Effects of proteolysis and transglutaminase crosslinking on physicochemical characteristics of walnut protein isolate

In order to improve the functional properties of walnut protein isolate (WPI), the effect of partial hydrolysis with trypsin, crosslinking with transglutaminase (TGase), and combinations of both treatments on the physicochemical characteristics of WPI was investigated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that WPI could be easily digested into smaller peptides by trypsin and could also be crosslinked into aggregates with high molecular weight. Hydrolyzing crosslinked WPI significantly increased its solubility from 9.73% to 54.54%. Scanning electron microscopy indicated that treatment with trypsin or TGase alone had a notable effect whereas the mixture of enzyme and TGase had a relatively lower effect on the microstructure of WPI. Hydrolysis did not affect the sulfhydryl (-SH) content while TGase crosslinking significantly reduced (-SH) content from 92.9 μmoL/g to 47.06 μmoL/g. Hydrolysis markedly increased the surface hydrophobicity (H0), while crosslinking did not significantly change H0. TGase treatment increased the viscosity of hydrolysates, but the viscosity of trypsin hydrolysates was not altered. Treatment of WPI with both TGase and trypsin showed the lowest initial G′ and G″ values due to the highest solubility. Thus, WPI with proteolysis and transglutaminase treatment has great potential for applications in the food industry.