Heat stability of transglutaminase-treated milk

Heat-induced coagulation of unconcentrated (9%, w/w) and concentrated (18%, w/w) reconstituted skim milk was determined after incubation with transglutaminase (TGase). Cross-linking ~20% of κ-casein strongly increased the heat stability of unconcentrated milk at pH >6.9, presumably by preventing heat-induced dissociation of κ-casein, whereas increased heat stability of unconcentrated milk at pH 6.6-6.8 was only observed when >80% of casein was cross-linked. Treatment with TGase reduced heat stability of unconcentrated milk at pH <6.6, presumably due to the increased susceptibility of partially cross-linked casein micelles to coagulation arising from heat-induced acidification. A low degree of cross-linking increased the heat stability of concentrated milk at pH >6.8, but more extensive cross-linking progressively reduced heat stability. The degree of cross-linking studied did not increase the heat-stability of concentrated milk at its natural pH. The outcomes of this study substantiate the crucial roles of heat-induced acidification and casein dissociation in heat stability of milk.