The Effect of Heat Treatment of the Nutritive Value of Milk Proteins IV. The Biological Value of Unheated and Autoclaved Dried Skim Milk

Abstract

Three samples of dry skim milk supposedly processed at different temperatures were found to have similar solubilities and identical protein efficiencies for the growth of young dogs. After 56 days, two of the dogs were fed a diet containing milk autoclaved 15 minutes at 118°C., but again no striking changes in growth, appetite, nitrogen retention or serum protein resulted. Two of the unheated milk samples and one autoclaved for 15 or 25 minutes at 120°C. were then fed at a 12% protein level to groups of rats, alone and supplemented by lysine, methionine, valine or all of these amino acids at the 1% level. The milk autoclaved for 15 minutes lost from one-half to two-thirds of its growth efficiency but this was largely restored by the lysine or lysine, methionine and valine supplement. The sample autoclaved for 25 minutes did not support growth except when lysine-supplemented. The efficiency for liver and kidney protein production of these diets was not identical, since lysine supplementation increased liver growth in all cases more than kidney growth. Kidney growth was less depressed in all cases by the autoclaving than the growth of the liver or total carcass. The weights of the eyes were remarkably constant regardless of growth failure. The serum protein levels were depressed in the groups fed the autoclaved diets but lysine supplementation restored these levels to normal. The loss of protein value of dry milk proteins exposed to steam autoclaving for 15 or 25 minutes is thus contrasted with the relatively mild changes produced in liquid milk concentrates or in casein and lactalbumin by dry heating.