and other diseases of malnutrition than when raw or pasteurized milk is used. No satisfactory explanation for this has yet been given, but there seems some reason for believing that it has to do with changes in- duced in the character of the calcium salts present. Probably much of the difference of opinion regarding the value of boiled milk is due to individual peculiarities. Some healthy per- sons who can not eat raw milk with comfort have no trouble with it boiled, while others whom the taste of boiled milk sickens are very fond of it raw. Foods whoso taste offends the palate do not stimu- ttArcb. Hyp., 2S (1897), p. 43. ''Jour. Physiol. [Loudou], 27 (1901-2), p. 2G. 363 THE USE OF MILK AS FOOD. 25 late the flow of digestive juices as do pleasant ones, and so it doubt- less would be wisest for the ordinary healthy person to consult his taste in choosing- between the raw and cooked milk. As for infant feeding, when artificial feeding is necessary, probably the majority of physicians recommend raw or pasteurized milk vmless the Aveather is very hot, or the quality of the milk very suspicious, or unless it is necessary to keej) the milk a long time, as in traveling. It goes with- out saying that in infant feeding the use of cow's milk should always presuppose a product as free as possible from bacterial contamination. MILK FOR INFANTS— MODIFIED MILK— HOMOGENIZED MILK. It is universally admitted that the best food for infants is milk from strong, healthy women. When this is not obtainable the more nearly the substitute resembles it the better. The milk of the ass and the mare are in many important respects more like human milk than is that of the cow, and their milk used frequently to be given to babies. Goat's milk, too, is highly thought of under ordinary cir- cumstances. At present, however, cow's milk is the most common substitute, and when necessary this is artificially modified to make it resemble human milk more closely. A glance at the table on page 8 shows that while cow's milk con- tains about the same proportion of total solids and fat as woman's milk it carries more protein and less milk sugar. Besides this it has larger fat globules and there is more casein in proportion to the albumin and the casein is said to form a tougher curd than that of human milk. The deficiency in milk sugar in the cow's milk can easih'^ be made good by adding either milk sugar itself or some other digestible carbohj'drate, such as rice flour or arrowroot. The casein may be made more easy of digestion by the addition of limewater, or may be artificially predigested by peptonizing. But nothing can exactly reproduce the protein of human milk, and for this reason, perhaps more than for any other, cow's milk, no matter how skill- fully modified, is never quite so satisfactory as human milk for infants. Fortunately, most healthy babies thrive on good cow's milk or cow's milk simply modified. It is the sickly who recj[uire special preparations, and their needs vary so greatly that only the physician acquainted with the case, and not always he, can say what change is necessary. Eecognizing the important bearing of the size of the globules of the milk fat upon digestion, so-called homogenized milk is now pre- pared, though it is not so well known or so generally used as modi- fied milk. In such milk the fat globules are broken down by mechanical means into very fine particles which show no marked 3C3 26 THE USE OF MILK AS FOOD. tendency to rise to the surface as do. the fat globules of ordinary size. This change in the fat globules is usually accomplished by forcing the milk through capillar}' tubes and against a resisting surface. The force of impact causes the breaking up of the globules and thus makes a more perfect emulsion of the milk. SPECIAL INFANT FOODS. There are numerous patent infant and invalid foods on the market, some of which contain cow's milk as a basis combined with varying amounts of carbohydrates or other constituents, and others which seem to be made of farinaceous materials without milk. In some cases the carbohydrates have apparently been malted l)efore being combined with milk or else malt extract is added during the process of manufacture. The jDcrcentage composition of the two types of infant foods is shown in the table on page 9. The chart on page 37 shows in graphic form how commercial infant food, such as is sold in dry form, in bottles, or in cans, and is made of milk and starch, compares with some other milk products. It is also interesting to compare this product with the data showing the composition of whole milk, skim milk, etc., included in Chart II, p. 30. Experience has shown that these special milk foods (when they really contain the nutrients of milk) are useful and valuable for infants where it is necessary to resort to some method of artificial feeding, but every one recognizes that where possible mother's milk is the best food for the young child. Too much credence should not be given to the extravagant claims made for some brands of infant foods. The safest course is undoubtedly to follow the advice of a competent physician in selecting the substitute for natural feeding. It is often wiser to use modified cow's milk in preference to these commercial foods, and it can be easily prepared at home imder a physician's directions. There are also milk laboratories in many of our large cities and towns where modified milk of all sorts can be. procured on prescription. All babies fed on raw cow's milk are in more or less danger from the undesirable bacteria which it may contain and which cause diarrhea and other serious infant disorders. The question of pasteurizing and sterilizing milk in order to avoid this danger has already been spoken of (p. 16). When raw milk is used, great pains should bo taken not only to obtain fresh, clean milk, but also in caring for it scrupulously after it is purchased. It is usually more important that the milk should be i:)ure than that it shoidd be rich in cream, especially as the fat in very creamy milk may be less digestible on account of the size of the fat globules. 