HEAT-INDUCED AGGREGATION AND DISSOCIATION OF PROTEIN AND FAT PARTICLES IN RECOMBINED MILK

Heat-induced changes in the proteins of recombined milk were studied u sing centrifugation, electrophoresis and electron microscopy technique s. The results showed that, during the initial stages of heating at pH s 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 mi celles, the extent of association being lower at a higher pH of the mi lk at heating. The dissociation of kappa-casein from the fat globule s urface and casein micelles increased with increasing pH of the milk al heating; the dissociation from the fat globule surface was more sensi tive 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 t he aggregation of protein and fat particles until, just before coagula tion, a rapid aggregation occurred. The caseins and whey proteins, bot h at the fat globule surface and in the serum, were modified through t he formation of new non-disulphide covalent cross-links as indicated b y SDS-electrophoresis, Electron microscopic examination of heated milk s showed that, when recombined milk was heated at below pH 6.7, the su rface of casein micelles, both the free micelles in the serum and thos e adsorbed on to the fat globule surface, developed many appendages (p ossibly aggregated whey proteins). However, the surface of adsorbed an d free casein micelles in recombined milk heated at pH 7.1 was not int act 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 fa t globules and casein particles largely retained their original shapes . The fat globules appeared to be linked in chains via the adsorbed ca sein micelles.