MODELING THIN-LAYER MICROWAVE DRYING OF SOYBEANS

The drying behaviour of thin layers of Natto soybeans exposed to diffe rent levels of microwave power at 2450 MHz was investigated using a si ngle-mode (TE10) microwave apparatus designed and constructed to monit or instantaneous power absorption and mass loss during the drying proc ess. Samples with initial moisture contents (IMC) from 24.6 to 15.2% ( dry basis) were each dried at absorbed powers of 0.76, 0.57, and 0.36 W/g. The drying rate of Natto soybeans decreased with decreasing absor bed power. For the same absorbed microwave power, the drying rate was the same for IMC of 22% and 24.6% but decreased with IMC below 22%. So ybean was found to be a hydro-diffusionally simple material. The time- moisture shift technique was therefore used to develop drying master c urves for the power levels investigated. A two-term semitheoretical se ries solution of Fick's equation described the drying behaviour over t he entire drying period. Overall drying equations which predict the dr ying behaviour as a function of IMC, time, and absorbed power are pres ented.