Sedimentation of Milk Proteins from Heated Milk

Abstract

Summary Milk is a very sensitive system and the most marked changes produced by heat treatments can be measured only when the time and temperature of fractionation are carefully controlled. Some of the observations which have previously been made and that appear contradictory may be attributed to differences in one or both of these factors. Immediately following the heat treatment of skimmilk, sedimentation reveals such a marked drop in supernatant nitrogen that it must be concluded whey proteins are being sedimented. As pointed out by Edmondson and Tarassuk (1) , the measurement of acid-coagulable nitrogen in the supernatant will not reveal this if the whey proteins are sedimenting uniformly with the casein. During 24 hours of storage in the cold, rapid shifts in equilibria occur, which tend to restore the supernatant nitrogen to a degree dependent on the severity of the heat treatment. These results were confirmed, using skimmilk containing added S 35 -labeled whey proteins. Radioactive analyses demonstrated 52% of the whey proteins were present in the gels from a skimmilk which had received a strong heat treatment. In this case, considerable amounts of the denatured whey proteins remained centrifugable even after 24 hours' storage.