Modelagem da resistência à desidratação de grãos de arroz em casca

Abstract

The paddy rice is composed of several structures, which, have different anatomical and chemical characteristics. Due to this diversity, each structure provides distinct influence on the drying process. This study was conducted in Agricultural Products Processing Laboratory of the Department of Agricultural Engineering of the Federal University of Lavras. The objective of this study was to assess the kinetics of drying paddy rice and their constituent parts; to determine its thermodynamic drying properties; to determine sorption isotherms and to develop a drying model by resistance imposed by each individual constituent of paddy rice. The grains were mechanically harvested, manually cleaned with the aid of sieves and selected, also manually, only healthy and ripe grains. The processing of rice-benefiting samples was carried out to obtain brown rice, polished rice, rice bran and rice-husk. By peeling the equipment, samples of brown rice and rice husk were obtained. Through peeling followed by polishing, samples of polished rice and rice bran were obtained. Paddy rice, brown rice, polished rice, rice bran and rice husk were submitted to drying in a flat-bed dryer at temperatures of 35±1oC, 45±1oC, 55±1oC, 65±1oC and 75±1oC with air flow of 10m3 .min.m-2 . Mathematical models of drying kinetics were adjusted to experimental data and effective diffusivity, activation energy, and the thermodynamic properties of enthalpy, entropy, and Gibbs free energy were calculated. To obtain the sorption isotherms of paddy rice and their constituent parts, the static method was used at temperatures of 20, 30, 55 and 70°C for water activities ranging from 0.1075 to 0.8511. The mathematical models frequently used to represent the hygroscopicity of agricultural products were adjusted to experimental data. The Midilli model was the one that best adjusted to the drying kinetic curves of paddy rice at all temperatures; for brown rice at 45oC; for polished rice at temperatures of 35, 45 and 65oC; for rice bran at temperatures of 45, 55 and 65oC; rice husk at 55oC. The Diffusion Approximation model was adapted to the experimental data of the drying kinetics of brown rice at 35, 55, 65 and 75oC; polished rice at 55 and 75oC; 35 and 75oC rice bran and rice husk at 35, 45, 65 and 75oC. The effective diffusion coefficients of all treatments increased with temperature increase and their dependence on temperature was described by the Arrhenius Equation. The values of enthalpy and entropy reduced with the rise of the drying temperature, The Copace model is the one that best represents the sorption isotherms of paddy rice, polished rice, rice bran and rice husk, and the Sigma Copace model for brown rice, in the temperature ranges and water activity studied. The resistance to dehydration increased with the rise of the drying temperature, for husked rice and their constituent parts. Each part of the paddy rice prints a different resistance, in which the rice bran has higher resistance, followed by the rice husk and polished rice, respectively.