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

Skim milks, adjusted to pH 6.48, 6.60 or 6.83, were heated for various temp erature-time combinations in a pilot-scale ultra-high temperature (UHT) pla nt. Heat-treated samples were ultracentrifuged and their supernatants analy sed 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 increas ed. In contrast, there was no apparent trend for alpha -1a. The extent of b eta -1g and alpha -1a association with the micelle increased with heating t ime and temperature. The association of both proteins with the micelle was markedly affected by the milk pH. The rate and extent of association were g reatest 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 w as possible that alpha -1a interacted at the micelle surface with beta -1g that had previously associated with the micelle. A pseudo-first-order mathe matical model was used to calculate the apparent rate constant for beta -1g association with the micelle. (C) 2001 Elsevier Science Ltd. AII rights re served.