Influence of transglutaminase protein cross-linking on the rennet coagulation of casein

The influence of protein cross-linking on the rennet coagulation of casein was investigated using a microbial transglutaminase (TG) preparation containing glutathione (GSH). As unheated milk is normally less reactive towards TG, GSH was applied in order to enable non-inhibited cross-linking. Thus, interactions between TG cross-linking and rennet coagulation and their impact on the renneting properties were studied independently from a pre-heat treatment of milk beyond pasteurisation. Protein cross-linking was carried out either prior to or simultaneously with the addition of rennet. Coagulation times tC and gel firmness assessed as the storage modulus G60min′ could be related either to TG pre-incubation times or to TG concentrations. The results show that enzymatic cross-linking affects both the primary and the secondary stage of rennet coagulation. The impact of protein cross-linking on the primary enzymatic phase can be mainly related to increased coagulation times due to inhibition of the caseinomacropeptide release. Consequently, a higher extent of protein cross-linking leads to reduced gel firmness. The simultaneous reaction of TG and rennet, however, results in markedly increased yield after centrifugation due to the enhanced serum binding of the gel network stabilised by additional covalent bonds. In conclusion, the cross-linking has an intense impact on the entire renneting process leading to rennet gels with modified functional properties.