Effects of Soy Protein Isolate on Pasting and Gelling Properties of Corn and Wheat Starches

Starch and Soy Protein Isolate (SPI) have numerous applications in food products mostly as gelling and texturizing agents. The main purpose of this research was to investigate the pasting and gelling properties of the mixtures of starch and various levels of SPI (0, 5, 10, 15, 20 and 25%). To determine the effects of starch sources on the results, wheat and corn starches were tested in this study. It was found that with increasing the quantity of SPI, the viscosities obtained from Rapid Visco Analyser (RVA) including final, holding, and setback decreased while pasting temperature remained unchanged. Increasing the SPI concentrations enhanced the peak viscosity of the wheat starch-SPI samples, while it had opposite effects on corn starch-SPI samples. The colour of both starch-SPI gels became darker and more yellowish with increasing the SPI concentration. Although gel hardness increased during storage for 1 and 3 days at 4˚C, the addition of SPI reduced the hardness of the gels. The residual modulus of the Maxwell model from stress relaxation data showed that gels became softer and less elastic with increasing the SPI concentration. The corn starch-SPI gels exhibited darker color, higher firmness, cohesiveness, springiness, gumminess and chewiness compared to the wheat starch-SPI gels. The results may be useful to obtain the desired quality in food products containing starch and SPI.