Heat-induced interactions of beta-lactoglobulin and alpha-lactalbumin with the casein micelle in pH-adjusted skim milk

Skim milks, adjusted to pH 6.48, 6.60 or 6.83, were heated for various temperature-time combinations in a pilot-scale ultra-high temperature (UHT) plant. Heat-treated samples were ultracentrifuged and their supernatants analysed by quantitative polyacrylamide gel electrophoresis in order to measure the extent of beta -lactoglobulin (beta -1g) and alpha -lactalbumin (alpha -1a) denaturation and their subsequent association with the casein micelle. The activation energy of beta -1g denaturation decreased as the pH increased. In contrast, there was no apparent trend for alpha -1a. The extent of beta -1g and alpha -1a association with the micelle increased with heating time and temperature. The association of both proteins with the micelle was markedly affected by the milk pH. The rate and extent of association were greatest at pH 6.48, and least at pH 6.83. alpha -La continued to associate with the micelle although most of the beta -1g had already associated. It was possible that alpha -1a interacted at the micelle surface with beta -1g that had previously associated with the micelle. A pseudo-first-order mathematical model was used to calculate the apparent rate constant for beta -1g association with the micelle. (C) 2001 Elsevier Science Ltd. AII rights reserved.