Occurrence of major whey proteins in the pH 4.6 insoluble protein fraction from UHT-treated milk

A clear picture of the protein rearrangement in milk following UHT-treatment was drawn by a comparative analysis of the pH 4.6 soluble protein fraction (SPF) and the pH 4.6 insoluble protein fraction (IPF) recovered from raw and UHT-treated milk samples. The two protein fractions were analyzed by mono- or bidimensional gel electrophoresis underreducing and nonreducing conditions, and protein bands were identified by specific immunostaining. Results showed that bovine serum albumin, beta-lactoglobulin, and, to a lesser extent, alfa-lactalbumin coprecipitated with caseins in UHT-treated milk samples at pH 4.6. These proteins were almost exclusively involved in high molecular weight aggregates held together by disulfide bonds. Partition of alfa-lactalbumin and bovine serum albumin in the protein fractions obtained upon acidification of milk at pH 4.6 was evaluated by competitive immunoassays. The ELISA-based results suggested the possibility of using pH 4.6 insoluble alfa-lactalbumin and bovine serum albumin, in addition to pH 4.6 insoluble beta-lactoglobulin, as indicators of the intensity of the heat treatment applied to milk.