Association of β-lactoglobulin and α-lactalbumin with the casein micelles in skim milk heated in an ultra-high temperature plant

Skim milk was subjected to various time-temperature treatments using a direct steam injection (DSI) system in a pilot-scale ultrahigh temperature (UHT) plant. The heated samples were ultracentrifuged and the supernatants analysed using quantitative polyacrylamide gel electrophoresis to determine the extent of denaturation of beta-lactoglobulin (beta-lg) and alpha-lactalbumin (alpha-la) and their association with the casein micelles. The extent of beta-lg and alpha-la denaturation and association increases with a an increase in both heating time and temperature; the rate of association was markedly less than that of denaturation. The association behaviour was affected by heating temperature; during the initial stages of heating in the range 80-130 degrees C, mainly beta-lg appeared to associate with the casein micelles, but after prolonged heating, alpha-la began to associate with the micelle. In contrast, below 80 degrees C both beta-lg and alpha-la appeared to associate simultaneously with the micelles. The maximum level of association of alpha-la varied with heating temperature, similar to 40% of total in the range 95-130 degrees C and similar to 55% below 90 degrees C. For beta-lg, the maximum level of association was similar to 55% of total regardless of the temperature. A pseudo-first-order model was used to calculate the reaction kinetics of the association of beta-lg with the casein micelle. (C) 1999 Elsevier Science Ltd. All rights reserved.