Effect of heat treatment at alkaline pH on the rennet coagulation properties of skim milk

Abstract

Reconstituted skim milk was heated at 90 °C for 30 s at pH values ranging from 6.6 to 8.1, stored overnight at 5 °C then renneted at pH values ranging from 6.2 to 6.6. The heat-induced disimprovement of the rennet coagulation properties of the milk (longer rennet coagulation time and lower gel firmness) were partially reduced as heat-treatment pH increased, except at pH 6.6. These properties were related to the increased heat-induced dissociation of micellar κ-casein and subse- quent formation of serum, rather than micelle-bound, heat-induced whey protein/κ-casein aggrega- tes. It was postulated that such distribution of the denatured whey protein and κ-casein slightly reduced the detrimental effects of heating on the enzymatic coagulation, as well as helped earlier destabilisation of the micelles. These results are discussed with respect to the amplitude of the effect of the heat-treatment pH, as well as the side effects of alkalinisation and heat treatment on casein dissociation. whey protein / heat treatment / rennet coagulation / pH