Properties of waxy rice starch and rice grain
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Soaking the waxy rice grain before cooking to raise the moisture content to 56% (db) before cooking in steam ensures that the applied heat is adequate to gelatinize the starch at the center of the grain. The moisture content of the rice grains increased a maximum of 10% (db) after steaming. The computer simulated moisture diffusivity of raw rice soaked at 1, 24, 40 and 60°C were 1.85, 3.88, 6.36, and 6.90 ´ 10 -9 m 2 /min, respectively. Rice soaked above 60°C gelatinized at the surface therefore grain integrity was altered, and a moisture- impervious layer of gelatinized starch was formed on the surface. Moisture absorption poorly fitted the diffusion model when the soaking temperature was higher than the gelatinization temperature. The desirable soaking condition was either overnight (12 hrs) at room temperature or 45 min at 60°C. Differentia l scanning calorimetry of whole rice grain showed an onset temperature of starch gelatinization from 58 to 67 °C , while the peak temperature was 69 to 72°C. The energy absorbed by the gelatinized transition was lowest on samples soaked the longest time and increased with decreasing sample moisture. Gelatinization transition was still exhibited by samples cooked 3 hr. Instant waxy rice was produced by a process called the "soak-steam-chill- separate-dry" method. To make instant rice, soaked rice was gelatinized in steam for 20 min followed by drying at 1 to 4°C for 48-72 hrs. The slow drying resulted in dense-transparent grains having moisture content 28-32% (db). The clump of gelatinized rice was separated into individual grains then further dried at low temperature until its water activity (aw) was 0.6- 0.7. Scanning electron micrographs of samples showed that an improperly dried produc t exhibited large hollow cavities in the grain and densely packed material near the grain surface. Texture profile attributes of reconstituted instant waxy rice was not significantly different from those of conventionally prepared waxy rice. A coconut flavored reconstituted instant rice was not significantly different in sensory attributes from a similar product prepared conventionally from regular waxy rice.