Heat-induced association of β-lactoglobulin (β-lg) and casein micelles, mainly at 85–90 °C, was studied by means of preparative ultracentrifugation, using [3H]-labelled β-lg. Qualitative aspects were studied by gel electrophoresis and electron microscopy. In this association, the formation of intermolecular S–S bonds between β-lg and κ-casein plays a role, but hydrophobic bonds are also involved.
After heating mixtures containing 25 g/kg casein and 4 g/kg β-lg at 90 °C for 20 min, the amount of β-lg sedimented with the casein micelles by ultracentrifugation decreased by approximately 30% when the milk salt buffer system was reduced to 0·25 of its normal concentration; it decreased by about 20% when the pH was increased from 6·8 to 7·3 and increased by 15% when the pH was reduced from 6·8 to 5·8. When the β-lg concentration was decreased from 4 to 2 g/kg, the amount of sedimented β-lg decreased by 25% after heating at 90 °C for 6 min. After heating in milk salt buffer, the amount of sedimented β-lg was 15% higher than after heating in a salt solution which contained neither Ca nor citrate ions.
Although α-lactalbumin (α-la) is involved in the heat-association of β-lg and casein micelles, no influence of α-la on the amount of associated β-lg was found. In addition to the heat-association product of β-lg and whole casein micelles, an association product consisting mainly of β-lg and κ-casein was also formed. This product was observed by electron microscopy as noodle-like particles. The size of these particles depended on the β-lg/κ-casein ratio.