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.

[1]  Terry J. Siebenmorgen,et al.  Rice fissuring response to high drying and tempering temperatures , 2003 .

[2]  Terry J. Siebenmorgen,et al.  The glass transition temperature concept in rice drying and tempering : Effect on milling quality , 2000 .

[3]  R. Lu,et al.  MOISTURE DIFFUSIVITY OF LONG-GRAIN RICE COMPONENTS , 1992 .

[4]  L. Slade,et al.  A food polymer science approach to structure-property relationships in aqueous food systems: non-equilibrium behavior of carbohydrate-water systems. , 1991, Advances in experimental medicine and biology.

[5]  R. P. Singh,et al.  Theoretical and Practical Aspects of Rough Rice Tempering , 1980 .

[6]  F. W. Bakker-Arkema,et al.  Drying cereal grains , 1974 .

[7]  D. Jayas,et al.  A METHOD OF DETERMINATION OF THE DIFFUSION COEFFICIENT BASED ON KERNEL MOISTURE CONTENT AND ITS TEMPERATURE , 1992 .

[8]  T. Siebenmorgen,et al.  RELATING DRYING RATE CONSTANT TO HEAD RICE YIELD REDUCTION OF LONG-GRAIN RICE , 1997 .

[9]  R. Singh,et al.  Liquid Diffusivity of Rough Rice Components , 1980 .

[10]  A. Cihan,et al.  A liquid diffusion model for drying rough rice , 1993 .

[11]  Terry J. Siebenmorgen,et al.  Effects of Oven Drying Temperature and Drying Time on Rough Rice Moisture Content Determination , 1987 .

[12]  Terry J. Siebenmorgen,et al.  Thermomechanical Transitions of Rice Kernels , 2002 .

[13]  O. R. Kunze,et al.  Fissuring of the Rice Grain After Heated Air Drying , 1979 .

[14]  Terry J. Siebenmorgen,et al.  Glassy state transition and rice drying: development of a brown rice state diagram. , 2000 .

[15]  S. M. Hendorson.  Grain Drying Theory (I) Temperature Effect on Drying Coefficient , 1961 .