Fissure characteristics related to moisture adsorption stresses in rice

Abstract

Rice kernels may develop fissures due to internal stresses when subjected to moisture adsorbing environments. Rice kernels were initially conditioned to three equilibrium relative humidities (ERH) of 46, 62, and 80%, and then were exposed to high relative humidity (RH) environments of 65, 86, and 100% for different periods. All experiments were conducted at 21$spcirc$C. Results showed that grains equilibrated to 46 and 62% ERH would fissure when exposed to either 86 or 100% RH. For rice grains at 46% ERH, the cumulative percentage of fissured kernels (CPFK) increased as the exposure humidity increased with all of the kernels fissuring at 100% exposure humidity. The CPFK decreased sharply as the initial moisture level increased with no kernels developing fissures when rice grains at 80% ERH were exposed to 86 and 100% RH. The cumulative number of fissures (CNF) in rice grains increased with exposure RH and exposure time. The retardation times were analyzed by using the experimental values and finite element techniques. The moisture adsorption rates (MAR) for different forms of rice and RH were determined. The experimental results were compared with the finite element analysis. The moisture gradients in a rice kernel during moisture adsorption were calculated. The causes and characteristics of rice fissures from moisture stresses were investigated. The finite element method was used to obtain numerical solutions to the moisture diffusion equation and stress-strain relationship equation. Therefore, two finite element models were derived from the mathematical analysis. The first model solved the moisture diffusion within the rice kernel while the second simulated the internal expansion and contraction of the kernel and evaluated the magnitude of the local strains and stresses. The three major stress components, namely axial, radial, and tangential stresses were studied in relation to failure tensile strength and local strain energy density of distortion by the finite element program. Axial stresses are the primary reason for fissures to develop in a rice kernel; tangential stresses may cause a rice grain to fissure parallel to the longitudinal axis; radial stresses do not cause a rice grain to fissure. The largest tensile stresses occur at the center of a kernel where the fissure usually starts.