Effects of rice endogenous proteins on the rheological and retrogradation properties of rice flour gel

This study aims to elucidate the regulatory role and mechanisms of rice protein in the retrogradation and gel properties of rice starch. By directionally regulating the protein content in rice flour using alkaline protease, the effects of protein content (ranging from 8.27% to 1.77% on a wet basis) on the rheological properties, gel texture characteristics, retrogradation behavior, water distribution, and microstructure of starch were systematically investigated. The results indicate that a decrease in the protein content of rice flour was found to result in a significantly increase in the starch setback value rising from 1 119 mPa·s to 1 593 mPa·s. Dynamic rheological analysis demonstrated that samples containing proteins exhibited substantially lower storage modulus (G') and loss modulus (G″) values compared with deproteinized systems, suggesting that proteins retard short-term retrogradation by disrupting the ordered rearrangement of starch molecules. After 14 days of storage, the retrogradation enthalpy (ΔH) increased from 6.07 J/g to 8.79 J/g. Texture analysis revealed that during a 21-day storage period, the hardness of high-protein gel (8.27%) increased from 41.93 g to 153.30 g, while that of low-protein gel (1.77%) rose more sharply, from 62.28 g to 326.66 g. Low-field nuclear magnetic resonance (LF-NMR) and Fourier-transform infrared spectroscopy (FT-IR) further confirmed that proteins inhibit water migration and amylopectin recrystallization by extending water relaxation time (T₂) and reducing the R_{1 047/1 022} and R_{995/1 022} ratios. Scanning electron microscopy (SEM) observations revealed progressive densification of the gel structure during aging, with lower protein content leading to more pronounced compactness. Collectively, these findings demonstrate that rice proteins effectively delay starch retrogradation through dual mechanisms: competitive water absorption and network structure inhibition. This research provides a critical theoretical foundation for the development of anti-retrogradation technologies in rice product processing.