Heat-induced interactions of whey proteins and casein micelles with different concentrations of α-lactalbumin and β-lactoglobulin

The effect of the addition of individual whey proteins to skim milk on the interaction between casein micelles and whey proteins was studied, during heat treatment at 75, 80, and 90 degrees C. Not only temperature and time but also concentration affected the extent of the heat-induced reations of the whey proteins with casein micelles. In general, higher concentration of alpha-lactalbumin (alpha-la) in skim milk caused an increase in the amount of this protein associated with the micelles. On the other hand, a further addition of beta-lactoglobulin (beta-lg) hardly affected the maximum incorporation of this protein with the casein micellar fraction. To determine the effect of a lower concentration of whey protein than that present in natural skim milk, purified alpha-la and beta-lg were added to separated casein micelles suspended in milk permeate from ultrafiltration, and the reconstituted mixture was heated at 80 degrees C. In the absence of beta-lg, very little alpha-la was found associated with the micellar pellet after heating. When comparable amounts of whey proteins were present, the interaction behaviors of alpha-la and beta-lg with casein micelles were very similar, resulting in an alpha-la/beta-lg ratio close to 1. In general, the ratios of alpha-la/beta-lg associated with the heated micelles were significantly affected by the amount of protein present, either in skim milk or in the presence of resuspended micelles. In milk, at temperatures <90 degrees C, alpha-la and beta-lg may interact together in the serum phase, with a ratio depending on the original protein concentration, before interacting with casein micelles; this hypothesis could explain most of the observations made in the study.