Association of β-Lactoglobulin and β-Lactalbumin with the Casein Micelles in Skim Milk Heated in an Ultra-high Temperature Plant

Abstract

Abstract Skim milk was subjected to various time–temperature treatments using a direct steam injection (DSI) system in a pilot-scale ultra-high temperature (UHT) plant. The heated samples were ultracentrifuged and the supernatants analysed using quantitative polyacrylamide gel electrophoresis to determine the extent of denaturation of β-lactoglobulin (β-lg) and α-lactalbumin (α-la) and their association with the casein micelles. The extent of β-lg and α-la denaturation and association increased with 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°C, mainly β-lg appeared to associate with the casein micelles, but after prolonged heating, α-la began to associate with the micelle. In contrast, below 80°C both β-lg and α-la appeared to associate simultaneously with the micelles. The maximum level of association of α-la varied with heating temperature, ∼40% of total in the range 95–130°C and ∼55% below 90°C. For β-lg, the maximum level of association was ∼55% of total regardless of the temperature. A pseudo-first-order model was used to calculate the reaction kinetics of the association of β-lg with the casein micelle.