Characteristics of Rice Dried in Superheated-Steam Fluidized-Bed

Abstract

Characteristic properties of rice in a superheated-steam fluidized-bed dryer have been investigated using RVA, DSC, and SEM. The experimental results have shown that the reduction in moisture content from 41–42.5 to 25% d.b. using steam temperatures of 150–170°C was initially linear with drying time followed by the exponential decay when the moisture content was below 25% d.b. During the latter period, the water transport was controlled by the internal diffusion. Condensation of steam, along with the use of high temperature, enabled the development of gel layer as obvious from SEM. The formed gel layer caused the very low effective diffusivity when compared with the case of no gel formation in paddy kernel. The pasting properties and DSC measurements have shown that paddy dried at a higher temperature and a thinner bed depth provided a greater extent of gelatinization during the drying time, the first factor showing a profound effect. Kinetics of gelatinization is reasonably described by the zero-order reaction. For the paddy quality, the head rice yield was 70% higher than the reference value and color of white rice became yellow and low luminous after reducing moisture content to 18% d.b. The percentage of white belly was less than 1.5%.

Keywords:

• Gelatinization
• Superheated steam
• White belly

Acknowledgments

The authors would like to express their sincere thanks to the Thailand Research Fund for financial support, Faculty of Engineering and Industrial Technology Silpakorn University for supporting RVA instrument, Division of Agricultural Engineering, Department of Agriculture for testing quality of milling, and Pathum Thani Rice Research Center for testing the physical quality of rice. Phillip Semiconductor (Thailand) for supporting SEM instrument.