單粒稻穀內部水分遷移之有限元素分析與磁振影像驗證

Abstract

本研究探討單粒稻穀內部的水分遷移情形，使用有限元素分析的方法進行模擬，並且利用磁共振影像進行模擬結果的驗證。模擬過程使用了兩種不同的稻穀模型，其中一種是扁橢圓模型，而另一種則是真實稻穀模型。所使用的真實稻穀模型是以高解析度的磁共振影像經COMSOL Multiphysics 3.2軟體內附加的程式指令在MATLAB 6.5上運算後取得。模擬結果顯示稻穀使用熱風乾燥時，熱風的溫度是一個影響乾燥速率的重要因子，使用較高的溫度進行稻穀乾燥時，稻穀表面的水分會快速地被帶走，而使穀粒內水分的分佈呈現不均勻的現象，位於穀粒表面的稻殼與穀皮水分明顯比胚乳處的水分低。稻穀乾燥前的初始溫度對於單粒稻穀乾燥的結果影響不顯著，不同初始溫度的稻穀分別置於熱風中經過18小時的乾燥，初始溫度為25°C與45°C之稻穀內部的長軸穀皮外緣、短軸穀皮外緣與胚乳中心的含水率的差異皆在±0.1% w.b.之內。若使用間歇式乾燥，即乾燥一段時間、均化一段時間的方式，隨著乾燥時間的加長，稻穀的平均乾燥率可明顯的提升，但相對地快速乾燥會造成穀粒內水分梯度的累積，但是若將均化時間拉長，即可減少穀粒內的水分梯度，降低稻穀產生胴裂的情況。因此，若不考慮完成乾燥所須的總時間長短，而僅考慮耗費能源多寡的情況下，使用較短的乾燥時間搭配稍長的均化時間，如乾燥20分鐘—均化120分鐘此一方式來進行間歇式乾燥將可得到較高的乾燥效率，且可減少穀粒因水分梯度過大導致大量胴裂使得整米率下降的情況發生。除了乾燥與均化之外，本研究也探討浸泡時稻穀內水分遷移的情形，分別對帶殼稻穀與去殼稻穀進行浸泡模擬後，分析模擬結果得知在相同溫度下將稻穀浸泡於水中，水分在稻殼的擴散速率比在穀皮慢，稻殼會阻礙穀粒快速地吸附水分，當穀粒經過10小時的浸泡後，帶殼稻穀胚乳中心的水分吸附率為2.55%d.b./hr，去殼稻穀胚乳中心的水分吸附率為7.94% d.b./hr。

In this study, we discuss the intra-kernel moisture migration of individual rice kernels by finite element method, and verify the simulated results with magnetic resonance imaging (MRI). Two kinds of rice kernel models were used in our study. The one is the prolate elliptical model and the other is the model based on real rice geometric structure. Functions provided by the COMSOL Multiphysics 3.2 software were used to obtain the real rice models from the MRI images of rice kernel. The simulation results show that the air temperature is the major parameter affecting the drying rate for rice drying. The moisture content on the surface of a rice kernel decreases rapidly and results in significant moisture gradient within the rice kernel when the air temperature for drying is high. The initial temperature of grain and the air velocity are less significant than the air temperature. The differences of moisture content between the bran and the central endosperm were only ±0.1% w.b. after 18 hours drying when the initial temperatures of grain were 25°C and 45°C. The average drying rate increases when the portion of drying time in a circulating drying is increased but the moisture gradient inside the rice kernel also increases at the same time. The moisture gradient can be reduced by increasing tempering time however, and then the fissuring of rice kernel can be improved. If we consider the energy consumption but not the total time of rice drying, the drying efficiency of circulating drying is better when it is implemented by the cyclic processes of short drying time followed by longer tempering time. For example, 20 minutes drying followed by 120 minutes tempering yields better drying efficiency. Beside simulations of drying and tempering process, we discuss the moisture migration of soaking process as well. From the simulating results of soaking of brown rice and rough rice, we found that the diffusion rate of the bran is higher than the hull. Hull layer provides a barrier to rapid absorption of water in rice soaking. The water absorption rate is 2.55% d.b./hr in the endosperm of a rough rice and 7.94% d.b./hr in the endosperm of a brown rice after 10-hour soaking.