Modelling some engineering properties of walnut kernel undergoing different drying methods with microwave pre-treatment

This work aimed in modelling some physical and thermal properties of walnut kernel undergoing various drying techniques include microwave semi-industrial continuous drying (M-CD), microwave infrared-vacuum drying (M-IVD), and microwave infrared-fluidised bed drying methods. By using the verifications of experimental parameters, ten drying models for describing combined effects of independent variables were derived and the parameters of these models were determined by multiple regression analysis. In the case of the model, comprised of collective effects of drying independents, the Midilli model gave the best result in describing drying behaviour of walnut kernel in all experimental domains. Minimum values of shrinkage (6.53%) and colour change (5.54%) were obtained in the M-CD with the microwave power of 270 W, air temperature of 45 degrees C, and belt linear speed of 10.5 mm/s, however maximum shrinkage (14.65%) and colour change (18.28%) were related to the M-IVD with microwave power of 630 W, drying temperature of 75 degrees C, and absolute pressure of 60 kPa. The lowest and highest values of effective moisture diffusivity (1.04x10(-8) and 9.74x10(-8) m(2)/s, respectively) belonged to the M-CD. Based on the quality indices including shrinkage and total colour difference M-CD was found an adequate arrangement among applied techniques in walnut kernel drying, hence it produced the most similar dried samples to fresh ones.