Development of mathematical models is important to understand the impact of factors affecting rice drying processes, without spending huge amount of time in conducting experiments. Moreover, such models can describe moisture gradients within the rice kernels, which are believed to cause the kernels to fissure and thus reduce the economic value of the rice crop.
In this study, mathematical models were developed to describe heat and moisture transport phenomena in the rice kernels during sorption, convective air drying and infrared drying processes. Experiments were conducted to validate the model and to determine the dependence of moisture diffusivity on rice temperature and instantaneous moisture content (MC). Additional experiments were performed to determine drying characteristics and milling quality of rice in different drying methods. Magnetic resonance imaging (MRI) was used to visualize moisture distribution within rice kernels during the drying process.
Mathematical models developed in this study successfully predicted MC and temperatures in the rice kernels during different drying and sorption processes. Root mean square errors between predicted and measured values for MC and temperature were less than 1% (dry basis) and 4ºC, respectively. The temperature dependence of moisture diffusivity was successfully described by Arrhenius equation. During convective drying, highest moisture gradients were predicted along the shortest axis of kernel on both sides of bran layer.
During infrared drying, rice became heated very rapidly, for example, within a minute of infrared exposure, its temperature rose by about 25ºC. Moisture loss in rice samples during the infrared drying increased linearly with increase in infrared heating time. Total moisture removal per pass in infrared drying was in 5% to 8% (d.b.) range, while it was only 3% to 4% (d.b.) in heated air drying. Despite higher moisture removal, active drying period in infrared drying took less than two minutes, which was about one-tenth of the active drying time in the heated air drying. Milling yields of rice dried by infrared drying methods were statistically similar to the rice dried by convective drying methods, however, milled rice dried by infrared drying was slightly darker in color.
Moisture diffusivity of rice components strongly depended upon its instantaneous MC or water activity (aw) and whether rice is absorbing or desorbing. Diffusivity of rice endosperm was higher during desorption than absorption at aw higher than 0.20. Between aw values of 0.20 and 0.80, endosperm diffusivity increased with an increase in aw. Diffusivity of bran remained almost constant with change in aw and was the lowest among all rice components. Diffusivity of husk decreased with an increase in aw value.
MRI of rice kernel showed that embryo region had higher MC than rest of the brown rice kernel, which was lost very rapidly in the first 30 minutes of drying. However, rest regions of the kernel lost moisture at almost steady rate throughout the first hour of heated air drying.
Knowledge gained in this study provided deeper understanding of heat and moisture transport within the rice grain. The mathematical model developed here can be used to optimize and improve drying process. Rice breeders can also use such models to compare drying and sorption characteristics of different rice stocks and develop fissure-resistant varieties.
|Commitee:||McCarthy, MIchael J., Upadhyaya, Shrinivasa K.|
|School:||University of California, Davis|
|Department:||Biological Systems Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 73/01, Dissertation Abstracts International|
|Subjects:||Food Science, Agricultural engineering|
|Keywords:||Convective drying, Infrared drying, Rice kernels, Sorption|
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