以磁振影像探討單粒稻穀之水份分佈

Abstract

本研究應用磁振影像探討單粒稻穀內部靜態水份分佈，以及在均化與浸泡過程中，內部水份經擴散與對流作用所產生之動態變化。實驗過程分別使用3.0T MRI系統之SPI脈衝序列，以及9.4T MRI系統之3D Spin echo脈衝序列進行磁振造影。首先藉由磁振造影機之參數設定試驗以取得較佳的影像，再由影像的分析探討稻穀內部水份之分佈與動態遷移。 單粒稻穀在平衡含水率下，磁振影像訊號強度並非均質且分成幾個部份，其中胚的訊號強度最高，糊粉層的強度次之，胚乳相較前兩者其訊號強度較低。在均化試驗中，稻穀以55℃熱風乾燥30分鐘之後進行均化，由於內外層之水份梯度，造成水份的擴散作用，過程中內層之擴散速度高於外層，呈現雙項指數函數的關係，兩項指數代表之意義，一者為內部水分子的擴散現象，另一者為通風環境下，受對流作用迫使水分子加速往外遷移。其結果顯示良好的通風將有助於單粒稻穀內部之均化，均化時間大約在7小時，其內外層水份可達平衡，若無通風的情況下均化時間將高達15小時以上，此遷移多發生在胚乳上，而胚很快即達到平衡，糊粉層的水份差異性並不大。 在稻穀直接浸泡的實驗中，水份經由稻殼滲透與珠孔的傳導而進入穀體內，而於胚及胚乳間擴散開來。期間米心之水份吸收可分為4個時期，由最初之遲滯時期，而到達較快速的水份增升期，而後為另一緩升時期，最後到達飽和之含水率。稻穀直接浸泡、珠孔導水、封住珠孔稻穀浸泡與封住稻殼僅以珠孔導水等四種不同的浸泡模式中，米心達飽和含水率67%需時分別約為7、17、22與32小時，在浸泡過程的兩個傳輸通路中以珠孔的傳水效率較快，而水份透過稻殼的傳輸較慢。

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.