Cross-Linking and Glucosamine Conjugation of Casein by Transglutaminase and the Emulsifying Property and Digestibility In Vitro of the Modified Product

Microbial transglutaminase was applied as a biocatalyst and glucosamine as an acyl acceptor to modify casein by cross-linking and glucosamine conjugation. Electrophoretic analysis revealed that transglutaminase-induced cross-linking and glucosamine conjugation occurred simultaneously during reaction, and some polymers of glycoproteins with higher molecular weights were formed in the modified casein product prepared. HPLC analysis demonstrated that about 1.2 mol of glucosamine was conjugated to 1 mol of casein, under the preparation conditions as follows: casein concentration of 3% (w/v), molar ratio of acyl donor in casein to glucosamine acceptor of 1:3, transglutaminase addition level of 10 U/g casein, reaction temperature of 37°C, and reaction time of 4 h. The evaluation results also showed that the surface hydrophobicity of the modified casein product decreased. The emulsifying activity index and emulsifying stability index of the modified casein product were 100.9 m²/g and 84.3%, about 12 and 20% higher than that of original casein. The digestibility in vitro of the modified casein product was the same as that of original casein or had an increase of 9.8%. Meanwhile, cross-linking of casein by transglutaminase in the absence of glucosamine showed adverse impact on the emulsifying properties of the cross-linked casein prepared. The new modification method developed might have the potential as an effective approach to improve the functional properties of food proteins.