Characterization of Whey Protein Isolate–Soymilk Complexes Modified by Transglutaminase and Their Application inYuba Film

Transglutaminase (TGase) improves protein structure by facilitating cross-linking reactions. However, the effects of TGase on the physicochemical properties of whey protein isolate (WPI)–soymilk complexes and their applications in yuba remain unclear. Therefore, the impacts of TGase concentration on the free sulfhydryl content, free amino content, particle size, and structure of WPI–soymilk complexes and their film-forming properties were studied. The results showed that the physicochemical properties of the composite soymilk were changed by the TGase-induced cross-linking reaction of protein. Compared with the composite soymilk without TGase modification, the particle size of the WPI–soymilk complexes increased from 707.99 ± 9.47 nm to 914.41 ± 2.8 nm as the TGase concentration increased, and the complexes remained relatively stable at low TGase concentrations. TGase modification changed the tertiary structure of the WPI–soymilk complexes. The composite yuba with 0.01% and 0.03% levels of TGase had a higher β-sheet content than composite yuba without addition of TGase. The surface hydrophobicity of composite soymilk was decreased by all the addition levels of TGase. Meanwhile, the TGase-modified composite protein with 0.03% TGase had the lowest free sulfhydryl (35.92 μg/g) and amino groups (0.46). Additionally, the tensile strength of the composite yuba with 0.05% TGase addition reached a peak of 1.66 ± 0.02 MPa, which was 7.8% higher than that of the composite yuba without TGase addition. The SEM results revealed that the composite yuba with 0.01–0.03% TGase addition exhibited a dense and non-porous film structure. Moreover, all the composite yuba with TGase addition had a reduced rate of yuba cooking loss. This study contributes to enhancing the yield and mechanical properties of traditional yuba.