Effect of pH and Gluten Content of Wheat Flour on the Physicochemical Properties of Fermented Wheat Flour-Soy Protein Isolate Films

To enhance the properties of soy protein isolate (SPI) films using wheat flour (WF), medium-gluten WF-SPI films were prepared using film-forming solutions with pH values of 7, 8, and 9. The WF-SPI films with low, medium, high, and ultrahigh gluten concentrations were prepared when the pH of the film-forming solution was 9. The effects of pH and WF gluten content on the physicochemical properties of the fermentative WF-SPI films were investigated by measuring the microstructure, Fourier transform infrared spectroscopy, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, mechanical properties, water vapor permeability, and thermal stability. The surface structure of the WF-SPI films was smooth and dense, and the interactions among the proteins increased with increasing pH. With increasing pH, the tensile strength (TS) and glass transition temperature (Tg) of the WF-SPI films increased, while the elongation at break (EAB) and water vapor permeability (WVP) decreased. Additionally, the protein structure of the ultrahigh gluten WF was not fully unfolded to interact with SPI, thus, the protein aggregated on the lower surface of the WF-SPI films and disrupted the dense network structure. The TS and Tg of the WF-SPI films with medium gluten strength WF reached maximum value of 12.53 MPa and 50.94 ℃ respectively, while the lowest WVP, transparency value and solubility were observed. Therefore, the composite films prepared from medium gluten WF and SPI, after fermentation at pH9, exhibited excellent physicochemical properties. These results offer a new approach to developing WF-SPI films.