3G3 THE USE OF MILK AS FOOD, 27 NUTRITIVE VALUE OF MILK COMPARED WITH OTHER FOODS. The value of milk for noiirisliment is not as generally understood as it should be. Many people think of it, for adults at least, as a bever- age rather than a food, and do not realize that a glass of it adds as much to the nutritive value of a meal as a quarter of a loaf of bread or a good slice of beef. A quart of average milk contains the same amount of nutritive ingredients as 0.75 of a pound of beef or 6 ounces of bread. To put it in another way, about one-eighth of the whole weight of the milk, one-third of the beef, and two-thirds of the bread consist of actually nutritive ingi'edients. The other seven-eighths of the milk and one-third of the bread are water, while the two-thirds of the meat which is not actual nutriment is mainly water, though in part is bone. Chart I shows the average composition of milk, milk products, and some other common food materials. In this chart the percentage amount of refuse, water, protein, fat, carbohydrates, and mineral matter or ash. is indicated by shaded portions corresponding to these different constituents, each full division' of the lines repre- senting constituents being equivalent to 10 per cent. The heavy black line indicates the number of calories per pound, each full division of this space corresponding to 400 calories. As compared with the animal foods, it Avill be noted from this chart that milk contains more carbohydrates and has no refuse. In these two resj^ects it resembles more nearly many of the vegetable foods, such as flour, oatrheal, and the like. The amount of mineral matter is much the same as in the other fresh substances given. There is a larger proportion of water in milk than in most other food materials except very succulent fruits and vegetables, so that a given weight contains less dry matter or nutrients than most foods. If we wish to compare the food values of the actually nutritive ingredients (the dry matter contained in the edible portion) of dif- ferent food materials, the calculations may be made on the basis of 1 pound of this water- free edible portion, and the following figtires show such a comparison of milk and a few other foods : Nutrients and energy in 1 pound of the water-free edible portion of several food materials. Food materials. Protein. Fat. Carbohy- drates. Mineral matter. Pound. Pound. Pound. Pound. 0.25 0.31 0.39 0.05 .36 .03 .56 .06 .33 .06 .53 .08 .39 .52 .03 .06 .57 .26 .13 .40 .06 .01 .03 .08 .01 .85 .15 .02 .82 .01 .10 .01 .85 .01 .03 .03 .92 .02 Fnel value. Whole milk Skim milk (0.3 per cent fat) Buttermilk Cheese Beef, round Smoked ham , Wheat flour Wheat bread , Potatoes , Apples 363 Calorics. 2,475 1,835 LS-IS 2,990 2,750 3,275 1,8C5 1,865 1,790, 1,885 28 THE USE OF MILK AS FOOD. Chart I. — Percentage composition and fuel value of milk, milk products, and some other foods. (The constituents are expressed in per cent, the fuel values In calories per pound.) Protein. Tats. Carbo- IJinoral hydrates, mailers. Water. Refuse. Fuel value. Oysters Corn meal Oatmeal- Beans, di'ied Potatoes- M:^mmMmm mmmmmmmmmmmmmmm. Sugar- 3G3 THE USE OF MILK AS FOOD. 29 One of the most common statements regarding milk is that it is a " perfect food." In one sense this is true, but in another it is not. The normal milk of any mammal may be perfectly adapted to the nourishment of its offspring, but the milk of one mammal, such as the cow, is not necessarily a perfect food for the young of another species, such as the human infant; nor is any milk a perfect food for a healthy adult of any species. Though an extrehiely valuable food, difl'ering from all others except possibly eggs in that it contains fairly good proportions of all the ingredients necessary for the build- ing and repair of the body and for supplying it with energ}' for its activities, the ingredients are so diluted with water that it would require a large quantity (4 or 5 quarts each day) to meet the needs of the adult body, and to get the required amount of energy from milk unnecessary quantities of protein must be consumed. Further- more, healthy digestive organs can do their work better when at least part of the food is in solid form. Even for adults, however, milk alone can support life for a considerable time, if not indefinitely, but its chief value is in combination with other foods; not as a beverage merely, but to supply in part the material needed for the body. For infants it is almost indispensable, and for invalids it is a nutritious, easily digested food, Avhich the physician can readily control. NUTRITIVE VALUE OF SKIM MILK. As every one knows, skim milk is the milk which remains after the butter fat — that is, cream — has been removed. In the ordinary proc- 1 ess of skimming the cream is allowed to rise in pans or other conven- ient receptacles, and the la^'er of cream on top is removed with a skimmer or some other convenient device. Formerly hand skimming was universal, but at the present time milk separators are in common use, which embody in their construction the physical principle that with rapidly rotating material the heavier body moves from the cen- ter to the circumference more rapidl}' than the lighter body. Milk separators permit of the convenient removal of any desired propor- tion of the butter fat, and in general remove the fat much more com- pletely than is possible by hand skimming. In Chart II (see p. 30) the outline figure, representing the glass of milk or some other convenient quantity, is so divided that the per- centage an.iount of the different constituents is indicated graphically. In the case of fuel values, the rectangular figure, representing 1,000 calc>ries or some multiple of 1,000, is so divided thai the black