CHAPTER II

ANIMAL FOODS: MILK

The animal foods used by man are not of great variety and source. They include the flesh and various organs of the herbivorous animals, swine, domestic and wild fowl, eggs, fish, shellfish, insects and their products (honey), milk, and milk products. The flesh of carnivorous animals, except that of fish, is unpalatable and, therefore, undesirable as a food.

The most important animal foods from the standpoint of the sanitarian are milk and meat.

MILK

Milk is responsible for more sickness and deaths than perhaps all other foods combined. There are several reasons for this: (1) bacteria grow well in milk; therefore, a very slight infection may produce widespread and serious results; (2) of all foodstuffs milk is the most difficult to obtain, handle, transport, and deliver in a clean, fresh, and satisfactory condition; (3) it is the most readily decomposable of all our foods; (4) finally, milk is the only standard article of diet obtained from animal sources consumed in its raw state.

The total milk production in the United States in 1911 was ten billion gallons. One-quarter of this is consumed as milk and the remaining three-quarters is used for butter and cheese. The average

per capita consumption of milk in the United States is 0.6 of a pint daily. More milk is used in the North than in the South; very little in the tropics, and practically none at all in China, Japan, and some other countries. About 16 per cent. of the average dietary in the United States consists of milk and milk products.

Fresh milk products may be quite as dangerous as the milk from which they are made. Milk laws which ignore milk products are incomplete from the sanitary side, and will fail to accomplish their purpose from the economic side.

Milk is a perfect food for the suckling. It contains all the essential elements of a well-balanced diet for the adult, and at prevailing

prices it is one of the cheapest of the standard articles of diet. Furthermore, it is readily digestible and is capable of a great variety of modifications. The sanitarian, therefore, has every reason to encourage the use of pure milk as well as to discourage the use of impure milk. Composition. Milk is the secretion of the mammary gland. In composition it is exceedingly complex, consisting chiefly of water; several proteins in colloidal suspension; fats in emulsion; sugar, and a number of inorganic salts in solution; also enzymes, as well as antibodies, cells, gases, and other substances. Milk from all animals shows a general agreement in physical properties and composition, containing essentially the same ingredients, but exhibiting differences in the amounts of the several constituents.

In the fresh state milk is a yellowish white, opaque fluid. Cow's milk has a specific gravity of 1.027 to 1.035; it freezes at a temperature somewhat lower than the freezing point of water (-0.554° C.); the electrical conductivity is 43.8X10-4 for cow's milk, and 22.6×10-4 for human milk. In other words, 58 per cent. of the molecules in cow's milk and 26 per cent. in human milk are dissociated. The specific heat of milk containing 3.17 per cent. of fat is 0.9457. The coefficient of expansion is greater than that of water. Milk shows no maximum of density above 1° C.

Freshly drawn milk of carnivorous animals is, as a rule, acid in reaction. This is probably due to CO, and acid phosphates. Human milk and that of most of the herbivora are slightly alkaline; cow's milk has been described as amphoteric.

Under the microscope milk is found to contain fat globules and cells, as well as bacteria, débris, and other objects.

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The gases dissolved in milk are oxygen, nitrogen, and carbon dioxid (3 to 4 per cent. by volume). Oxygen and nitrogen are carried into milk mechanically from the air in the process of milking. Other substances found in milk, but in small quantities, are lecithin, cholesterin, citric acid, lactosin, orotic acid, and ammonia.

The composition of cow's milk may be understood from the scheme prepared by Lucius L. Van Slyke, given on page 496.

PROTEINS. The three proteins constantly found in milk are casein, lactalbumin, and lactoglobulin. A trace of fibrin, mucin, and other proteins sometimes occurs.

The proteins in milk of a given species are quite constant both in composition and amount; it is, therefore, not necessary, as a rule, to make a special analysis for them. They may be estimated by subtracting the fat, sugar, and ash from the total solids.

Casein is a highly specialized protein found in the secretion of the milk glands of all mammals, but nowhere else in nature; it is a nucleo'When not otherwise specified in this section milk refers to cow's milk.

albumin, and as such contains phosphorus. It is soluble in water, and by virtue of its property as an acid it forms soluble salts with alkalies. There are two series of casein salts, basic and neutral; solutions of the latter have a milky appearance. In milk, casein is found dissolved in the form of a neutral calcium salt, which accounts in part for the white opalescent appearance of milk. Casein exists in milk in the form of caseinogen, that is, casein in combination with calcium phosphate. The caseinogen is held in solution by the calcium phosphate. It is not coagulated by heat, but is precipitated by acids, for the reason that acids take the calcium from the calcium phosphate, and thus throw the casein out of solution as a curd. This flaky or lumpy precipitate is again soluble in lime water and dilute alkalies. Casein is also thrown out of solution by rennin.

