Human milk contains a smaller percentage of caseinogen than cow's milk, although different investigators differ somewhat as to the relative proportions.12 The caseinogen of human milk differs from that of cow's milk in being more difficult to precipitate by acid or to coagulate by rennin. Moreover, the curd formed from human milk is looser and more flocculent, and consequently more easily digested than the curd of cow's milk.13

Lactalbumin contains a higher proportion of sulphur and a smaller proportion of phosphorus than caseinogen. It is soluble in weak acid, but precipitated by addition of large quantities of mineral acids or metallic salts. At ordinary temperature it is soluble in half-saturated « ammonium sulphate solution. It is not precipitated from neutral solution by saturation with sodium chloride or magnesium sulphate at ordinary temperatures, but separates on addition of acetic acid to this solution.

12 See Lane-Claypon, 1. c. p. 84.

12 Hawk, Pract Physiol. Chem. 6th. Ed. p. 844.

Lactoglobulin has been less studied than caseinogen or lactalbumin. Osborne and Wakeman14 suggest that it may be a lecithalbumin similar to the vitellin of hen's eggs, or a mixture of proteins, one of which belongs to this group. It differs considerably from most other globulins in its solubilities, but appears to be biologically identical with the seroglobulin of the lactating animal.15

The rapid growth of the young on a milk diet indicates that the proteins of milk are of excellent nutritive quality, and this is borne out by the experience of many investigators.

Thomas16 calculated what he termed the "biologic values " of different proteins by finding the minimum amount of each necessary to prevent loss of tissue protein from the body on a diet of starch and sugar. Assigning the value 100 to milk proteins, the proteins of other common foods can be arranged in the following order on the same scale:

Milk .......................... 100

Ox Meat ...................... 104

Fish ........................... 95

Rice .......................... 88

Potatoes ...................... 79

Yeast ......................... 71

Casein ........................ 70

Peas .......................... 56

Wheat flour ................... 40

Corn meal..................... 30

Thomas' results have been widely quoted, but are apparently not entirely reliable.17 More valuable evidence is offered by later experimenters.

McCollum18 gives the following table showing the lowest plane of protein intake derived from a single seed which just suffices to maintain an animal in body weight, when the factors other than protein are properly constituted:

14 Osborne and Wakeman, J. Biol. Chem. 33, 13, 1918. 15 See Lane-Claypon, Oleomargarine, p. 37. 16 Thomas, Archiv. fur. Physiol. 1909, 219.

17 Hindhede, Skand. Arch. Physiol. 31, 259, 1914; Sherman, J. Biol. Chem. 41, 97, 1920.

18 McCollum, "Newer Knowledge of Nutrition," N. Y. 1919, p. 75.

These maintenance experiments were of three to six months' duration.

The value of milk or the separate milk proteins as a supplement to cereal grains in animal husbandry has been repeatedly emphasised. In determining the relative values for milk production of different supplements, Hart and Humphrey 19 found that milk proteins had a percentage efficiency of about 60; oil meal, 61, distillers' grains, 60, gluten feed, 45, corn, 40, and wheat, 36. Later, Hart and Steenbock,20 on the basis of their experience with swine, stated that either the cereal proteins, a protein mixture from cereals and alfalfa, or a mixture of proteins derived from cereals, cabbage, and potato, could be effectively supplemented by additions of either milk or whey or meat or fish proteins, milk or whey proteins being the most satisfactory for the purpose. A mixture of proteins from corn, alfalfa, and whey, in which only 16 per cent of the protein nitrogen came from the whey, was nearly equal in value to a similar mixture in which 27 per cent of the nitrogen came from skim milk instead of from whey. The proteins from tankage (animal residues from meat packing) were not quite so effective in their supplementary relations as those from either milk or whey, although the inferiority was slight.

A protein mixture drawn from five common sources, rice, wheat, corn, potatoes, and cabbage, with the addition of sufficient wheat gluten to supply 36 per cent of the total nitrogen, showed a production value of 19.4 per cent; that is, from 100 pounds of protein, 19.4 were retained for growth. When the nitrogen from the wheat gluten was displaced by an equal amount of nitrogen from meat crisps the efficiency rose to 32.7 per cent, and when it was replaced by an equivalent in milk nitrogen it rose to 47 per cent.

Recently, Hart and Steenbock21 have asserted that when milk is used as a supplement in corn feeding, a highly efficient protein mixture is obtained only when the proportion of liquid milk to corn meal reaches 1:1.

With regard to the practical application of these observations Sherman states:22 "It is plainly desirable in all cases that grain products be supplemented by milk products, and it is clear that in providing for the needs of growing children and of pregnant or nursing mothers, the proportion of milk in the diet should be more liberal than it need be when only maintenance is concerned; this both because of the superior amino acid makeup of the milk proteins and to provide amply for the mineral elements and vitamins as well".

19 Hart and Humphrey, J. Biol. Chem. 21, 239, 1915; 26, 467, 1916. 20 Hart and Steenbock, J. Biol. Chem. 38,267,1919. 21 Hart and Steenbock, J. Biol. Chem. 42, 167, 1920. 22 Sherman, J. Biol. Chem. 41, 97, 1920.

Osborne and Mendel23 endeavored to ascertain the nutritive value of the separate milk proteins as compared with certain vegetable proteins by careful experiments with rats, in which the experimental animals were all of the same sex, and the same initial body weight, and the same total food intake was furnished daily in precisely equivalent and increasing amounts in successive periods of the experiment in accord with the anticipated needs of increasing body weight. The diets used were made up of purified protein, 8.0 to 20.45 per cent, "protein-free milk," 28 per cent, starch and sucrose, 23 to 36 per cent, butter-fat, 18 per cent, lard, 7 to 12 per cent. Comparing lactal-bumin, casein, and edestin (a vegetable protein), lactalbumin was found to be far superior, 50 per cent more casein and nearly 90 per cent more edestin being required to produce the same gain in body weight. Good growth was secured with as little as nine per cent of lactalbumin in the ration. The superiority of lactalbumin for maintenance as well as for growth was shown by tests with adult animals.

It was found that casein could, however, be supplemented by the addition of a particular amino acid, cystine, in which it was known to be lacking, so as to render it equal to lactalbumin.