A New Model and Quality of Unfrozen and Frozen Cooked Rice Dried in a Microwave Vibro-Fluidized Bed Dryer

Abstract

This study aimed to develop a suitable drying model for microwave vibro-fluidized bed drying in a single-mode applicator (MVFB-SMA drying) of cooked rice with and without prefreezing treatment and to investigate the effects of prefreezing treatment and drying temperature (110–185°C) on quality of dried cooked rice. During the process of drying cooked rice from 60 to 10% (wet basis), results indicated that drying rate increased, whereas drying time decreased with prefreezing treatment and increased drying temperature. The drying rate and drying time of unfrozen and frozen cooked rice ranged from 0.196 to 0.497 g water/g dry matter/min and 0.228 to 0.554 g water/g dry matter/min; and from 7 to 2.5 min and 5.5 to 2 min, respectively. A new model was proposed in this study (MR = exp(−k t ⁿ ) + bt + c) to compare with 11 commonly used drying models. The new model describes the MVFB-SMA drying data most satisfactorily. The values of effective diffusivity were between 1.70 × 10⁻⁷ and 5.72 × 10⁻⁷ m²/s for the unfrozen sample and between 1.99 × 10⁻⁷ and 5.86 × 10⁻⁷ m²/s for the frozen sample. Their activation energy values were 23.66 and 21.19 kJ/mol, respectively. Prefreezing treatment provided a whiter product with a less uniform porous structure and higher bulk density. Slower ability to rehydrate was also observed for the frozen cooked rice dried at 160 and 185°C. An increase in drying temperature resulted in changes in whiteness, microstructure, bulk density, and rehydration capability. No prefreezing treatment and drying at 160°C seemed to be the optimal process condition for cooked rice, ensuring whiteness, a porous structure, low bulk density, and high rehydration capability.

Keywords:

• Cooked rice drying
• Drying kinetics
• Microwave drying
• Prefreezing treatment
• Single-mode applicator
• Vibro-fluidized bed drying

ACKNOWLEDGMENT

The authors are pleased to acknowledge the Royal Thai Government and Ampol Food Processing Ltd. for supporting this work under the Royal Thai Government Joint Industrial Research Program, Fiscal Year 2008.