Influence of protein content and storage conditions on the solubility of caseins and whey proteins in milk protein concentrates

Milk protein concentrate (MPC) powders with protein content ranging from 35% (MPC35) to 87% (MPC90) (w/w) protein in dry matter were prepared on a pilot scale. The solubility of the MPCs and the individual caseins and whey protein therein was determined as a function of storage time and temperature. Insolubility developed more rapidly in high protein MPCs and at higher storage temperatures and was primarily related to insolubility of the micellar caseins. Non-micellar caseins retained solubility throughout storage, whereas development of insolubility of whey proteins was less extensive than that of caseins, but was promoted in MPC80 prepared from milk with significant amounts of denatured whey proteins. It is hypothesised that the development of insolubility in MPCs arises due to aggregation of casein micelles at high temperature during drying. High calcium ion activity and the gradual conversion from an aqueous to a non-aqueous surrounding contribute to this instability.