The Glass Transition Temperature Concept in Rice Drying and Tempering: Effect on Drying Rate

Research on rice drying and tempering has shown that high drying temperatures (up to 603C) and high moisture removal rates (up to 6 percentage points moisture content) can be used without reducing milling quality as long as sufficient tempering at a temperature above the glass transition temperature (Tg) is allowed between drying passes. Using drying air temperatures above the Tg of the rice significantly reduces drying and tempering durations since kernel moisture diffusivity is much higher above Tg. Understanding the effects of glass transition is important in optimizing the drying and tempering processes in terms of overall required drying durations to achieve given moisture removals. The objective of this study was to investigate the effect of Tg on drying rates when using drying air temperatures above and below Tg. Both medium–grain and long–grain rice was harvested during 2000 and dried under various air conditions above and below the Tg of the rice. Results showed that rice dried significantly faster above Tg than below Tg. In addition, high temperature/low relative humidity drying air conditions, which result in a low equilibrium moisture content, apparently caused the surface of the kernel to transition from a rubbery to a glassy state and reduced the drying rate.