Drying kinetics of apple cylinders under combined hot air-microwave dehydration

Apple cylinders (Granny Smith) were dried in a combined hot air-microwave system. Drying experiments were carried out at various air temperatures (25, 30, 40 and 50 degreesC) combined with different levels of microwave incident power (0, 3, 5, 7 and 10 W/g) until 0.11 (d.b.) moisture content was obtained. Vacuum impregnation with an isotonic solution was used as a pretreatment before drying. Sample weight changes were determined for different drying conditions. When drying with combined air-microwave system, the drying rate curve could be divided in four periods limited by four critical points. An empirical model was proposed to estimate the drying kinetic constants as a function of the air temperature and the microwave power level for both sorts of samples, fresh apples and impregnated apples. At the end of the process different tissue matrices were obtained depending on the different drying conditions. (C) 2003 Elsevier Ltd. All rights reserved.