Heat-induced association of beta-lactoglobulin and casein micelles

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