The Mechanism of Bursting of Chalky Rice Kernels on Cooking

Bursting rice is a concern for consumer acceptance, but the mechanisms are still relatively less explored. In this work, the factors that contribute to rice bursting are determined by comparing healthy (translucent) and full chalky rice grains through a single-grain experimental approach, which prevents the averaging-out effect in bulk samples. Time-series imaging is employed to monitor the longitudinal expansion and transverse cracks of rice grains during soaking, as well as the bursting of rice during cooking. Scanning electron microscopy is employed to analyze the internal cracks of rice grains prior to soaking. Microstructure analysis shows that 85% of full chalky rice grains have internal cracks larger than 3.2 µm, which are significantly larger than those of healthy rice grains. These cracks are found to cause biphasic expansion during soaking due to the rapid entry of water and reduced mechanical strength, resulting in a cooking expansion rate that is $2.7\times$ higher. Kinetic instabilities cause rice bursting during cooking in 67% of full chalky rice grains compared to 13% of healthy rice grains, which predominantly occur on the ventral side, which is a natural developmental seam formed during centripetal cellularization, becoming mechanically weak due to their porous and crack-rich microstructure.