Preservation of Head Rice Yield Under High‐Temperature Tempering as Explained by the Glass Transition of Rice Kernels

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 is an effective way to preserve the whole kernel percentage for rice dried at a raised temperature (e.g., 60°C) 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 80°C and a tempering duration of 80 min, the whole kernel percentage for the rice with an initial moisture content of 20.4% wb dried at 60°C and 17% rh for 120 min down to 10.2% wb (10.2 percentage points of moisture content removal in one drying pass) was preserved to a level close to that of the control sample.