Molecular Mobilty and Food Preservation with Relevance to Glass Transition

近年来，玻璃化转变引起了食品科学家的极大兴趣，因为许多与食品制造和保存有关的现象都可以用玻璃化转变的概念来系统地解释：例如，食品材料在玻璃态比在橡胶态更好地保持其物理和化学稳定性。许多食品的玻璃化转变温度T_g是通过热分析如差示扫描量热法（DSC）测定的。然而，热分析不能提供分子迁移率的信息。在这项研究中，玻璃和橡胶材料之间的分子流动性的差异进行了分析，通过脉冲核磁共振，可以检测系统中的组分的流动性，通过分析质子的弛豫行为，和水对分子流动性的影响进行检查。此外，还讨论了玻璃化转变对食品粉末的两个重要质量变化--结块和颜色变化的影响， ...更多信息 用差示扫描量热法（DSC）测定了食品的玻璃化转变温度T_g。由于水的增塑作用，所检查的食品的T值随着水分含量的增加而降低。对于小麦面筋蛋白，其变色临界点AE可视为对应于玻璃化转变点。谷朊粉的DSC数据也支持谷朊粉在颜色变化临界点以上处于玻璃态。所研究的食品的吸水等温线与许多食品材料相似，呈S形;在玻璃-橡胶转变点以上，吸附水量急剧增加。在水活度0.5- 0.6附近，吸水速率达到最大值。NMR的FID（Free Induction Decay）谱线可以用高斯、指数和高斯三种谱线的组合来描述。M_2有一个或两个转变温度。在低含水量下，不动组分占主导地位，但随着含水量的增加，不动组分的百分比下降。高水分时，直接连接在分子碳链上的质子不移动的，羟基上的质子相对移动的，而在水中的质子移动的较多。少

The glass transition has attracted great interest from food scientists in recent years, because many phenomena related to food manufacturing and preservation could be systematically explained with a concept of glass transition : for example, food materials maintain their physical and chemical stability better in a glassy state than in a rubbery state. The glass transition temperature T_g for many foods has been determined by thermal analysis such as differential scanning calorimetry (DSC). The thermal analysis, however, does not provide information on molecular mobility. In this study, the difference in molecular mobility between glassy and rubbery materials was analyzed by a pulse NMR that can detect the mobility of the components in the system by analyzing the relaxation behavior of the protons, the and the effect of water on the molecular mobility being examined. In addition, the influence of glass transition on two important quality change of food powders, caking and color change w … More as investigated.The glass transition temperature T_g for foods was determined by DSC. The values of T, for the foods examined decreased with increasing moisture content, due to plasticizing effect of water. As for wheat gluten, the critical point of color change AE can be taken as to correspond to the glass to rubber transition point. The DSC data of for gluten also support that gluten is in glassy state above the critical point of color change.Water sorption isotherms of foods examined were sigmoidal similar to many food materials ; the amount of sorbed water increased drastically above glass to rubber transition point. The water sorption rate takes a maximum value near water activity 0.5-0.6.FID (Free Induction Decay) of NMR was satisfactory described by a combination of two or three components, Gaussian, exponential, and Gaussian lineshapes. M_2 showed one or two transition temperatures. At low moisture content, the percentage immobile component was dominant, but it decreased as moisture content increased. At high moisture content, the protons directly attached to the carbon chains of molecules seemed to be immobile, those in the hydroxyl group to be relatively mobile, and those in water to be more mobile.Powders stored at a temperature above T_g were caked after cooling to room temperature. Less