Enzymatic cross-linking of whey proteins by a Ca 2+-independent microbial transglutaminase from Streptomyces lydicus

Abstract

In this study, a Ca 2+-independent microbial transglutaminase derived from Streptomyces lydicus was used to cross-link whey proteins in whey protein isolate ( WPI) and β-lactoglobulin. The course of the reaction was followed with size-exclusion (SE)-HPLC. In WPI solutions, no enzyme-catalysed decrease in monomeric/dimeric β-lactoglobulin was observed in the absence of a reductant, whereas a decrease in α-lactalbumin content andformation of higher molecular weight products was found without a reductant in the presence of transglutaminase. In the presence of the reductant dithiotreitol (DTT), both α-lactalbumin and β-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 transglutaminaselg whey protein. Cross-linking of pure β-lactoglobulin in the presence of DTT was followed as a function of reaction time. With SE-HPLC, a decrease in β-lactoglobulin was monitored during 180 min of reaction at 40°C, after which the concentration of β-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% β-lactoglobulin solution incubated with transglutaminase increased with reaction time and after ~100 min of reaction gelation was evident, as indicated by a steep increase in apparent viscosity.