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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  036044,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.15050) @2003
Authors:   Qingling Zhang, Wade Yang, Canchun Jia
Keywords:   Rice, drying, tempering, glass transition temperature, head rice yield, fissure, internal crack, internal stress work, viscoelasticity

Tempering has been shown in literature to preserve head rice yield after heated air drying. Most reported tempering work was done adiabatically at a temperature below that for rice drying. In this study, the effect of a tempering temperature above that for rice drying on the whole kernel percentage was investigated. High temperature tempering was shown to be an effective way to preserve the whole kernel percentage for rice dried at a raised temperature (e.g., 60oC) at which head rice yield would otherwise incur a pronounced reduction without tempering. Tempering helped relax the strains inside a rice kernel induced by internal stresses developed during the drying process. The strains had two components (elastic component and viscous component) due to the viscoelasticity of rice kernels. The reduction of moisture content gradients inside a rice kernel during tempering helped eliminate the elastic component of the strains due to the elasticity of the rice kernel. Results showed that to effectively eliminate the viscous component of the strains due to the viscosity of the rice kernel, tempering temperatures must be kept well above the glass transition temperature of the rice kernel. A tempering temperature below the glass transition temperature failed to preserve the whole kernel percentage. For example, with a tempering temperature of 80C and a tempering duration of 80 min, the whole kernel percentage for the rice with an initial moisture content of 20.4% w.b. dried at 60oC and 17% relative humidity for 120 min down to 10.2% w.b. (i.e., 10.2 percentage points of moisture content removal in one drying pass) was preserved to a level close to that of the control sample.

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