73C-29 |
Computer simulation of intra-kernel stress pattern of rice during drying and explanation by glass transition hypothesis |
C. JIA, W. Yang, T. J. Siebenmorgen, and A. Cnossen. Department of food science, University of Arkansas, 272 Young Avenue, Fayetteville, AR 72704 Rice kernels involve both expansion and contraction during the drying process. Expansion and contraction inside a rice kernel would produce non-uniform strains, which may form a complicated internal stress. Stress cracking/fissuring would appear when the thermal-hydro stresses of a rice kernel exceed its failure strength. Stress cracks reduce both the quality and the market value of rice. The objective of this study was to simulate intra-kernel stress patterns during drying using finite element method and computer simulation technology. Explanation was also given from the glass transition perspective. Currently, no experimental method is available to directly determine the internal stresses. The moisture and temperature distribution, stress pattern are verified against the measured temperature and moisture profile and the crack/fissure observed experimentally, which has been a common procedure adopted by many researches. In this study, thin-layer drying tests of a long-grain rice variety Cypress were conducted in the conditions of 60°C and 17% relative humidity. The initial moisture and temperature was 21.4% (wet basis) and 27.3°C. The moistures were determined by an oven method (125°C, 24 hrs.). Internal temperature of the kernel was measured using a 0.1-mm diameter thermocouple that was inserted through a small hole drilled into the kernel. The results showed that the predicted moisture and kernel-center temperature agreed well with the measured values. The maximum stress and the maximum strain energy of distortion density always appeared along the short axis of the rice kernel. The closer to the kernel surface, the greater the stress. The results were explained in terms of the glass transition hypothesis. This study demonstrates the potential of computer simulation technology to model the complex stress problems for a viscoelastic material like rice undergoing moisture and temperature changes. This study has put forward some new ideas for optimizing the rice drying process.
Session 73C, Food Engineering: Transport Processes and Kinetics
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