Physicochemical properties and microstructures of soy protein isolate gels produced using combined cross-linking treatments of microbial transglutaminase and Maillard cross-linking

Soy protein isolate (SPI) was incubated with microbial transglutaminase (MTG) enzyme for 5 (SPI/MTG(5)) or 24 (SPI/MTG(24)) h at 40 °C and the gel obtained was freeze-dried, and heated with 2% (w/v) ribose (R) for 2 h at 95 °C to produce combined-treated gels. Control experiments were run with sucrose. Stress relaxation experiments indicated that combined-treated gels were higher in the compressive gel strength, viscoelasticity properties (K1 and K2), solidity (Ea), and were lower in % deformation as compared to single-treated or control gels (gels produced without cross-linking treatment). At most of pHs tested, the solubility of single-treated or control gels were always higher than that of the combined-treated gels. The pH ranges at which the gels were least soluble were wider in the combined-treated gels compared to those of other gels. Field emission scanning electron microscope (FESEM) analysis revealed a denser and finer network of combined-treated gels as compared to that of other gels. The occurrence of covalent cross-links within the network of the gels as well as changes in net charge of soy protein had all affected the physicochemical properties and the microstructure of the combined-treated gels.