portion indicates the number of calories per pound supplied b}^ the food material under consideration. When whole milk and skim milk are compared, as in the table on page 9, and the chart on page 30, it will be seen that though the latter 363 30 THE USE OF MILK AS FOOD. is deficient in fat this is not the case as regards protein, carbohydrates, and ash. This is even more evident when tlie two sorts of milk are compared on a dry-matter basis. Such a comparison suggests a very important topic, namely, the great nutritive value of skim milk. Even after average milk is skimmed — that is, after the bulk of the fat is removed — the solids, or nutritive ingredients, still make up nearly one-tenth of its entire weight. The amount of fat left in skim milk varies greatly with the method of creaming. Ordinary open. Chart II. — Percentage composition and fuel value per pound of whole milk and some milk products, (The constituents are expressed in per cent, the fuel values in calories per pound.) W/^Ol£ M/IH S/(/M M/LK P/fors/A/: 3.3 ■c^RBOHYOM/tres :5. o ^OT£-/A/ : 3.4- rU£l VALU£: UNSWEETENED CONDENSED MILK 3IO C/ILOM/HS PSTR POUND FUEL VALUC: f6S C4LOP/SS P£^ POUNO. BUTTERMILK FAT.a /iSH:l.7 780 CALOPiES P£f( POUND pp6re//v:3.o -C/^/fBOHYOPATSS.'^.a ppor£//v:2.S <:/l/^BOf/yDP/tTSS:4: S FUEL VALUE: Pt/SL value: /60 CALORIES PER POUND 86S CALOR/ES PER POUND shallow pan setting leaves anywhere from one-tenth to one- fourth of the original fat of the milk in the skim milk. Deep, cold setting removes the fat much more completely, and separator skim milk has usually less fat than that from deep, cold setting. It is not far out of the way, however, to say that a pound of skim milk contains on the average 0.034 pound protein and has a fuel value of 170 calories, or a little greater proportion of protein than the same weight of whole milk and about one-half the fuel value. 363 THE USE OF MILK AS FOOD. 31 In the ordinary mixed diet a sufficient amount of fat is supplied by meat, butter, lard, etc., so that the loss of this ingredient is of rela- tively little importance. Protein, on the other hand, is the most costly of the food ingredients and the one most likely to be lacking in inexpensive meals, and this is the nutrient which skim milk supplies in a cheap and useful form. The value of skim milk as food is not generally appreciated. Taken by itself it is rather " thin " and, to use a common expression, " does not stay by." It is not as rich in flavor as Avhole milk, but when taken with bread or used in cooking, it forms a very nutritious addition to the food. A j^ound of lean beef (round steak, for example) contains about 0.18 pound of pro- tein and has a fuel value of 870 calories. Two and a half quarts or 5 pounds of skim milk will furnish nearly the same amount of protein and have about the same fuel value as the pound of round steak, while it would cost hardly a quarter as much. Two quarts of skim milk have a greater nutritive value than a quart of oysters ; the skim milk has 0.14 pound of protein and a fuel value of 680 calories, while the oj^sters contain only 0.12 pound of protein and have a fuel value of 470 calories. The nutriment in the form of oysters would cost from 30 to 50 cents, while the 2 quarts of skim milk Avould liave a market value of from 4 to 6 cents and a value on the farm of from 2 to 4 cents. A lunch or meal of bread and skim milk is very nutritious in pro- portion to its cost and convenience, as the following computation shows : Composition and cost of a lunch or meal of bread and skim milk. Food materials. Amount. Estimated cost. Protein. Fuel value. Bread Ounces. 8 8 Cents. 3 2 Pound. 0.05 .04 Calories. 601 255 Total 5 .09 859 The commonly accepted standard for a man at moderately active muscular work calls for 0.28 pound of protein and a fuel value of 3,500 calories per day, so that the above lunch furnishes very nearly one-third of a day's nutriment and at a cost of but 5 cents. If whole milk were used instead of skim milk, the cost would be about 7 cents and the fuel value 1,080 calories, while the protein would remain the same in amount. 363 32 THE USE OF MILK AS FOOD, The following lunch, such as might be obtained in a restaurant or lunch room, will serve for the purpose of comparison : Estimated cost and nutrients of a restaurant lunch. Food materials. .\mount. Estimated cost. Protein. Fuel value. Soup Ounces. 8.0 2.0 2.0 1.0 3.0 .5 1.0 .5 Ctnls. Pound. 0.01 .02 Calorics. 75 Beet 275 100 15 Bread .02 225 Butter 100 Coffee: Milk 20 65 Total 15-20 .03 865 It will thus be seen that the 15-cent lunch, containing nine different food materials, would not have any greater nutritive value than the 6-cent lunch of bread and skim milk; though it contained a few calories more of energy, it w-ould yield only about half as much protein. The idea that only whole milk is fit to use, which is rather errone- ously held by housewives, is perhaps ascribable to the esteem in which cream is held as an ingredient of " rich food," and may lead to quite needless waste or expenditure. For growing children, who need large quantities of j)rotein and carbohydrates, 2 quarts of skim milk would supply more of these constituents and more ash than 1 quart of whole milk, and it has alread}'^ been suggested that fresh skim milk of good qualit}^, sold under its real name and at a reasonable price, is preferable to suspicious whole milk. Many families who are in the habit of drinking whole milk and buying cream would doubtless be quite as Avell off if the top of the milk, say 2 or 3 inches in a quart bottle, were poured into the cream instead of the milk pitcher; the milk ought still to be far from thin and blue, and there would be a marked saving in the cost of cream. On the farm skim milk is often considered fit only for the pigs; but it really may be used in cookery, etc., as a useful and economical food, though it is only fair to say that on a farm milk is often so abundant that the fresh whole milk will naturally be selected for culinary and table purposes. The fresh whole milk seems to most persons more tempt- ing as a beverage, perhaps, than older skim milk, though this is a matter largely of cultivation and habit; but if no one cares to drink it the skim milk can always be used in cooking, as will be discussed in a later section, a fact of more importance from the standpoint of economy in families where milk is purchased than in those where it is produced. 