The constituents of milk are fat, carbohydrate, protein, salts, and vitamins, with 83 to 90 per cent of water. The carbohydrates, proteins, salts, and the vitamins B and C are in aqueous solution, while the fat is held in suspension as minute droplets which form an emulsion, in which the vitamin A is dissolved.

The milk of different species of animals varies considerably in the relative proportions of the constituents present; even within the same species, milk of different animals shows rather wide fluctuations round a mean. The following table1 shows the general difference in the composition of human milk and cow's milk, from which it appears that human milk differs from cow's milk chiefly in its much higher sugar content and lower protein:

Goat's milk has a higher content of protein than cow's milk, slightly lower sugar, and somewhat higher fat. Sheep's milk has a high content of both protein and fat.2

Fat is the most variable constituent in milk. Many factors influence the fat content of cow's milk, the breed, the individual characteristics of the animal, the period of lactation, and the season being the most influential. Sherman 3 gives the following figures as fairly representative of the breeds indicated:

Jersey ............................. 5.10 per cent fat.

Guernsey...........................5.16 " " ".

Durham............................4.05 " " ".

Ayrshire............................3.64 " " ".

Holstein............................3.43 " " ".

The fat content of human milk probably does not differ greatly from that of average dairy milk, although the variability noted makes it difficult to generalize correctly. The results of a great many determinations gives an average of 3.9 per cent of fat in human milk. The composition of the milk fat varies considerably in different species,4 owing to variations in the fatty acids present, but this appears to be of no importance in nutrition.

1 Meigs and Marsh, J. Biol. Ghem. 16,167, 1913-14. 2 See Folia, Denis and Minot, J. Biol. Ghem. 37, 349, 1919, for analyses of milk of different species.

3 Sherman, Chemistry of Food and Nutrition, N. Y., 2nd Ed. p. 59.

The lactose content of milk shows much less variation within the species than does the fat, but there is greater difference in this respect between different species, than is the case with the fat. Mathews5 suggests that the high content of lactose in human milk may be correlated with the very much greater brain development of human beings. Myelin, a compound which develops rapidly in the brain fibres during the first six weeks of life, contains as one of its constituents galactose, which is produced by the hydrolysis of lactose. It may be, therefore, that one function of the lactose in human milk is to furnish this necessary substance for the development of the brain. If this is correct it is obvious that the deficiency of lactose in cow's milk cannot be satisfactorily remedied by the addition of cane sugar. In any case lactose is less susceptible to fermentation than is either glucose or cane sugar; consequently it is less liable to produce digestive disturbances than are these sugars.6

There is some uncertainty as to how many proteins are present in milk. In addition to the well-characterized caseinogen, and lactal-bumin which make up by far the greatest proportion of the protein material, there is a third, lactoglobulin, which is more closely related to lactalbumin than to caseinogen7 and also, according to Osborne and Wakeman,8 an unidentified alcohol-soluble protein present in extremely small amounts. Besides these there are traces of non-protein nitrogen-containing substances, such as urea, creatine, creatinine and uric acid,9 and possibly small quantities of proteoses (protein derivatives of lower molecular weight) as well.10 The total proteins other than caseinogen are sometimes grouped together as "whey proteins," since they remain in solution in the whey when the casein is coagulated by acids or rennin.

4 Arnold, Zeit. Unters. Nahr. u. Genussm. 23, 433, 1912. 5 Mathews, Physiological Chemistry, 1915, p. 307.

6 See also p. 267.

7 Lane-Claypon, Oleomargarine, pp. 31-32, 38-41.

8 Osborne and Wakeman, J. Biol. Chem. 33, 17, 1918.

9 Dennis and Minot, J. Biol. Chem. 37, 363, 1919.

10 Full details of the method of separation of the proteins of milk are given by Osborne and Wakeman, 1. c.

Caseinogen is distinguished from the other milk proteins by its relatively high percentage of phosphorus and by the fact that it is coagulated by rennin. It is precipitated by heating in faintly acid solution, but redissolves in excess of acid or alkali. The action of the rennin enzyme is supposed to be a partial hydrolysis or digestion of the caseinogen, by which it is transformed into a new protein, casein, of lower molecular weight. The calcium salt of casein is insoluble and precipitates, forming a curd.

There is an unfortunate confusion in the nomenclature used by different authorities in connection with this phenomenon. While that given above is in accordance with English usage, the Germans give the name casein to the protein present in normal milk and distinguish the product of rennin action as paracasein. Many American writers follow the German usage, while others use the English nomenclature.

The clear fluid from which the coagulated casein has separated is known as whey, and contains the remaining proteins and other nitrogenous constituents as well as the milk sugar and inorganic salts.

Caseinogen probably exists in normal milk in the form of neutral calcium caseinogenate, Cas Ca4.11 The relation between this and the coagulum obtained by acidifying is not quite clear. It is completely precipitated from its neutral solution by saturating the solution with sodium chloride or magnesium sulphate or by half saturation with ammonium sulphate, or by the action of certain metallic salts such as alum, zinc sulphate, or copper sulphate.

11 Van Slyke and Boswortfa, J. Biol. Chem. 20, 136, 1915.