Study on the Functional Properties of Soybean Protein Isolate Cross-Linked with Gelatin by Microbial Transglutaminase

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A microbial transglutaminase was used to cross-link soybean protein isolates in the presence of gelatin to prepare a cross-linked composite soybean protein-gelatin. Cross-linking conditions, such as total proteins, the ratio of soybean protein isolates to gelatin, and original pH, were fixed at 5% (w/v), 4:1 (w/w), and 7.5, respectively. The 4-hydroxyproline content in the modified product was measured and used as an index to select suitable transglutaminase addition, reaction temperature, and time by single factor experiments, which were found to be 20 U/g proteins, 40°C, and 2 h, respectively. Under the identified conditions, a modified product with 4-hydroxyproline content of about 36 mg/g proteins was prepared, and its functional properties were evaluated. SDS-PAGE analysis showed that several protein polymers existed in the modified product. Compared to the original soybean protein isolates or a cross-linked soybean protein isolates, the modified product had a better emulsifying stability and water holding capacity but a poor enzymatic digestibility in vitro, emulsifying activity and oil binding capacity; meanwhile, the dispersions formed by the modified product exhibited the highest apparent viscosity, storage modulus, and viscous modulus.