Improvement of enzymatic cross-linking of casein micelles with transglutaminase by glutathione addition

The impact of glutathione (GSH) on cross-linking of micellar casein and sodium caseinate by microbial transglutaminase (TG) was investigated. Micellar casein was obtained by removing whey proteins from skim milk by means of membrane separation techniques. The addition of GSH (0.05–0.1 mm) was found to enhance the cross-linking of micellar casein suspended in milk serum. Cross-linking of sodium caseinate remained unaffected by GSH addition. Mixing TG and milk serum before addition of substrate proteins, however, resulted in an almost complete inhibition of the enzyme. When GSH and TG were present in the system before substrate addition, the reactivity of TG can be maintained. Hence, it was concluded that the addition of GSH mainly affected interactions between TG and an indigenous TG inhibitor present in milk serum. The addition of GSH allows cross-linking in milk products by TG without requiring a prior heat treatment of the milk beyond pasteurisation conditions.