Lactalbumin is very similar to the serum albumin of the blood, but it appears to differ from this in some particulars. It coagulates by heating to 70° C., but not with dilute acids, and is precipitated by a saturated solution of ammonium sulphate, but, like all other albumins, is not precipitated in a neutral solution of sodium chlorid and magnesium sulphate. Lactalbumin contains sulphur but no phosphorus. It is present in amounts varying from 0.2 to 0.8 per cent., but is much. more abundant in colostrum.

It

Lactoglobulin occurs in milk in very small quantities, merely in traces, while colostrum is comparatively rich in this protein. coagulates at 75° C., it is precipitated in the same way as serum globulin, and, like serum globulin, is insoluble in water, but is soluble to some extent in weak salt solution.

FAT. The fat is suspended in the milk serum in the form of an emulsion. The droplets or globules vary in size. On the average they are smaller in milk from Holstein than from Jersey, Guernsey, or shorthorned breeds. Under the microscope some of the fat globules seem to have an albuminous membrane, but this interpretation is now questioned. The fat droplets are lighter than the milk serum, hence they rise on standing (gravity cream), or they may readily be separated by centrifugal force (centrifugal cream). Cream, or top milk, does not consist of fat alone, but contains all the constituents of the milk; it is simply milk rich in fat. Upon shaking the fat globules gradually coalesce into larger drops and lumps to form butter.

The first milk drawn from the udder is commonly poor in fat. This is known as "fore" milk. The middle portion contains about the average percentage of fat, and the last, known as "strippings," is always the richest in fat. The strippings may contain as much as 9 or 10 per cent. butter fat.

Heat increases the viscosity of milk, and hence hinders the rising of the fat drops; 68° C. is the critical temperature; if heated above this

point for any length of time the formation of the cream line is retarded or prevented. For this and other reasons the richness of milk, therefore, cannot always be judged by the depth of the cream layer.

Milk fat consists of a mixture of different neutral fats, the principal of which are olein, palmitin, and stearin. These are neutral triglycerids of the corresponding fatty acids. Besides these are found the triglycerids of miristic, butyric, and caprylic acids. The last two are volatile and give to butter its characteristic odor and flavor. The composition of the fat is subject to variation, depending upon racial or individual peculiarities, also upon the character of the food and other conditions.

The percentage of butter fat in milk has long been one of the standards by which milk is tested. The richness of milk gaged by the amount of fat it contains is more of an economic than a sanitary question. Milk with a low percentage of fat from Holstein cows is relatively just as nutritious a food as richer milk from Jersey and Guernsey cows; even skimmed milk containing little or no fat is a valuable food. The problem is one of honest labeling and the marketing of various grades at prices corresponding to their nutritive contents. When the standard for butter fat in milk is relatively low, say 3.25 per cent., it is a temptation for dairy men to remove the excess. This is a fraudulent practice which should not be countenanced. A high fat standard encourages the breeding of better cows; requires caution in their feeding and care, and puts a premium upon good dairy methods.

In normal milk the larger proportion of the fat droplets agglutinate into tiny clusters or masses. At a temperature of 65° C. or above these clusters are broken up and the globules are more homogeneously distributed throughout the liquid. When milk is subjected to a pressure of about 3,000 pounds at a temperature of about 75° C. the individual fat globules are broken up into fine particles, which remain as a uniform and permanent emulsion known as "homogenized milk." This process applied to cream increases its viscosity, so that cream containing 20 per cent. butter fat appears to have the body and richness of a 30 per cent. cream.

Researches of Heubner, Keller, and Czerny show that the fats and not the proteins are the cause of much of the digestive disturbances in infants. When the fat is excessive in amount the infant at first seems to thrive, but sooner or later loses weight and appetite, and shows other symptoms, associated with stools composed largely of fat soaps and of a pale gray, hard, and dry constituency. The alkaline bases are so largely drawn upon from the body to saponify the excessive amount of fat in the intestines, that a condition resembling acidosis may appear; furthermore, fermentative changes take place in the intestines and the "catastrophe" ensues.