大米小包装储藏过程中脂类及微观结构的变化

【目的】由于胚乳失去保护，大米在储藏过程中易发生陈化、霉变等问题，致使其品质劣变甚至丧失食用价值。通过测定储藏过程中大米游离脂肪酸、总酸含量、脂肪酸组成和含量，观察大米横断面胚乳细胞、淀粉颗粒微观结构，探讨大米品质劣变原因，为大米储藏技术提供参考。【方法】以粳稻99号大米为对象，进行人工模拟储藏试验：采用编织袋、自然密闭缺氧、抽真空3种包装方式，置于温度为15、25和30℃（60%湿度）的人工气候箱中储藏180 d，每个月进行一次品质指标测定；测定大米储藏过程中脂肪酸值、总酸值的变化，利用气相色谱分析脂肪酸组成及含量变化，利用扫描电镜观察大米横断面胚乳细胞形态，探索脂类及横断面胚乳结构导致大米品质陈化的机理及品质劣变后淀粉颗粒微观形态的变化规律。【结果】①储藏过程中，编织袋及自然密闭缺氧包装的大米游离脂肪酸及总酸含量皆呈现急剧上升趋势，当品质劣变严重时两者含量反而下降；抽真空包装的大米游离脂肪酸及总酸含量均保持在较稳定的水平，且储藏温度对其含量影响不显著；②脂肪酸组成中含量最多的是亚油酸、油酸和棕榈酸，这3种脂肪酸含量占总脂肪90%以上。包装方式及储藏温度对各脂肪酸组成影响不同。除了花生酸（C20:0）及二十二烯酸（C22:1）外，其余脂肪酸受包装方式影响较大；而储藏温度仅与豆蔻酸（C14:0）和山嵛酸（C22:0）有显著性关系，对其余脂肪酸影响不大。脂肪酸变化规律符合：高不饱和脂肪酸含量随温度升高而降低，多不饱和脂肪酸与饱和脂肪酸及单不饱和脂肪酸间呈现显著负相关，具体表现为硬脂酸（C18:0）与亚油酸（C18:2）、亚麻酸（C18:3）；油酸（C18:1）与亚油酸、亚麻酸皆呈现明显的“此消彼长”现象。③新鲜大米淀粉粒以复粒形式存在，形状较大，表面有较厚的膜，排列整齐有序，两侧以片状的细胞间断面居多，放射状结构清晰可见。陈化加剧的大米胚乳细胞横断面结构松散无序，细胞内断面多于细胞间断面，横断面上可观测到“小孔”及一些分散在淀粉体周围的蛋白体和脂肪滴；细胞壁的完整性受到破坏致使复粒崩解并可见大量单粒细胞，一部分单淀粉粒形状近圆型，棱角较钝；细胞间的棱角模糊，放射状结构变得模糊甚至消失。【结论】编织袋及自然密闭缺氧包装的大米陈化劣变较为严重，储藏过程中其脂肪酸值、总酸值急剧上升后下降，胚乳表面光滑度变差、小孔数量及表面蛋白质膜的翘起变多，辐射状趋势渐渐消失。抽真空包装可以有效延缓大米品质劣变进程。

Lipid and Microstructure Changes of Small Packaged Rice During Storage

【Objective】Due to the loss of protection of endosperm, rice prones to aging during storage, mildew and other issues, which resulted in edible quality deterioration or even lost edible value. The aim of this study was to explore the reasons for the deterioration of rice quality by measuring the content of total acids and free fatty acids, changes in fatty acid composition and content, microscopic structure of cross-sectional endosperm cells or starch granules of rice, and to provide reference for storage technology of rice. 【Method】The No. 99 japonica rice packaged in woven, natural airtight anoxia and vacuum was simulated storaged in artificial climate chamber at a temperature of 15, 25, and 30℃ under 60% RH for 180 d, then the quality indicators and morphology of rice were measured every month. The aging mechanism of rice and variation of starch granules morphology caused by lipids and cross-sectional structure of endosperm after deterioration were explored by determining the variation of fatty acid and total acid value, the changes of fatty acid composition and content were analyzed by gas chromatography, and the cross section of rice endosperm cell morphology was observed by scanning electron microscopy.【Result】It was found from the study that the free fatty acids and total acid contents of rice packaged in woven and natural airtight anoxia increased sharply in initial process while decreased with a serious deterioration of the quality. The free fatty acids and total acid contents of rice packaged in vacuum maintained at a relatively stable level while the storage temperature had no significant effect on its content. The most abundant fatty acids were linoleic acid, oleic acid and palmitic acid,which accounted for more than 90% of total fats. Packaging methods and storage temperature had a great impact on various fatty acids. In addition to arachidonic acid (C20:0) and docosenoic acid (C22:1), the other kinds of fatty acids were greatly influenced by packaging methods. The storage temperature had a significant relationship with myristic acid (C14:0) and behenic acid (C22:0) while had little effect on the rest of the fatty acids. The changes of fatty acids agreed with the results that the amount of highly unsaturated fatty acids decreased with increasing temperature, these data also demonstrated negative correlations between polyunsaturated fatty acids (stearic acid (C18:0) and oleic acid (C18:1)) and saturated or monounsaturated fatty acids (linoleic acid (C18:2) or linolenic acid (C18:3)). The starch grains of fresh rice exist in a compound grain form with a large shape and a thick film on the surface, neat and orderly arranged with clear radial section in both sides of the sheet between the cells. The structure of endosperm cell cross-section became sloppy and disorderly, while the number of intracellular sections were more than that of intercellular sections. In addition, some “small holes” can be observed in sections with some protein and fat droplets scattered in the amyloplast. The compound starch granule broke into a large number of single granulocytes due to the damage of cell wall. Part of the shape of single starch granules became nearly circular with some blunt edges. And the radial structure became blurred or even disappeared when aging seriously.【Conclusion】The value of fatty acids and total acid showed an upward trend first and then a downward trend, the smoothness of endosperm surface deteriorated, and the number of holes and tilt of surface proteins film increased, as well as the radial structure trend disappeared gradually during storage. The rice quality deterioration seriously with woven and natural hypoxic packaging, it could delay the deterioration process of rice quality with vacuum packaging effectively.