Mechanical properties of sound and fissured rice kernels and their implications for rice breakage

Mechanical properties of brown rice kernels that were thin-layer dried at 60 °C and 17% relative humidity for 0, 10, 30, 50, 70, 90 and 120 min were measured by three-point bending tests after rice kernels were cooled to room temperature. The apparent modulus of elasticity, bending strength, and fracture energy of the sound brown rice kernels increased with longer drying, while the same properties of the fissured kernels were smaller than those of the sound kernels, especially for the bending strength and fracture energy. This explains why fissured kernels are much easier to break during milling. The finding that the bending strength and the fracture energy of the fissured brown rice kernels decreased with longer drying indicated that fissures were either created or propagated during the drying process, which weakened the fissured kernels as drying proceeded. Results indicated that mechanical properties measured on the sound kernels at different drying durations would not be affected by the loss of the sound kernels to fissured or broken kernels.