The Effect of Heat Treatment on the Nutritive Value of Milk Proteins I. Evaporated and Powdered Milks

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Abstract
The nutritive value of the proteins of two commercially prepared, evaporated milks has been shown, in two different rat growth experiments, to be slightly but significantly lower than that of whole, fresh, pasteurized milk. It was found, by paired feeding for 4 weeks, that the growth of rats was less on evaporated milk given additional heat treatment at 120°C. for 30 minutes than on the same milk unheated. These rats, when continued on the same diets ad libitum, grew steadily, but the difference in weights between the two groups was somewhat increased at the end of 16 months. The protein efficiencies of evaporated milk sterilized at 115 to 116°C. without removal of the oxygen, and of milk in which the oxygen was replaced with nitrogen and the milk subsequently sterilized, were not significantly different from one another, but both were significantly lower than the protein efficiency of the same evaporated milk unsterilized. Storage for one year at room temperature did not alter the value of the proteins of these sterilized milks. The changes which took place in the protein (probably a protein-lactose reaction) did not appear to be dependent upon the presence of oxygen. The efficiency of the protein of preheated skim milk powders, when compared with that of a powder which was not preheated, was decreased in proportion to the degree and time of heating. These studies justify the conclusion that the ordinary sterilization or preheating temperatures used in the preparation of evaporated and powdered skim milks decrease the protein efficiency of the milks in proportion to the degree of heat used. While the damage is not marked at the temperatures commonly used, it may be increased by careless overheating in commercial preparation of the products and in cooking.