Hogan,84 using rations of butter-fat, starch, agar, protein-free milk, and casein, obtained results at variance with those of McCollum and Davis. Even when the casein had been heated for two hours at 45 pounds pressure its nutritive value was unimpaired.

Emmett and Luros85 suggest that the discrepancy between the observations of Hogan and of McCollum and Davis may be due to a growth-promoting vitamin which is supplied in the protein-free milk and in the unheated casein but is destroyed by heat. These investigators tested lactalbumin which had been heated in an air oven at 90° to 100° C. for 16 hours, or in an autoclave at 15 pounds pressure for six hours, and found no alteration in its growth-promoting power.

Lane-Claypon 86 states that: "It seems certain that when milk is boiled some of the albumin is precipitated, but where the same vessel is used for storage and feeding purposes, there is no difficulty in maintaining the same amount of nitrogen in the food. There is no indication that there is any difficulty in the digestion of the coagulated albumin; in fact it is probably more readily digested than in the raw state".

It is a well-known fact that the calcium content of milk is reduced on heating, owing to the precipitation of insoluble calcium salts. Daniels and Loughlin 87 in the course of an investigation into the nutritive defects of heated milk obtained some very interesting results which seem to indicate that this precipitation of calcium may be the primary cause in rendering heated milk insufficient for the nutrition of certain animals. It is improbable that this would have any effect in infant feeding, since cow's milk contains considerable excess of calcium as compared with human milk.

84 Hogan, J. Biol. Chem. 27, 193, 1916; 30,115, 1917. 85 Emmett and Luros, J. Biol. Chem. 38, 267, 1919. 86 Lane-Claypon, 1. c., p. 230. 87 Daniels and Loughlin, J. Biol. Chem. 44, 381, 1920.

The experimental animals used by Daniels and Loughlin were healthy young rats. It had previously been observed by Daniels and Stuessy 88 that rats fed on milk which had been boiled for even one minute grew subnormally and never reproduced, but that the addition of well-washed coagulated egg yolk or white or casein induced normal growth, and, in the case of the egg yolk at least, successful reproduction. In these experiments the milk was placed in glass containers holding one pint, surrounded with cold water, and brought to a boil. Since the length of time the milk was held at boiling point (1, 10 or 45 minutes) appeared to make no appreciable difference in the results, in the second set of experiments the milk was brought quickly to the boiling point in an open kettle and then maintained at boiling temperature for one minute. To the surprise of the investigators the rats fed on milk so treated grew normally and appeared perfectly nourished. Repetition of the earlier experiments confirmed the observations first made. It was evident therefore that milk which had been slowly heated to boiling point was not equivalent for nutritive purposes to that which had been heated rapidly to the same temperature.

Experiments were then undertaken with milk pasteurized by heating gradually (in the course of 30 to 45 minutes) to 65° and 82° C. respectively, the milk, being held at this temperature for 40 minutes and then cooled with running water. Condensed and evaporated milk were also tested. The condensed milk had been prepared by heating to approximately 200° F. and then evaporated in vacuum pans to the desired consistency, the temperature during this part of the operation averaging 150° F., but dropping to about 120° before its completion. In the preparation of the evaporated milk it was held at the boiling temperature for 10 minutes, then evaporated at 130° to 140° F., cooled, canned, and sterilized at 240° F. for about 20 minutes. The pasteurized milk failed to induce more than one-half normal growth, and the animals on the evaporated milk grew very slowly but in a miserable condition for a longer or shorter period and then died. It is noteworthy, however, that they showed no signs of xerophthalmia.

Additions of butter-fat or extract of wheat embryo were without effect * in stimulating growth, and separate tests on purified rations deficient in B but supplemented with fresh and evaporated milk indicated if anything a rather higher content of B in an amount of evaporated milk supposed to be the exact equivalent of the fresh milk used. Addition of calcium lactate, however, produced much better growth, both with the evaporated and the slowly pasteurized milk. When calcium glycerophosphate was used instead of calcium lactate the results were even better, growth and reproduction both being highly successful. The best growth of all was obtained when tricalcium phosphate was made into a paste with starch and fed with evaporated milk.

88 Daniels and Stoessy, Am. J. Dis. Child. 11, 45, 1916.

These results are in line with those obtained by Hittcher 89 in feeding calves on boiled milk, in which he found that boiled milk with the addition of tricalcium phosphate was much more efficient for growth than raw milk or milk supplemented with any other salt. Daniels and Loughlin suggest that on long heating sufficient insoluble calcium salt precipitates to lower the nutritive value of the milk, a hypothesis which is confirmed by the fact that when especial care was token to include the insoluble material by incorporating the whole in a paste, results comparable to those on raw milk were obtained. The better growth of the animals on condensed milk as compared with evaporated milk is explained by the fact that the condensed milk which is of a thick viscous consistency was fed undiluted and consequently the insoluble material remained fairly well distributed through it. The improvement noted in the work of Daniel and Steussy on addition of egg yolk and casein would naturally follow from the amount of calcium and phosphorus in these substances. It may be noted in this connection that Soldner 90 found a loss of from 13 to 15 mgs. of calcium and 10 to 13 mgs. of phosphorus per 100 cc. of milk which was just brought to a boil.

Solomin91 found that at 130° to 140° C. about one-half of the ash of milk, including all of the calcium phosphate, was precipitated.

89 Hittcher, Landwirtsch. Jahrb. 1909, quoted by Lane-Claypon, p. 166.

90 Soldner, Landw. Vermicha. 1888, p. 861, quoted by Lane-Claypon, l. c. d. 861.

91 Solomin, Arch, f. Hyg. 28, 43,1897.