Heat-of-Aging Measurements of Boiled Rice by Isothermal Microcalorimetry

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Aging enthalpies were found for boiled rice at 10ーC with an isothermal microcalorimeter (Sweden Thermometric AB, Therm al Activity Monitor 2277 (TAM2277)). The corresponding texture data (hardness, adhesiveness, and cohesiveness) were also obt ained with a creep meter (Yamaden RE-3305). The rice used were Hitomebore", product of Miyagi Pref. and Koshihikari", product of Niigata Pref. They were polished before use, immersed in water for 30 minutes before boiled in an electric rice co oker, and then retained in it for 15 minuets. The rice such cooked we will call "the standard cooked rice", and the latter treat ment "Murashi 15 min". We examined the standard cooked rice, "Murashi 5 min" rice, and the rice cooked with two kinds o f emulsified oil (KAO Delicalight and RIKEN VITAMIN Ricegood) and sucrose fatty acid ester (abbreviated as sugar ester). Heat measurements : The aging was exothermic process continuing for more than 1 week. The heat peaks appeared after 16 - 29 hours, and then gr … More adually decreased to zero. The time at which the peak appeared and the heat-of-aging per Ig of coo ked rice were shown below. "Hitomebore" : standard cooked (18 h, 2.2 J), 1% Delicalight added (23 h, 1.8 J), 2% Ricegood added (24 h, 1.7 J), 0.5% sugar ester added (19 h, 1.6 J), Murashi 5 min (16 h, 1.8 J) "Koshihikari" : standard cooked (29 h, 1.6 J), sugar ester added (reproducibility being examined, 1 - 1.2 J) The addition of Delicalight, Ricegood, or sugar ester to "Hitomebore" retarded the aging and suppressed its degree. The small heat of "Murashi 5 min" is to due to the insufficient gelatinization. The "Koshihikari", known as superior quality rice for eating, had the smallest aging enthalpy. The smaller value than the heats of the "Hitomebore" with aging retardant added should kbe noted. Texture measurements : The aging accompanied the noticeable increase in hardness and the frequent occurrence of brittleness. It was also characterized by large decrease in adhesiveness and cohesiveness. The addition of Delicalight, Ricegood, or sugar es ter was effective to suppress the increase in hardness. The hardness (80% compression) of "Koshihikari" was similar to that of "Hitomebore". On the other hand, the adhesiveness and the cohesiveness of the former were larger than those of the latter on the first day, and after the third day the hardness (25% compression) was smaller and the adhesiveness (25% compressio n) was larger than those of "Hitomebore". We could not find the heat-of-aging at 25℃. Texture changes were also very small at this temperature." Less