Heat Stability of Milk: Aggregation and Dissociation of Protein at Ultra-high Temperatures

The aggregation and dissociation of protein in milk heated at 140 degrees C was studied using high performance liquid chromatography on a TSK-GEL G4000SW column in 6M urea. Initial heating gave rise to the formation of high molecular weight complexes of whey proteins and x-casein which eluted at or close to the void volume of the column. With continued heating, the quantities of these complexes remained more or less constant but the amounts of intermediate-sized protein material cross-linked through covalent (non-disulphide) bonds increased gradually. The size of the whey protein/x-casein complexes decreased with increase in pH at heating. Most of the whey protein/x-casein complexes remained associated with the micelles on extended heating at the natural pH of milk (6.67) although some protein (monomeric in 6M urea) dissociated from the casein micelles. Increasing the pH at heating resulted in increased quantities of whey protein/x-casein complexes and monomeric protein in the ultra-centrifugal supernatant. The implications of the above changes are discussed in relation to heat stability of milk.