Microwave heating and mass transfer characteristics of white beans

The drying characteristics of white bean seeds exposed to constant microwave (2450 MHz) absorbed powers ranging from 0.28 to 0.44 W/g (dry basis) was investigated with a single-mode (TE(10)) microwave drying apparatus. The effect of absorbed power level on moisture and temperature history during drying are presented. Microwave drying of white beans was found to be a falling rate process. At constant absorbed power, seed temperature increased rapidly during the initial stages of drying to a maximum value and began to decrease gradually during the latter stages of drying. To maintain a constant drying temperature the absorbed power had to be increased progressively with further reduction of moisture content. The gradual decrease in seed temperature when drying rate was decreasing is opposite what is observed during hot air drying. It was considered as resulting from a progressively increasing heat of desorption with drying, a phenomenon common to hygroscopic solids. Thus, the microwave heating characteristics observed for white beans is likely to apply to similar hygroscopic solids. (C)1996 Silsoe Research Institute