Changes in Thermal Conduction and Mechanical Properties during Cooking Procedure of Foodstuffs due to Phase Trnsition

1）从上一个项目继续进行了一系列实验研究，以描述加热过程中相变对食品机械性能和热传导的影响。2）尝试跟踪淀粉颗粒在糊化过程中水体系中宏观和微观状态的变化，即光学显微镜检查、DSC测量和小应力下机械响应的观察。结果表明，马铃薯和小麦淀粉颗粒在糊化过程中的溶胀程度远高于糯米、粳米和玉米淀粉颗粒。然而，DSC 测量表明，在待测样品中，马铃薯淀粉的表观转变热最高（-27.3 mJ），而小麦淀粉的表观转变热最低（-15.2 mJ）。所有样品的机械响应，即瞬时弹性和稳态流动粘度，在转变过程中都可以用一种凸曲线来表征，并且每个最大值出现在70℃左右，对应于每个淀粉分子中的晶体区域的消失。3)测量了加热过程中成分的相变对食品中传热的影响，并将其应用于金属容器中沿x轴加热平面的温度增加曲线。研究发现，随着可可粉、绿茶纤维、小麦淀粉颗粒等样品中热不敏感物质的量增加，由于相变（例如脂肪熔化或亲水胶体溶胶转变）对温度升高曲线的干扰趋于减少。结果表明，加热过程中，转变吸热在温度升高中被分散，而温度升高速率的过程可能受到热不敏感材料的存在的抑制。

1)A series of experimental studies have been continued from the last project so as to describe the effect of phase transition on the mechanical properties and thermal conduction in foodstuffs during heating procedures.2)An attempt was made to follow the changes in macro- and microscopic states of starch granule in water systems during gelatinization, i.e.optical microscopic examination, DSC measurement, and observation of mechanical responses under small stresses. The results obtained indicated that the potato and wheat starch granules show much higher degree of swelling in the course of gelatinization than those from glutinous and non-glutinous rices and corns. The DSC measurement, however, revealed that the apparent heat of transition was highest with potato starch (-27.3 mJ) among the samples to be tested, while that for wheat starch was lowest (-15.2 mJ). The mechanical responses, i.e. instantaneous elasticity and steady flow viscosity, of all the samples could be characterized by a types of convex curves during the process of transition, and each maximum appeared at around 70℃ corresponding to disappearance of the crystal region in each starch molecule.3)Measurements were made to clarify the effect of phase transition of ingredients on the heat transfer in foodstuffs during heating applied to the increasing curve of temperature along the x axis from heating plane in a metal vessel. It was found that the disturbance to the increasing curve of temperature due to phase transitions, e.g.melting fats or sol transition of hydrocolloids, tends to decrease with increasing amount of thermo-insensitive materials in the samples such as cocoa powder, fiber of green tea, wheat starch granule, etc. The results suggest that the endothermic heat of transition is scattered in the increasing temperature during heating, while the course of increasing rate of temperature may be restrained by the existence of thermo-insensitive materials.