3G3 THE USE OF MILK AS FOOD. 33 COST OF NirTRIENTS IN WHOLE MILK AND SKIM MILK. Just as the nutritive value of a given food depends not simply on its chemical composition but on its proportion of digestible nutrients, so its real cheapness or dearness depends not on the price per quart or per pound, but on the amount of digestible nutrients which a given sum will jiurchase. Milk is economical in the sense that it contains no refuse, such as the bone and gristle of meat, the shells of eggs, and the skins and seeds of vegetables. Moreover, the proportion of its nutrients which can be digested and utilized by the body is, under ordinary circumstances, larger than that from most food materials. The thing which might make milk expensive is its diluteness — that is, the large amount of water which it contains in proportion to its solid matter. The accompanying chart will help in showing the real cheapness or dearness of milk at various prices as compared with other foods. In this chart the amount of nutrients is indicated by the shaded portions corresponding to the protein, fat, and carbohydrates, the length of each portion representing the number of pounds or frac- tions of a pound of the constituent which is supplied, each full division of the chart being equivalent to 1 pound. The heavy black line indicates the number of calories per pound, each full division of the chart corresponding to 2.000 calories. The whole milk represented contains the average am.ounts of solid matter, regardless of price. Milk selling at 10 cents a quart ought to contain larger proportions than that selling at 5 or 6, besides having fewer bacteria; on the other hand, milk at the lower prices may occasionally contain larger proportions than the more expensive, but it is impossible to represent such variations satisfactorily. For the purposes of the present comparison the average composition will be sufficiently accurate. Of course, in comparing milk and such material as dried beans, one must take into account the fact that the beans will absorb con- siderable water in cooking and that a pound of them ready to eat would not contain as much nourishment as is represented in the chart, while milk is consumed as bought. In addition, the cost of cooking must always be considered in connection with foods which are not eaten raw. Bearing these things in mind, we see that milk at all but the highest prices assumed is a cheaper source of protein than any of the animal foods except cheese, very cheap meat, and salt fish. At usual prices skim milk furnishes protein more cheaply than any common animal food except salt fish. The protein of vegetable foods is less expensive, but, on the other hand, as prepared for the table is less thoroughly digested. Moreover, it is accompanied by such large amounts of 363 34 THE USE OF MILK AS FOOD. Chart III. — Pecuniary ecoriomy of milk and other foods. Amount of actual nutrients obtained in di^erent foods for 10 cents at certain assumed prices per pound. PROTEIN. /TtT. C^RBOMYDP/irES. FUEL VALUE. 303 THE USE OF MILp AS FOOD. 35 carbohydrates that to secure much vegetable protein in the diet usually means an excess of the carbohydrates. Under ordinary mar- ket conditions milk, and even skim milk, is a cheaper source of body fuel than any of the usual animal foods except cheese and salt pork, but is a dearer one than the usual vegetable foods. Here again, how- ever, the milk furnishes the ingredients in a form more readily and thoroughly digested than the vegetable foods as ordinarily served. Milk, then, is fully as economical a source of nutrients as most animal foods, but is dearer than most vegetable foods. It has the decided advantage of having no waste, requiring no time for preparation, and being more digestible than the vegetable foods. Skim milk answers the most important purposes of milk in the ordinary mixed diet, and under usual market conditions costs at most only half as much as whole milk. Both whole and skim milk at moderate prices arc there- fore to be ranked among the most economical of our foods, not only when taken as beverages, but also when used in preparing other foods. THE TJSE OF MILK IN COOKING. If freely used in the preparation of other foods, milk can be made to add considerably to the food value of the meals. Many dishes are, of course, richer in flavor if whole milk is used rather than skimmed, but for the purpose of increasing the food value of the diet the main nutritive ingredient — that is, the nitrogenous material — as we have seen, is in the skim milk ; if the extra fat is needed, it may be supplied in the form of butter, which is usually a more economical source than whole milk, or in the form of lard or other culinary fat. Most persons consider that the choice between bread made with milk and that made with water depends simply upon the taste and appearance. There is, however, a difference in the food value, as will be seen from figures showing the average composition of various foods prepared with milk and of similar dishes in which it is not used. According to the figures which were taken from anal3'ses made at the University of Minnesota,* bread made with skim milk is richer in total solids, protein, and fats than otherwise similar bread made with water. The dilTerences are not very great, but they are well worth considering, especially where skim milk is a drug in the mar- ket. The importance of skim milk