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Glassy State Transition and Rice Drying: Development of a Brown Rice State Diagram¹

¹Published with the approval of the director, University of Arkansas Agricultural Experimental Station. ²Former research assistant; professor and head, Food Science Department; and associate professor, respectively, Agricultural Statistics Laboratory, University of Arkansas, Fayetteville, AR. ³Corresponding author. E-mail: tsiebenm@comp.uark.edu

The effect of moisture content (MC) on the glass transition temperature (T_g) 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 T_g: 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 (β) of the kernel during glass transition. TMA-measured T_g at similar MC levels for both cultivars were not significantly different and were combined to generate a brown rice state diagram. Individual kernel T_g for both cultivars increased from 22 to 58°C as MC decreased from 27 to 3% wb. Linear and sigmoid models were derived to relate T_g to MC. The linear model was sufficient to describe the property changes in the MC range encountered during rice drying. Mean β values across both cultivars in the rubbery state was 4.62 × 10^-4/°C and was higher than the mean β value of 0.87 × 10^-4/°C in the glassy state. A hypothetical rice drying process was mapped onto the combined state diagram generated for Bengal and Cypress.