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

Results of transglutaminase reaction on the susceptibility of individual mi lk proteins in skim milk towards crosslinking and on the distinction betwee n crosslinking and other transglutaminase-catalysed reactions, are presente d. In unheated milk, transglutaminase had a small effect on proteins wherea s in the preheated milk, considerable crosslinking, deamidation and/or amin e incorporation occurred. Direct measurement of the dipeptide epsilon-(gamm a -glutamyl)lysine showed that a rapid crosslinking of proteins occurred du ring the first 30 min of transglutaminase reaction. Results from the SDS an d capillary gel electrophoresis showed that although the crosslinking was p revalent 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 reactio n with transglutaminase, suggesting that these two proteins were most susce ptible 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 sh own that only preheated beta -lactoglobulin was susceptible to transglutami nase action, while alpha -lactalbumin was crosslinked with or without prehe ating to the same extent. (C) 2001 Elsevier science Ltd. All rights reserve d.