tion of approximately 100 per cent, while those of the sort shown in Table 5 always fall considerably short because a part of each amino acid is unavoidably lost in the process of their separation and determination. It is certainly largely and probably chiefly because of the unavoidable underestimation of each known amino acid that the summation in all analyses of this kind fall so far short of showing 100 per cent. The deficit does not prove the presence of amino acid radicles of other kinds, though it is still possible that some such may be present.

On whichever basis the amino-acid make-up of the milk proteins is expressed, it is plainly apparent that the proteins of milk are richer than those of most other foods in the amino-acid lysine which is important in growth. Such quantitative estimations as are available show the milk proteins to be also relatively rich in tryptophane, which is extremely important both for growth and maintenance and which is furnished in only small amounts, if at all, by some of the other important food proteins. This richness in lysine and tryptophane has important bearings upon the place of milk in the diet, as have also the richness of milk in calcium and in fat-soluble vitamin.

The fat of milk is characterized both by its physical form and by its chemical composition. As shown above (Fig. 6) it exists in the form of small droplets floating in the "serum" of the milk and tending to rise to the surface and form a cream layer when the milk is allowed to stand. This makes of milk an emulsion varying in physical property with the percentage of fat and the size of the fat globules. The so-called "homogenizing" machines are devices for breaking up the fat globules into particles of small and more uniform size with the object of getting the fat into such a finely divided and highly dispersed form that it will remain uniformly distributed through the milk on standing. Milk or cream which has been thus treated is said to be "homogenized," though it is, of course, still a heterogeneous system in the scientific sense.

Chemically milk fat is characterized by its relatively high content of butyric acid, and relatively low content of stearic acid. The percentages of the different fatty acids found by Browne in butter fat are shown in Table 7.

TABLE 7. PERCENTAGES OF DIFFERENT FATTY ACIDS OBTAINED
FROM BUTTER FAT BY BROWNE

Butter fat, like most other food fats, contains considerable quantities of palmitic and oleic acids, but it differs in containing very little stearic acid, and notable quantities of butyric and caproic acid, of which other food fats contain very little. The fact that butter contains more of these fatty acids of low molecular weight (which are liquids) and less stearic acid (which is solid) makes milk fat softer than most food fats, and this softness together with its emulsified form both help to give it its high digestibility.

For convenience we have here spoken of fatty acids as contained in the fat. It is not implied that the fatty acids exist in the free state, nor even necessarily in the form of their simple glycerides. Doubtless both simple and mixed glycerides are present. It is believed that practically all of the butyric acid in butter fat exists in the form of mixed glycerides.

The most important difference between milk fat and most other fats as food lies not in the triglycerides, which, chemically, constitute the fat itself, but rather in the fact that milk fat is rich in fat-soluble vitamin.

The yellowish color of milk is due to a small amount of the natural coloring matter, lactochrome, dissolved chiefly though not entirely in the fat of the milk, and which is derived from the pigments of the green plant tissues consumed by the cow (Palmer).

Milk sugar, lactose, is the only known carbohydrate of milk. It is formed in the mammary gland, doubtless from glucose brought by the blood. While the proteins exist in milk in the form of colloidal dispersion, the fat in the form of distinct droplets easily visible under the microscope, the milk sugar is in true solution. It is to the milk sugar and the soluble salts of milk that its osmotic properties are due. Investigation has shown that the mammary gland secretes milk always of a very uniform osmotic pressure. Usually this means that the percentages of lactose and soluble salts in milk remain nearly constant even when the percentages of other constituents vary. Occasionally the percentage of lactose falls considerably below the normal average, and in such cases the percentage of salt in the milk rises sufficiently to keep the osmotic pressure at the normal figure. Since, weight for weight, the osmotic pressure of salt is much higher than of sugar, a sample of milk of the sort just mentioned will have an abnormally low percentage of solidsnot-fat, but since its osmotic pressure and therefore its freezing point remains normal, a determination of the freezing point will show that the milk is not watered. Conversely, watered milk always shows a different freezing point from genuine milk, even though the latter varies in chemical composition. For this reason the determination of the freezing point is frequently included in the analytical examination of milk for the purpose of determining whether or not it has been watered.

Citric acid occurs normally in milk to the extent of about 0.2 per cent. In the ordinary routine analysis, or in statements of composition based on such analyses, the citric acid is usually counted as carbohydrate.

The ash constituents of milk include all the so-called inorganic elements necessary to the normal nutrition of man. Some of these exist in the milk as salts, some as constituents of the organic matter, some in both forms.

Sulphur, of which milk contains about 0.03 per cent, exists almost entirely as a constituent of the milk proteins.

Phosphorus constitutes about o.10 per cent of the fresh weight of milk (equivalent to 0.23 per cent phosphoric acid) and is present in at least four forms. About 65 per cent of the phosphorus of milk is in the form of phosphate in the sense that it is precipitable by phosphate reagents, but to what extent this is free phosphate and to what extent loosely combined with organic matter has not been determined; about 25 per cent exists as an essential organic constituent of the casein (the latter containing 0.8 to 0.9 per cent of phosphorus after having been purified by dissolving and reprecipitating until ash free); about 10 per cent in the form of phosphatids (the so-called phosphorized fats, including lecithin) and other organic compounds. References to the recent work of Osborne and Wakeman upon the phosphatids of milk will be found at the end of the chapter. Chlorine exists in milk in the form of sodium chloride, possibly in part also as potassium chloride.

The base-forming elements, sodium, potassium, calcium, and magnesium, are present in milk in slightly greater amounts than would be necessary to neutralize the acids obtainable from the sulphur, phosphorus, and chlorine present, and in distinct excess over what would be required to combine with the ready-formed acid radicles. This surplus of base is combined in part with the casein and in part with citric acid, a small quantity of which is a normal constituent of milk and is counted with the carbohy

drates in the usual proximate analysis. The percentages of these elements, calculated as oxides, in average cows' milk are as follows: calcium oxide, o.168 per cent; magnesium oxide, o.019 per cent; potassium oxide, o.171 per cent; sodium oxide, 0.068 per cent.

Noticeable here are the high calcium content as compared with other foods and the richness of milk in calcium and potassium as compared with magnesium and sodium. In these respects the composition of milk ash resembles that of the ash of the animal body.

The iron of milk is small in amount (about 0.0002 per cent) but of high food value. It will be considered in the section on the nutritive value of milk and the place of milk in the diet.

Since iodine is now known to be essential to normal nutrition and since animals have grown normally and even reproduced themselves when kept from weaning time upon exclusive milk diet, we must conclude that milk contains some compound of iodine, although the amount is too small for satisfactory determination by present analytical methods.

Table 8 summarizes the mineral elements or ash constituents of milk, calculated in terms of the element and not of its oxide.