Study On Influence Of Rice Protein And Its Composition On Rice Cooking And Eating Quality

Rice protein, as the second largest constituent of rice, has great influence on rice quality, especially for cooking and eating quality (CEQ). Therefore, further clarification of the influence of rice protein and its composition (RPC) on rice CEQ has significant importance to theoretical and practical study.Firstly, twenty-six rice varieties from the unified producing areas were taken as materials, and the relevance features between physicochemical properties, especially protein properties of twenty-six rice with the physical properties and sensory of their cooked rice was analyzed in this study. Then, eight cultivars with same amylase content (AC) but with different or same contents of RPC were selected to further reveal the relationship between protein and rice eating quality. In which, the correlation between RPC with rice quality related eating taste was systematically analyzed, and the change characteristics of protein during cooking, and the effect of RPC on rice cooking characteristics was also investigated, hoping to provide a theory basis for the quality control of rice and rice products in food industry, and also for breeding high-quality special rice. The main results are as follows:(1) Apart from the main constituents of rice, such as starch, protein and moisture, the free amino acids (FAA) could be a key physicochemical factor which had dual affects on rice CEQ. High FAA content was favorable for the physical properties, but reversed to the palatability of cooked rice.(2) Rice quality could be greatly effected by albumin, which significantly positively correlated with the aspect ratio (AR) (r=0.772*), significantly negatively correlated with the brown rice rate (r=-0.754*), and extremely significantly negatively correlated with the AC (r=-0.895**) of rice. And then, the content of prolamin significantly positively correlated with the AR (r=0.795*). The content of protein components negatively correlated with the gel consistency (r=-0.709*). Through the correlation analysis between contents of protein and its components with physical properties parameters of cooked rice, we could conclud that the rice physical properties of cooked rice would be better when the content of albumin was low while globulin was suitable. The negative influences on taste quality of cooked rice from prolamin were most significant in five compositions of protein. The content of prolamin extremely significantly negatively correlated with the viscosity (r=-0.886**), and significantly negatively correlated with the integrity, springinss and total value of cooked rice (r=-0.831*, r=-0.731*, r=-0.731*). And the influence on rice taste quality of cooked rice from total protein content (TPC) which significantly negatively correlated with the viscosity and taste (r=-0.725*, r=-0.719*) of cooked rice ranked second. The TPC had some negative impacts on other rice taste quality indicators, as well positive impacts on the color of cooked rice. The FAA content positively correlated with taste quality indicators but negatively correlated with the rice color, luster and cold rice texture. Especially, the FAA content significantly positively correlated with the viscosity of cooked rice (r=0.721*).(3) The change characteristics of protein contents and subunits molecular weight during cooking showed that protein mainly aggregated with the other constituents in rice rather than simply with protein molecules, and this aggregation upon prolamin was more obvious compared with glutelin, which reduced the protein solubility.(4) One of the reasons that protein digestibility of cooked rice were higher than the polished rice may be the sharply relative decline of contents of extractable and non-digestible prolamin after cooking. The average digestibility of japonica cooked rice was higher than the indica. And the higher TPC in the same kind of rice had lower average protein digestibility. Among indica rice with similar RPC, their differences in protein digestibility were not significant.(5) The RPC had great influence on rice cooking properties such as water absorption, pasting properties. The maximum absorbed water was affected by the TPC. When TPC was high, the absorbed water was less and vice versa. The water absorbing processes and results of cultivars with same TPC were not in the same may due to their differences in protein composition. The higher TPC, the harder gelatinized rice. Albumin and prolamin had great influence on the rice thermodynamic properties, and prolamin content was significantly positively correlated with the DSC offset temperature (Tc) (r=0.764*).