Amorphous state transition in rice during the drying process

Polymer science was used to formulate a hypothesis on the mechanism of kernel fissuring during drying. The effect of the moisture content (MC) gradient on the glass transition temperature, Tg, of individual brown rice kernels of Bengal, a medium-grain cultivar, and Cypress, a long-grain cultivar, was studied. Three methods were investigated for measuring Tg, differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA). Among these methods, TMA was chosen because it can also measure changes in the thermal volumetric coefficient, beta, of the kernel during glass transition. Individual kernel Tg for both cultivars increased from 22 to 58°C as MC decreased from 27 to 3% wb. The existence of an MC gradient within a kernel can result in regions with varying Tg. The MC gradient formed was demonstrated by measuring the apparent water activity of brown rice kernels at different drying stages. Gradients formed at drying conditions corresponding to 6% equilibrium moisture content (EMC) were greater compared to 9% EMC. At low EMC, the surface had lower MC, hence higher Tg, than the interior. The state of these regions depended on their temperatures. Kernel surface temperatures, monitored with thermocouples, were higher than the interior during the first three minutes of drying. The surface cooled down faster than the interior after drying. During these times, expansion between the surface and the interior might be different. Mean beta across both cultivars, when in the rubbery state was 4.62 x 10-4 °C-1 and was higher than the mean beta when in the glassy state, 0.87 x 10-4 °C-1. At high MC gradients, the difference in betas between the rubbery and glassy state could generate sufficient stress to cause the kernel to fissure. Drying conditions that generated greater MC gradients resulted in higher percentages of fissured kernels. When averaged across all harvest and drying conditions, Bengal had higher mean fissured kernels than Cypress, 9.5% compared to 2.8%. Bengal could experience greater stress, hence fissure more, because it has higher beta than Cypress when in the rubbery state. Its higher mean kernel width and thickness could also generate a greater MC gradient.