| Summary: | 博士 === 國立臺灣大學 === 生物產業機電工程學研究所 === 97 === The objectives of this research are to investigate the static moisture distribution within an individual rice kernel, and the dynamic moisture migration during tempering and soaking using MRI techniques. Experiments were performed separately using SPI pulse sequences in a 3T MRI system and 3D spin echo sequences in a 9.4T MRI system for image acquisition under various experimental conditions. The optimum MRI acquisition parameters were initially studied and determined for later experimentation for the assessment of static and dynamic moisture migration behaviors within individual rice kernels.
For rice kernel at equilibrium moisture content, the MR signals spatially non-homogeneous. A rice kernel has the strongest signal intensity at its embryo part. The aleurone layer has relatively weaker signal intensity while the endosperm part exhibits the least signal intensity. The signal intensity of the endosperm increases as the moisture content increases. In the tempering experiments, the rice kernels were air-dried for 30 minutes at 55℃ before image acquisition. The dynamic moisture migration was observed due to the existence of moisture gradient and the diffusion process. The transient change of the signal intensities in the endosperm was well fitted with a double exponential function suggesting that both convection and diffusion contributed to the reduction of the moisture gradient within the rice kernel during tempering. This hypothesis was further supported by the experimental data of the insulated rice kernel whose convective mass transfer was excluded. The tempering time was about 7 hours when the moisture gradient of the endosperm became minimal. The tempering time was about 15 hours for the insulated rice kernel without convective mass transfer.
MRI experiments were also designed to assess the moisture migration within rice kernels under various soaking conditions. Moisture can either transport across the rice husk or via the micropyle. The dynamic change of moisture content at the central part of the endosperm was observed to have four phases: initial lag phase, rapid increase phase, slow increase phase, and the saturation phase. For the four test conditions: direct soaking, guided micropyle transport, sealed micropyle, and insulated husk, the time required to reach 67% moisture saturation at the central endosperm was 7, 17, 22 and 32 hours, respectively. The rate of moisture transport via micropyle appeared to be faster compare with the moisture transport across the rice husk.
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