Excessive cross-linking of caseins by microbial transglutaminase and its impact on physical properties of acidified milk gels

By varying cross-linking intensity, the effect of microbial transglutaminase on acid gels made from casein solution and raw milk was studied. To avoid any impact of heating, N-ethylmaleimide was used for enzyme inactivation after appropriately checking its efficiency. Up to a specific degree of oligomerisation gel stiffness and firmness increased and tan delta, time at gelation onset and syneresis decreased. Above approximately 70% and 25% of cross-linked protein in casein solution and raw milk, respectively, these parameters showed an opposite behaviour, and weak gels with high syneresis were obtained. Substrate differences, such as preferred cross-linking of adjoining K-caseins on the surface of the micelle enhanced the effect of steric hindrance in raw milk and impaired proper rearrangements upon acidification at a much lower level of oligomerised protein. It is mainly dimeric and trimeric casein that successfully contributed to the enhanced properties of milk protein gels. (C) 2009 Elsevier Ltd. All rights reserved.