Validating the Glass Transition Hypothesis in Explaining Fissure Formation in Rough Rice Kernels During the Drying Process

Abstract

HighlightsX-ray imaging allows visualization of the fissuring that occurs at various drying air conditions.Drying air conditions that create severe intra-kernel material state gradients during drying result in kernel fissuring.The glass transition hypothesis was validated for explaining the fissuring of rice kernels during drying.Abstract. Fissured rice kernels tend to break during milling, leading to milling yield reductions. A hypothesis involving changes in material state properties has been proposed to predict kernel fissuring during the drying process. The hypothesis, referred to as the glass transition hypothesis, has been used to explain kernel fissuring during the drying process and has been supported by various milling studies. However, this hypothesis has not been validated from a fundamental fissuring standpoint. In this study, experiments were performed using drying air temperatures of 45°C, 50°C, 55°C, 60°C, and 65°C with relative humidity values that produced equilibrium moisture contents (EMCs) of 6%, 8%, 10%, 12%, and 14%. These EMCs would position the kernel surface at select regions on a rice material state diagram during drying. At the end of active drying, the kernels were tempered for 2 h at the drying air temperature. Fissures were viewed and detected in these kernels using X-ray imaging. Drying air temperature and EMC combinations that caused sufficient portions of the kernel surface to transition to the glassy region while the core remained in the rubbery region caused severe intra-kernel material state gradients. Such intra-kernel material state gradients caused severe fissuring, thus supporting the glass transition hypothesis in explaining fissure formation. At drying air temperature and EMC combinations that did not cause severe intra-kernel material state gradients, severe fissuring was averted, thus further supporting the glass transition hypothesis. Keywords: Glass transition hypothesis, Material state, Rice quality, State diagram, Tempering, X-ray imaging.