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

1）在上一个项目的基础上，继续进行了一系列的实验研究，以描述食品在加热过程中相变对机械性能和热传导的影响。2）试图跟踪淀粉颗粒在水体系中糊化过程中宏观和微观状态的变化，即光学显微镜检查，DSC测量，以及观察小应力下的机械响应。结果表明，马铃薯和小麦淀粉颗粒在糊化过程中的膨胀度明显高于糯米、糯米和玉米淀粉颗粒。然而，DSC测量显示，在待测试的样品中，马铃薯淀粉的表观转变热最高（约27.3mJ），而小麦淀粉的表观转变热最低（约15.2mJ）。所有试样的力学响应，即瞬时弹性和稳态流动粘度，在转变过程中都表现为一种凸曲线，在70℃左右出现最大值，对应于每个淀粉分子中结晶区的消失。通过实验研究了在食品加热过程中，原料相变对食品传热的影响，温度沿着x轴从金属容器中的加热平面。结果表明，随着样品中热不敏感物质（如可可粉、绿色纤维、小麦淀粉颗粒等）含量的增加，相变对升温曲线的扰动（如脂肪的熔化或水胶体的溶胶转变）趋于减小。而热不敏感材料的存在则会抑制温度的上升速率。

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.