Formation of soluble and micelle-bound protein aggregates in heated milk.

The formation of heat-induced aggregates of kappa-casein and denatured whey proteins was investigated in milk-based dairy mixtures containing casein micelles and serum proteins in different ratios. Both soluble and micelle-bound aggregates were isolated from the mixtures heated at 95 degrees C for 10 min, using size exclusion chromatography. Quantitative analysis of the protein composition of the aggregates by reverse phase high-performance liquid chromatography strongly suggested that primary aggregates of beta-lactoglobulin and alpha-lactalbumin in a 3 to 1 ratio were involved as well as kappa-casein, and alpha(s2)-casein in micellar aggregates. The results gave evidence that heat-induced dissociation of micellar kappa-casein was implicated in the formation of the soluble aggregates and indicated that a significant amount of kappa-casein was left unreacted after heating. The average size of the aggregates was 3.5-5.5 million Da, depending on the available kappa-casein or the casein:whey protein ratio in the mixtures. The size and density of these aggregates relative to those of casein micelles were discussed.