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Dairy Products Laboratory, Eastern Utilization Research and Development Division, USDA, Washington, D. C.
ABSTRACT
Under standard conditions of centrifugation, the concentration and composition of the nonsedimenting nitrogen content of heated milks are found to vary. It decreases in concentration with increasing temperature to between 103 and 110 C, where its concentration increased again with temperature. Below 103 C the nonsedimenting nitrogen fraction was found to consist of whey proteins and the proteosepeptone fraction; at 110 C and above the nonsedimenting nitrogen fraction consisted predominantly of caseinate. The amounts of nonsedimenting nitrogen produced above 110 C was dependent on the temperature of heating, the concentration of milk solids, and on the phosphate and citrate concentrations. It has been reported by others, and also found in this investigation, that the soluble calcium and phosphate concentrations of milk decreased during heating, whereas those of citrate remained constant. Soon after heating, the soluble calcium concentration of milk returned to its former value. Formation of dissociated caseinate at temperatures of 110 C and above may be attributed to removal of calcium from caseinate aggregates by the free citrate ion. Since the citrate concentration was not changed by heating, its formation of a soluble calcium citrate complex also returns the soluble calcium concentration to its value before heating.
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