稻米陈化中蛋白质对淀粉糊化的影响及其机理研究

Rice ageing during storage often leads to significant deterioration of its eating quality, but the mechanism is unclear. Our research shows that proteins have decisive contribution to changes in pasting properties during rice ageing; the oxidation of rice protein does occur during rice ageing, and it is the case not only for glutelin but also for albumin and globulin; the reducing agents significantly improve pasting properties of the aged rice. Thus, protein oxidation occurred in rice ageing could be the most important reason of pasting prpperty changes of rice. Clarifying the pathway of proteins influencing gelatinization of starch in rice during ageing would be most likely to reveal rice ageing mechanism and thus to provide a new approach to maintain good quality of rice during storage. This project intends, from the standpoint of gelatinization course changes of starch granule caused by protein oxidation, to investigate the mechanisms of protein influencing starch gelatinization of rice during ageing. Removing protein-rich outer layer of rice grain, pasting properties and the particle size distributions after gelatinization would be compared before and after outer layer removal, and the position in rice grain of the protein causing ageing changes could be located. Taking the removed outer layer of rice grain as experimental material, the changes of protein migration caused by rice ageing in the gelatinization course of starch granules will be compared; after removing each protein sequentially from outer layer fractions resulted from rice grain milling, the effects of individual protein on pasting properties of rice during ageing could be researched. Combined with the changes in protein structure/nature, the mechanisms of protein influencing starch gelatinization during rice ageing could be clarified. This project would be helpful to reveal the mechanisms of rice ageing, to regulate ageing deterioration and to improve eating quality of stored rice.

稻米储藏陈化常引起食用品质的劣变，但其机理不明。我们研究显示，蛋白对米陈化后糊化特性的变化有决定性贡献；米陈化后蛋白确实发生了氧化，且不只谷蛋白氧化，清蛋白和球蛋白也呈现出明显的氧化；还原剂可明显改善陈米的糊化特性。因此，蛋白氧化可能是稻米陈化后糊化特性变化的诱因。阐明稻米陈化中蛋白对淀粉糊化的影响机制，或许是揭示稻米陈化机理进而保持其新鲜品质的重要途径。本项目拟从蛋白氧化引起淀粉颗粒糊化历程变化的角度，研究稻米陈化中蛋白影响淀粉糊化的机制。逐步剥离富含蛋白的米粒外层，比较剥离前后糊化特性和糊化后粒度分布的变化，确定引起陈化变化的蛋白在米粒中的位置；以被剥离的米粒外层为材料，对比新、陈米中蛋白在淀粉颗粒糊化中的迁移变化，并逐一脱除各蛋白，研究它们对淀粉糊化的影响，结合蛋白结构、性质变化，阐明蛋白影响淀粉糊化的机制。本项目将对揭示稻米陈化机理、调控陈化劣变速度及改善稻米食用品质具有重要意义。

稻米陈化劣变，是引起粮食产后损失的重要原因。越来越多的研究表明，蛋白质对稻米的陈化劣变起着关键作用，但是蛋白质对淀粉糊化的影响机理不明。本项目从蛋白氧化引起淀粉颗粒糊化变化的角度，通过逐步剥蚀碾磨富含蛋白的米粒外层确定引起陈化变化的蛋白在米粒中的位置，通过激光共聚焦、染色等显微技术原位观察蛋白在淀粉颗粒糊化中的迁移变化，通过逐一脱除各组分考察它们对糊化中淀粉颗粒离解分散的影响，通过拉曼和红外光谱结构分析及蛋白功能性质的测定表征蛋白的氧化，以研究稻米陈化中蛋白影响淀粉糊化的机制。研究发现，15%米粒外层蛋白对米粒内部淀粉糊化的影响较大，米粒内部自身的陈化变化也很明显。淀粉糊化中，蛋白仍以蛋白体形式存在，本身并不迁移，只是受淀粉糊化后不同程度的挤压稀释和包埋黏连。新米淀粉糊化充分，破坏了蛋白体原先存在的位置——胚乳细胞壁和淀粉体膜，蛋白体被稀释冲散；而陈米中淀粉糊化不充分，蛋白体仍然存在于原先位置，集中可见于细胞壁之间，其占位作用和疏水性阻止了水的进一步吸收，抑制了淀粉的充分糊化。陈化后，清蛋白和球蛋白对糊化中淀粉颗粒的离解分散具有显著抑制作用，陈化贡献率分别为89%和82%，谷蛋白的抑制作用较小，醇溶蛋白几乎没有影响，而脂肪则具有明显促进分散作用。拉曼和红外光谱表明，陈米中的谷蛋白发生了明显的氧化，α-螺旋结构减少，含硫氨基酸氧化产物增多，酪氨酸残基更加暴露，色氨酸残基更加埋藏，蛋白与淀粉的结合变得更紧密。陈米中谷蛋白的溶解性、持水性、乳化性、乳化稳定性和起泡性均明显降低，持油性明显升高，仅泡沫稳定性变化不明显，而且新米谷蛋白被提取出来后单独陈化，其氧化程度更深。本项目的实施，为阐明稻米陈化机理提供了未来的研究方向，为表征稻米的陈化特征及理解稻米食用品质的形成提供了新的途径，从而为稻米产后减损奠定基础。