The causes and characteristics of pre-harvest stress-cracking ofrice kernels are reviewed. A mathematical formulation of the three physical phenomena involved (heat transfer, moisture transport and internal expansion/contraction ofthe grain) in that process is developed. Values and regression models for the relevant geometrical and physical properties ofrice kernels were obtained from the literature. Two finite element models are derived from the mathematical analysis. The first model solves the coupled diffusion ofheat and moisture within the kernel while the second simulates the internal expansion and contraction of the grain and evaluates the magnitude of the local strains and stresses. The model for the coupled diffusion incorporates boundary conditions that account for radiation, convection, diffusion and condensation/evaporation of dew at the kernel surface. The kernel is approximated as an axisymmetric solid (ellipsoid) and linear triangular elements with two degrees of freedom per node are used to discretize it. A failure criterion based on the local strain energy density ofdistortion determines when and where cracking is initiated and how it propagates.

Five simulation runs were conducted using weather data recorded in a rice field north of Davis, CA during the fall of 1987. Results showed that the coupled diffusion model correctly predicted the evolution ofthe kernel moisture content, including the daily cycles of diurnal drying and nocturnal rewetting. Failure ofthe endosperm was observed only after adjusting the liquid diffusivity ofthe hull and bran layers so that rewetting ofthe endosperm could occur. Cracking is initiated when the kernel is under tensile stress during moisture absorption phases. The simulation results showed that endosperm cracking first occurs at the center ofthe grain and can then progress both in the radial and longitudinal directions.