The physico-chemical aspects of the impaired rennet coagulation properties of heat-treated milk

This project investigated the factors that impair the rennet clotting properties of heated (90°C, 10 min) skim milk. Reconstituted skim milk powder (RSMP, 11% solids) was heated at pH values from 6.3 to 7.1. All heated samples coagulated poorly with rennet irrespective of the huge differences in the distribution of the heat-induced whey protein (WP)/kappa-casein complexes between the micellar and serum phases. Through a series of micelle/serum exchange experiments and by the dispersion of native and heat-altered casein micelles in the various sera (with and without restored ionic equilibrium) and protein-free ultrafiltrates, it was established that the casein micelles with their heat-modified surfaces, the soluble WP/kappa-casein complexes, and the dialyzable serum components were all part of the heat-impaired rennet clotting of milk. When the RSMP containing WP/kappa-casein complexes was treated with rennet, these complexes were found to progressively attach to the surfaces of both the unheated and the already heat-impaired casein micelles; this had adversely affected the clotting of the otherwise "perfect" native micelles and had further impaired the coagulation of heat-altered micelles. However, it was not clear why the rennet-induced attachment of WP/kappa-casein complexes to the native micelles (albeit to a lesser extent) was not detrimental to their clotting when the complex-rich serum (obtained from the heated RSMP) was extensively dialyzed against unheated RSMP.;Using transmission DWS, the rennet coagulation behavior of the unheated RSMP (9% and 11% solids) was compared with that of fresh cow's milk; the diffusion coefficient (D) and the turbidity parameter (1/l*) were monitored. The casein micelles in the RSMPs were found to be intrinsically less capable of aggregation with rennet than the 'native' micelles in fresh milk, possibly due to only a partial restoration to the 'native state' as in fresh milk. The micelles in the RSMPs also showed anomalous early increases in 1/ l* as the x-casein hairy layer was being progressively removed (15-70%) by rennet. Interestingly, heating the RSMPs restored the characteristic (of fresh milk) sigmoidal shape of the 1/l* curve against renneting time. The sera of the RSMPs and fresh milk were also found to contain factors that inhibited the activity of rennet.