Influence of transglutaminase treatment of skim milk on the formation of epsilon-(gamma-glutamyl)lysine and the susceptibility of individual proteins towards crosslinking

Results of transglutaminase reaction on the susceptibility of individual milk proteins in skim milk towards crosslinking and on the distinction between crosslinking and other transglutaminase-catalysed reactions, are presented. In unheated milk, transglutaminase had a small effect on proteins whereas in the preheated milk, considerable crosslinking, deamidation and/or amine incorporation occurred. Direct measurement of the dipeptide epsilon-(gamma -glutamyl)lysine showed that a rapid crosslinking of proteins occurred during the first 30 min of transglutaminase reaction. Results from the SDS and capillary gel electrophoresis showed that although the crosslinking was prevalent during the entire reaction time, most crosslinking occurred during the first 30 min. In both unheated and preheated milk, major reduction in the monomeric forms of kappa- and beta -caseins occurred due to the reaction with transglutaminase, suggesting that these two proteins were most susceptible to transglutaminase-induced crosslinking. The high susceptibility of kappa -casein towards crosslinking is likely to be at least partly due to its peripheral position in the casein micelles, and the high susceptibility of beta -caseins is hypothesised to be due to its dynamic nature and thus ease of accessibility in the micelle structure. In addition, it could be shown that only preheated beta -lactoglobulin was susceptible to transglutaminase action, while alpha -lactalbumin was crosslinked with or without preheating to the same extent. (C) 2001 Elsevier science Ltd. All rights reserved.