Rennet-induced coagulation of enzymatically cross-linked casein micelles

The enzymatic cross-linking of casein micelles with transglutaminase had an adverse influence on rennet-induced coagulation. Incubation with transglutaminase at 30 °C progressively reduced the levels of monomeric caseins and increased rennet flocculation time (RFT) in a Berridge test. For incubation up to 3 h at 30 °C, the reciprocal of the RFT was linearly correlated with the level of residual monomeric κ-casein, indicating that at complete cross-linking flocculation is absent. After treatment for 4–24 h at 30 °C, no residual monomeric κ-casein was detected and no rennet-induced flocculation of the casein micelle suspension was observed. Monitoring rennet-induced coagulation by diffusing wave spectroscopy revealed that transglutaminase-induced inhibition of rennet-induced coagulation of casein micelles is primarily due to an inhibition of the secondary phase of rennet coagulation, i.e., the gelation and gel-firming phase of the casein micelle coagulation. The gelation and fusion of κ-casein-depleted para-casein micelles as in normal milk appears to be absent if the casein macropeptide remains attached to the casein micelle.