A two-compartment drying model related to the fissuring in rough rice

Abstract

Rough rice drying has been studied for years and described mathematically by various researchers. Some have used diffusion equations or have adopted semi-theoretical models in explaining the drying process. But none whatsoever subdivided a drying model corresponding to the material constituents. Two-term and three-term models have been developed by some but the rationale behind those models has been mainly from the point of view of better curve-fitting. Rough rice grains have been shown by many to fissure from moisture adsorption. This is true if the rough rice is at a low moisture content (about 14 percent or less). Temperature effect has been studied independently by various researchers and it does not seem to be a factor as critical as moisture in the fissuring of grains. Research was conducted with the Labelle variety of long-grain rough rice to explain the drying process using a two-compartment model and to relate the drying constants to the percent of kernels which fissured in storage after drying. Cornell (1962) described a method of using partial totals to estimate the parameters of a multi-compartment system. Agha (1971) proposed a more consistent method that overcomes the disadvantages of Cornell's method. These methods are often useful in computing preliminary estimates for iterative solutions. In this research however, model parameters were determined using the non-linear iterative procedure of the Statistical Analysis System (SAS). Kernels after drying were then stored in air-tight vials for 6 days and observations were made on them every 12 hours to check for fissures. The phenomenon of fissure development with storage time followed a sigmoid trend and was described mathematically using the equation for enzyme kinetics which fitted the data adequately. The curves also confirmed the fissuring pattern as suggested mathematically by Nishiyama et al. (1979). Grains used in this study were re-wetted from storage moisture content (about 14 percent) by partially equilibrating them in a walk-in environment chamber. For the tests on fissures, good sound kernels were used before drying and were also inspected for fissures immediately after drying, before placing them in air-tight vials.