For simulating the process of cooking rice grains or rice starch-related foods, the knowledge of the water diffusion coefficient as a function of temperature and moisture content is essential. Unfortunately, however, Iittle reliable published data are available. In this paper, the water diffusion coefficient (WDC) in rice starch/water mixture with a selected moisture content was measured by PFG-NMR over a range of temperature. The WDC in a heated sample (0.42-0.89 g H20/g sample), which was prepared by heating at 90'C for OO min and cooling in advance, was measured over the temperature range 20-78'C, and that in a non-heated sample (0.465-0.55 g H201 g sample) was measured at 16-53'C. The correlations between WDC and temperature as well as between WDC and moisture content were examined, and some empirical equations were developed with which the estimation of the WDC at an arbitrary combination of temperature and moisture content was enabled.Keywords: starch, rice, diffusion, NMR, gelatinization, water content, temperature dependence Although we have a long history of cooking rlce grains, the mechanism of moisture percolation into the rice grain during cooking has not been fully analyzed. We measured the change in moisture distribution in a rice graln during boiling via an NMR T2 imaging method (Takeuchi et a/, 1996). For the analysis of this experiment, we tried to simulate the change in the moisture profile in a rice grain using a mathematical model, where we needed the water diffusion coefncient (WDC) in a rice grain during boiling. Unfortunately, however, there has been little literature data available for the WDC with its dependence on moisture and temperature, even though the WDC in food materials Is well known to depend greatly on temperature and moisture content (Saravacos 1986). In this paper, the pulse field gradient NMR (PFG-NMR) method was applied to measure the WDC in rice starch/water mixtures over a range of temperature and moisture content. The measured data were arranged to provide some empirical equations with which one can estimate the WDC at an arbitrary temperature and moisture content.
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