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

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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.