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系统地解释：例如，食品材料在玻璃化状态下比在橡胶状态下更能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已通过热分析如差示扫描量热法（DSC）测定。然而，热分析并不能提供分子迁移率的信息。在本研究中，通过脉冲核磁共振分析了玻璃和橡胶材料之间分子迁移率的差异，该方法可以通过分析质子的弛豫行为来检测系统中组分的迁移率，以及水对被测分子迁移率的影响。此外，还研究了玻璃化转变对食品粉两种重要品质变化——结块和颜色变化的影响。用DSC法测定了食品的玻璃化转变温度T_g。由于水的塑化作用，受测食品的T值随着水分含量的增加而降低。对于小麦面筋，颜色变化AE的临界点对应于玻璃到橡胶的过渡点。谷蛋白的DSC数据也支持谷蛋白在颜色变化临界点以上处于玻璃态。所测食品的吸水等温线呈s型，与许多食品材料相似；在玻璃到橡胶的过渡点以上，吸水量急剧增加。吸水率在水活度0.5 ~ 0.6附近达到最大值。用高斯、指数和高斯线形的两种或三种分量的组合描述了核磁共振的自由感应衰变（FID）。M_2有一个或两个转变温度。低含水率时，不动成分占主导地位，但随含水率的增加而降低。在水分含量高的情况下，直接附着在分子碳链上的质子似乎是不能动的，羟基上的质子相对能动，而水中的质子则更能动。在T_g以上温度下储存的粉末冷却至室温后结块。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