in bread making has also been demonstrated experimentally at the Maine Experiment Station. Milk soups furnish an excellent means of increasing the food value of a meal or of using up superfluous milk. Sometimes the milk is mixed wdth " stock " made from meat, and sometimes, as in vegetable purees, it forms the basis to which the pulp of some vegetable, such as beans, peas, potatoes, corn, or celery, is added to give flavor and « U. S. Dept. Agr., Office Expt. Stas. Bui. G7, 363 36 THE USE QF MILK AS FOOD. " b(x]y." Oyster stew made with milk owes its food value more to the milk than to the oysters. ]Milk 01* '' white "' and '" cream " sauces are also very useful, not only for the nutritive material they supply but also as a help in using up '• left overs." Bits of meat can often be made very attrac- tive by serving them on toast minced and " creamed," and many warmed-over vegetables are improved by the addition of milk or white sauce, while the same sauce also helps in giving variety in winter when not many kinds of vegetables are to be had. There is almost no end to the puddings and desserts in which considerable milk is used. Blanc mange is practically flavored milk, jellied with starch, Irish moss, or some similar material, and the simplest kinds of ice cream are milk and cream mixtures flavored and frozen. Junket is simply milk curd separated by rennet, as in cheese making, and eaten before the bacteria which give the cheese its flavor develop. Then there are all the puddings made of some form of cereal and milk and flavored in some way, as bread puddings, cornstarch, rice, and tapioca jouddings, and the countless forms of custard of which milk and eggs make the basis. Almost all of these, if carefully made, are nutritious, easily digested, and economical. For children and persons of weak digestion the simpler ones like blanc mange, corn- starch, and rice puddings are almost indispensable. MILK PRODUCTS. BUTTER AND CHEESE. These are by far the most important products of the dairy industry, but so much has been written of them in other publications of this Department that they need only be mentioned briefly here. Chart IV shows in graphic form the composition of butter, cheese, and other milk products. The data are given in tabular form for these and other foods on page 9. Butter, which is practically separated milk fat, is one of the most important sources of fat in our diet and certainly the most palatable and one of the most digestible. Its flavor depends upon such factors as volatile fats, cleanliness, the action of bacteria, and the amount of salt added. The price varies greatly, even during the same season, and is influenced by flavor, color, texture, etc. These factors affect quality rather than composition, and so far as is known have little effect on nutritive value or digestibility. As has been stated already, it is usually a cheaper source of pure fat than Whole milk. The butter on sale in the United States is usually salted, partly because of the general preference for the salt flavor and partly be- cause of improved keeping qualities. It is noticeable that butter is 363 THE USE OF MILK AS FOOD. 37 now less heavily salted than was the case before cold storage and other such conveniences became common. In Europe sweet or un- salted butter is very popular, and its popularity is growing in the United States. Such butter has a mild, creamy flavor, which verj'' many relish. As it does not keep well it must be used soon after it is made, and ranks as a delicacy. Many persons make sweet butter at home as needed, using for the purpose an egg beater or one of the devices similar in principle which are on the market for this purpose. Chart IV. — Percentage composition and fuel value per pound of butter, cheese, and other milJc products. (The constituents arc expressed in per cent, the fuel values in calories per pound.) CREAM CHEESE BUTTER AVATeR:II.O ^ FAT:8S.O K protein: 1.0^ MSH:3.0 FUEL VALUE: FUEL VALUE: ^PROTEIN : 25.9 CAPBO - HYDRATES: 2.4 3410 CALORIES PER POUND COTTAGE CHEESE fQ^Q CALORIES PER POUND CARBOHYDRATES : 4.3 /NFANT FOODS fat- i.o^ '^protein :20.9 (milk and starches) fuel VAL UE: WAT£R:4.3-^^ -^ /PR0TEIN:9.6 FAT:3.S ASH: 2.1- M/LH POWDER DRIED SHIM MILK WATER:3.0 510 CALORIES PER POUND FAT: 3.1 J ^ARBO- 1'^ HYDRATES: 60.2 ASHie.O 1780 CALORIES PER POUND xPROTEIN: 34.0 ^CARBO- r HYDRATES: 51.9 /7/S CALORIES PER POUND A peculiar form of butter, called " ghee," is commonly used in India and central Asia. The water is boiled out from freshly made but- ter and the resulting product is kept for general use, especially in cooking. In the cold, high regions of Tibet it is used in large quan- tities, lumps of it being put even into tea. This product is very similai: to the rendered butter which is used by orthodox Jews in kosher cookery. Rendered butter is also a common culinary fat in Egypt and some other countries. 363 38 THE USE OF MILK AS FOOD. Cheese consists of the casein of the milk and more or less of the fat and mineral matters which are precipitated along Avith it when rennet is added to milk. Thqre are countless varieties made not only from cow's milk but also from that of goats, ewes, and other animals. The flavor is due chiefly to the action of ferments, of bacteria, or of molds. The changes which these agents induce are now well understood, and the manufacture of cheese of different types is an important industry in which the expert cheese maker now utilizes them according to scientific methods to bring about the desired results with almost un- varying certainty. The literature of food and nutrition contains lit- tle definite information regarding the digestibility of cheese, but there is a general opinion that this food, particularly the very strong varieties, is less easily digested than most milk products. As regards thoroughness of digestion, a very large number of digestion experi- ments carried on^by the Office