Effect Of Protein Content On The Rheological Properties Of Early Idica Rice Flour And Preliminary Study On Its Mechanism

Using early indica rice as the raw material, this paper aimed to clarify the effects of protein content on gelatinization, rheological properties, the long-term retrogradation and gel properties of early indica rice flour, based on which, the mechanism of protein affecting early indica rice flour was also elucidated initially. Then a theoretical basis of specialized production in early indica rice flour could be found.Firstly, we studied the preparation method of early indica rice flour with different protein contents. And then with the mathematical equations’ simulation, the non-linear equations of the protein content and reaction time during the reaction process were acquired, based on which, we prepared our samples. As the protein content decreased from 7.64% to 0.56%, peak viscosity, the lowest viscosity, final viscosity, breakdown and setback values of early indica rice flour decreased by 36.50%, 25.04%, 27.41%, 58.75% and 30.66%, respectively, while pasting temperature almost unchanged. The enthalpy value after long-term retrogradation gradually reduced with the decrease of protein content, which indicated that low protein content could delay retrogradation. The results of dynamic rheological studies were consistent with gelatinization results. As the protein content decreased, the elastic modulus(G’) increased while the viscous modulus(G’’) gradually decreased. The static rheological studies showed that, when the protein content decreased, pseudoplastic behavior became weaker, and it was less likely to shearing thinning. Meanwhile, solubility and swelling capacity increased with the decrease of protein content. The gel strength also changed, when protein content decreased, the hardness, springiness, cohesiveness and chewiness of rice flour gel increased, but the adhesiveness decreased.Secondly, the mechanism of protein affecting early indica rice flour was elucidated initially. The X-ray diffraction patterns of these six samples showed that protein content had little impact on the indica rice’s crystallinity. The results of Scanning Electron Microscopy(SEM) also showed that when protein was removed, flour surface became smoother and microstructure of rice gel became tighter. By Fourier Transform Infrared Spectroscopy(FTIR) analysis, there was no new characteristic peak which meaned that basic structure of early indica rice flour was not damaged when protein content decreased. The wave number of O-H and C=O peak became larger, because the secondary structure of protein was broken and hydrogen bonding between protein and starch weakened. The 1H NMR was applied to study transverse relaxation time T2. As the protein content decreased, T21 which represents combined water would decrease, T23 which represents free water would increase. The change of T2 could explain very well why viscosity of early indica rice flour decreased during gelatinization and why the adhesiveness of rice gel decreased.Finally, early indica rice flour with different protein contents was applied to traditional rice product—fermented rice cakes. The sensory score and Texture Profile Analysis(TPA) parameters of fermented rice cakes were analyzed. These results showed that the changes of hardness, springiness, cohesiveness, chewiness and adhesiveness of fermented rice cakes were consistent with the changes of gel strength. So, the texture parameters of rice flour gel could predict the related parameters of fermented rice cakes accurately. Meanwhile, when protein content was about 5%, fermented rice cakes had the best quality.