Heat-induced aggregation and dissociation of protein and fat particles in recombined milk

Heat-induced changes in the proteins of recombined milk were studied using centrifugation, electrophoresis and electron microscopy techniques. The results showed that, during the initial stages of heating at pHs in the range 6.3 to 7.1 at 130 degrees C whey proteins denatured and associated with kappa-casein in the fat globule surface and casein micelles, the extent of association being lower at a higher pH of the milk at heating. The dissociation of kappa-casein from the fat globule surface and casein micelles increased with increasing pH of the milk al heating; the dissociation from the fat globule surface was more sensitive to pH changes in the range from 6.3 to 6.7. This was followed by a rapid increase in the percentage of total protein and fat sedimented at 18 000 g for 20 min; the effect was largely independent of the pH of the milk at heating in the range from 6.5 to 7.1. Continued heating of pH 6.7 milk at 130 degrees C caused a relatively small change in the aggregation of protein and fat particles until, just before coagulation, a rapid aggregation occurred. The caseins and whey proteins, both at the fat globule surface and in the serum, were modified through the formation of new non-disulphide covalent cross-links as indicated by SDS-electrophoresis, Electron microscopic examination of heated milks showed that, when recombined milk was heated at below pH 6.7, the surface of casein micelles, both the free micelles in the serum and those adsorbed on to the fat globule surface, developed many appendages (possibly aggregated whey proteins). However, the surface of adsorbed and free casein micelles in recombined milk heated at pH 7.1 was not intact and appeared to be free of whey proteins. Samples heated close to the coagulation time at pH 6.7 clearly showed the formation of chains consisting of fat globules and casein particles in which individual fat globules and casein particles largely retained their original shapes. The fat globules appeared to be linked in chains via the adsorbed casein micelles.