of Experiment Stations in cooperation with the Bureau of Animal Industry of the Department of Agricul- ture have shown that, when consumed even in relatively large amounts, cheese is very thoroughly assimilated. Furthermore, it caused no physiological disturbances in the large number of tests in which it was used. Cheese, containing, as it does, almost all of the protein and fat of the milk from which it is made and having a com- paratively low water content, is a very nutritious food, and the, cheaper kinds may well be used more abundantly than is commonly the case in this country as a part of the regular diet and not simply as a condiment at the end of the hearty meal. The cost of cheese varies greatly with the kind, but the higher prices are usually paid for distinctive flavors or texture rather than for food values. JUNKET. A favorite dish for invalid dietetics and for more general use is prepared by adding rennet to milk and allowing it to stand undis- turbed until it thickens or coagulates — that is, until the casein is precipitated. If the process is carefully carried out a thick, custard- like product results. If, however, it is stirred the casein readily breaks up and separates from the whey. There are a number of preparations of rennet on the market designed especially for making junket, all of which seem to give satisfactory results. COTTAGE CHEESE. Cottage cheese as commonly made at home from sour milk with or without the cream is a nutritious and palatable food, as may be seen from figures for its composition in the table on page 9, and the data given in Chart IV, page 37. Under ordinary conditions it is very inexpensive. There are several sorts of this cheese on the market 363 THE USE OF MILK AS FOOD. 39 which are made in large quantities as regidar commercial products and sold under such names as Neufchatel, or, less commonly, cream cheese. In many city markets the homemade product may also be purchased. Cottage cheese is used as a palatable addition to the diet, alone or seasoned in various ways, and is also used in the prej)aration of a number of dishes. CREAM. Cream is made up principally of water and the fat of milk, but contains also a little protein, carbohydrates, and mineral matters. When served with cereals, fruit, etc., it not only improves the flavor, but adds to the nutritive value of the dish. When added to the cus- tards, cakes, soups, and other food it materially increases the fat con- tent and makes the dish " richer," as does cream, either plain or whipped, served with a dessert dish or other food. Judging by available data, cream in large quantities is less easily digested than the same amount of whole milk, because it contains so much fat ; but, on the other hand, it is more thoroughly digested than most other forms of fat, and is often ordered by ph3^sicians when tlie amount of this ingredient in the diet is to be increased. At usual prices it is so expensive that it must be considered a luxury to most people who purchase their milk and cream. When cream is beaten or whipped the fine bubbles formed do not break readily, so that it may be whipped until it is stiff, almost like egg white. If the " whipping " is too long continued the fat globules separate, and butter results. If cream is too thin it does not whip well, and the same may be said of very thick cream. Several ex- planations of this property of " whipping " have been offered, but it seems fair to say that no entirely satisfactory explanation has been given. That the property may be dependent upon the calcium con- tent of the milk is suggested by the fact that cream to which a small amount of a solution of lime in sugar and water has been added may be readily whipped. This question has been carefully studied at the Wisconsin Station, and it was found that if a small portion of such a lime solution was added to pasteurized cream which ordinarily does not whip readily, or to thin cream, satisfactory results could be obtained. A cream product known as " Devonshire clotted cream," or simply as " clotted cream," and very popular in Great Britain and, to a less extent, elsewhere, is made by allowing pans of milk to stand undis- turbed in a cool place until cream rises and then scalding, care being taken not to disturb the cream on top af tlie milk. When this is properly done and the pans allowed to cool, the cream may be taken off in a thick, clotted condition, and is ready for use. It has a peculiar, nutty flavor, which most persons relish. 363 40 THE USE OF MILK AS FOOD. BUTTERMILK. Besides skim millv, there is another important by-product resulting from the manufacture of butter, namely, buttermilk. In butter mak- ing the fat globules are brought together by churning and removed, leaving a thin liquid. This buttermilk is very like skim milk in com- position, but it has usually a mild acid taste, because the cream is generally allowed to sour before churning. Buttermilk is often used as a beverage, and has much the same food value as skim milk. An ordinary ghiss would contain about as much nourishment as 2 ounces of bread, a good-sized potato, or a half pint of oysters. To many jDersons buttermilk is as palatable, or even more palatable, than whole or skim milk, but others find the sour taste very unpleasant. This sour taste is due mainly to lactic acid, and does not make the butter- milk less digestible. On the contrary, its casein forms a more flaky curd than that of ordinary milk. Buttermilk is frequently fed to babies, especially in Holland, and is sometimes prescribed when the protein of ordinary milk proves indigestible. Its general use is increasing in this country. Buttermilk ice cream is considered a delicacy in some sections, especially for invalids. Condensed buttermilk is manufactured to some extent, but seems to be best known in Europe, and is certainly not common even there. WHEY. Just as buttermilk represents the residue of milk from