Enzymatic cross-linking of whey proteins by a Ca2+-independent microbial transglutaminase from Streptomyces lydicus

In this study, a Ca2+-independent microbial transglutaminase derived from Streptomyces lydicus was used to cross-link whey proteins in whey protein isolate (WPI) and beta-lactoglobulin. The course of the reaction was followed with size-exclusion (SE)-HPLC In WPI solutions, no enzyme-catalysed decrease in monomeric/dimeric beta-lactoglobulin was observed in the absence of a reductant, whereas a decrease in alpha-lactalbumin content and formation of higher molecular weight products was found without a reductant in the presence of transglutaminase. In the presence of the reductant dithiotreitol (DTT), both alpha-lactalbumin and beta-lactoglobulin in WPI were extensively cross-linked by transglutaminase. Gelation was observed visually in the enzyme-treated WPI samples containing DTT at protein concentrations above 10% within 120 min using 10 U transglutaminase/g whey protein. Cross-linking of pure beta-lactoglobulin in the presence of DTT was followed as a function of reaction time. With SE-HPLC, a decrease in beta-lactoglobulin was monitored during 180 min of reaction at 40 degrees C, after which the concentration of beta-lactoglobulin monomers was reduced by 90%. The formation of covalently linked polymers was confirmed by gel electrophoresis (SDS-PAGE) under reducing conditions. The apparent viscosity of an 8% beta-lactoglobulin solution incubated with transglutaminase increased with reaction time and after similar to 100 min of reaction gelation was evident, as indicated by a steep increase in apparent viscosity.