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Rice cake ("Mochi" in Japanese) is the traditional Japanese food and also have a function of preserved food. It is made of sticky steamed rice with sever deformation by pounding. The grain boundary disappears after deformation and it has uniform structure like candy and show viscous flow. This viscosity comes from gelatinization in which amylopectin and water mixed uniformly. We often preserve the rice cake with preventing drying, then it becomes hard. When we reheat it up to 60-80ºC, it becomes soft again like the state just after pounding. Our target is to know how they become soft again. In other expression, where is the water when it is hard? Therefore, we measured small angle X-ray scattering (SAXS) of “hard” rice cake and measured the profile change when we heat up to 80ºC and cooling down again. All are the in-situ experiments. The SAXS profile shows shoulder like scattering around 0.4 nm^-1, which indicate that the existence of density different region with 10 nm in diameter in hard state. That scattering almost disappear when it becomes soft. These results suggest the existence of “water tank” with 10 nm in diameter in the preserved state and it supply to whole area when it becomes soft.