butter mak- ing, so whey represents what is left from cheese making, and consists mainly of water, milk sugar, and mineral matters. It is less nutritious than skim milk and buttermilk, but is occasionally useful as a mild laxative drink for invalids. In the " whey cures " for dyspeptics, of Avhicli so much was heard some years ago, whey was usually com- bined with a simple vegetable diet and the beneficial results were probably due as much to the simple, out-of-door life, which made part of the treatment, as to the whey itself. T\niey may be made at home by cooking sweet milk with some acid material, such as vinegar, lemon juice, sour wine, or cream of tartar, or even with sour milk. Such whey differs but little in composition from regular cheese whey. Fresh curds and whej is an old-fashioned dish still used to some extent, though less common than it was when cheese making was regularly carried on as a home industry. SOUR MILK OR CLABBER. Sour milk or clabber is a common article of diet in many parts of the United States and is wholesome, and to those who care for it very refreshing and palatable. It is certainly nutritious, since when 363 THE USE OF MILK AS FOOD. 41 made from whole milk it contains all the nutritive ingredients origi- nally present, and when only skim milk is used it contains the casein and other constituents except the fat. Usually no special methods are followed to prepare sour milk or clabber for table use, though most housewives recognize the fact that if the souring takes place too slowly the clabber may have a bitter or unpleasant flavor. This is, of course, due to the develoj^ment of undesirable bacteria along with those which cause the souring of the milk. Sour milk is much used in cookery, and adds materially to the nutritive value of the dish of which it forms a part. Before baking powders became so com- mon sour milk and baking soda were very commonly used to leaven doughs and batters of various sorts. Some cooks maintain that they can secure the best results by using only the whey of sour milk, but this naturally gives a less nutritious dish and therefore is not so desirable. KEPHIR, KOUMISS, AND OTHER FERMENTED MILK PRODUCTS. Since earliest times fermented-milk products have been used as beverages and articles of diet in Central Asia, Turkey, and other countries. These products are prepared by allowing special ferments or yeasts to develop in milk, and like all fermented beverages owe their sparkling or effervescent qualities to the carbon dioxid j^ro- duced by the action of organisms. The flavor differs with the process of manufacture. These fermented-milk beverages have proved very satisfactory in invalid dietetics and are now well known and com- monly used. Fermented-milk beverages may be made at home, but are perhaps more commonly commercial products in the United States. In this country cow's milk is almost universally used to make these beverages, but other milk, for instance, mare's milk, is more common in Central Asia and other regions. A carbonated milk, which is made by charging milk with carbon dioxid, is sometimes found on sale, but, of course, lacks the special qualities which all the fermented products contain. Some of these fermented-milk products contain lactic acid forming bacteria in great abundance, and their extensive use has been much discussed recently, owing to the prominence which has been given to the theory that very many diseases, particularly those incident to old age, are the results of the development of putrefactive bacteria in the intestine and the possibilities of hindering the growth of such organisms by the presence of lactic acid. The most recent observa- tions, however, make it doubtful whether it is as beneficial as has been claimed by some enthusiasts, 363 42 THE USE OF MILK AS FOOD. STJMMAEY. Some idea of the importance of milk as human food may be gained from the fact that about one-sixth of the total food of the average family is furnished by it and its products. Milk from various mam- mals is used in various parts of the world, but with us that of the cow so far surpasses all other kinds in importance that imless other- wise specified the w ord " milk " is taken to refer to cow's milk only. Few staple foods vary so much in composition, but, on the average, good, unadulterated milk should contain about 87 fter cent water and 13 per cent solids. About one-fourth of these solids are furnished by the protein compounds, casein, and albumin, the casein being five or six times more abundant than the albumin. Fats (butter fat) form one-third of the total solids. Butter fat occurs in globules throughout the milk, and it is upon the size and number of them that the creaminess of milk depends. The larger the globules the more easily and completely will they rise as cream to the surface of the milk. Carbohydrates make up 38 per cent of the solids, by far the most important of them being lactose, or milk sugar. The remain- ing 5 per cent of solids consists of mineral matters, the bulk of which are phosi:)hates and chlorids of soda, potash, and lime. Besides these chemical constituents, milk almost inevitably con- tains bacteria of many kinds and in varying numbers. They cause the souring of milk and the ripening of cream and cheese, and pro- duce many other changes in the appearance and flavor. The numl)er present in freshly-drawTi milk varies enormously with the conditions of milking, and, as they are greatly increased with dirty and careless handling, cleanliness in all matters pertaining to the milking and marketing of milk and keeping it in the home can not be too strongly insisted on. Disease germs, notably those of typhoid, diphtheria, scarlet fever, and tuberculosis, may also be carried in milk, so that the purity of the milk supply is of vital importance to every family and community. Although some of the bacteria in milk are essential in the manu- facture of butter and cheese, they are as a class a source of danger to the ordinary consumer. Without them milk would stay sweet indefinitely, and the problem of keeping milk is simply one of check- ing their growth. The less there are in the fresh milk, the slighter the danger, hence the superior keeping qualities of milk from a clean, well-ordered dairy. Tliey are comparatively inactive at a tempera- ture below 50° F., and therefore milk should be kept in a cool place. Freezing does not kill bacteria and produces undesirable changes in the milk. Extreme heat does kill them, but also produces other un- desirable changes. Nevertheless, heat is employed to preserve milk in the two common methods of pasteurization and sterilization. In 3G3 THE USE OP MILK AS FOOD. 43 the former the aim is to apply heat in such a way as to kill the most bacteria without producing the undesirable changes; in the latter, to apply enough heat to kill all the bacteria, but with the least pos- sible undesirable change. Bacteria require moisture as well as heat' for their growth, therefore by extracting the water, as in condensed milk, milk powders, etc., milk may be preserved indefinitely. An- other way of keeping it is by the use of chemicals to kill the bacteria, but as such chemicals may be injurious to human beings as well, such practices are not usually to be recommended. AVliat is commonly known as the richness of milk depends upon the amount of fat. This varies so greatly in milk from different animals and is so easily reduced by a fraudulent dealer that many efforts have been made to regulate the price of milk according to its fat content. Milk graded according to government standards is sold in some cities, especially in Europe. Certified milk — that is, milk in sealed jars from establisliments regularly inspected — is more commonly known in this country, and rightly commands a higher price than that from uncertain sources. Of course, cleanliness and care are as important in keeping milk in the home as in the dairy and market, and each housekeeper should see to it that all receptacles in which it is kept are thoroughly scalded each time they are used; if this is neglected, bacteria from the old milk will contaminate the fresh. Compared with other food materials, milk furnishes the nutritive ingredients in forms in which they may be easily and thoroughly digested by the normal, healthy person, and often by those of im- paired health. Boiling is believed by many to make the protein slightly less digestible, but as yet knowledge on this point is incom- plete. The digestibility of the fat seems to depend upon the size of the globules, the smaller ones being more easily absorbed. This ex- plains why rich, " creamy " milk, such as that given by the Channel breeds, sometimes causes digestive trouble in children. Mother's milk is best adapted by nature to the nourishment of in- fants, and differs from cow's milk mainly in the character of its pro- tein, in the smaller size of its fat globules, and in its greater amount of milk sugar. "N^Tien necessary, cow's milk may be artificially modi- fied to approach it more nearly in composition, but the results are not always satisfactory, especially as regards the protein. Milk contains too much water to be a perfect food for adults; nevertheless, its solids furnish all the necessary ingredients and in good proportions. A large glass of it yields as much nourishment as a slice of i:oast beef. Unless exceptionally high prices are paid for it, milk is fully as economical a source of nutrients as other animal foods, but dearer 3G3 44 THE USE OF MILK AS FOOD. than most staple A^egotable products. It should be borne in mind, however, that it requires no preparation, has no waste, and is more thoroughly digested than most vegetable foods. As a source of pro- tein, the most expensive of the nutritive ingredients, it is especially economical. Skim milk, which is whole milk minus part of its fat, and which costs only half as much as whole milk, furnishes protein about four times as cheaply as beef, and since fat is usually abundant in the ordinary mixed diet might most advantageously be used in the place of whole milk in dietaries where cost must be carefully con- sidered. The freer use of skim milk in cooking is also to be recom- mended. Of course, foods prepared with either skim or whole milk are by so much the more nutritious than those prepared with water. Butter and cheese are the most important milk products. Butter is one of the chief sources of fat in the ordinary diet and furnishes it in a very palatable and easily digested form. Cheese consists of the casein of milk plus more or less of the fat and mineral matters. The flavor and texture of the many varieties are due mainly to the peculiar bacteria and ferments which the various methods of manu- facture develop. The less expensive varieties make one of the cheap- est sources of protein, and might well be more freely used as part of the regular diet. Digestion experiments indicate that the common and milder varieties are more easily and thoroughly assimilated than is som.etimes supposed. The other milk products — junket, whej^ but- termilk, clabber, and the fermented forms, such as kephir and koumiss — are all nutritious foods and are often of especial value in invalid diet. In short, milk and its products are fully entitled to their prominent place in our food list as comparatively inexpensive, easily digested sources of all the necessary ingredients of our diet. [A list giving the titles of all Farmers' Bulletins available for distribution will be Beat free upon application to a Member of Congress or the Secretary of Agriculture.] 303 Q This book is DUE on the last date stamp UNIVERSITY OF CALIFORNIA LIBRARY Los Angeles This book is DUE on the last date stamped below. MAR 1 7 1S5f. WI\R 2 4 195B j£C 5 Form L9-50m-7, '54(5990)444 UC SOUTHERN REGIONAL LIBRARY FACILITY AA 001 169 904 8 TX M^m6 ■fel.^ ^/.: m