Effect of microwave pretreatment on some operating conditions for drying banana slices using infrared/convection dryer

Abstract

The objective of this study is it investigates the effect of the microwave pretreatment on drying of banana slices by combined infrared / convection dryer under different power levels and different thickness of banana samples. Three levels of infrared power, namely: 150, 200 and 250 W and three thickness of sample are 2, 4 and 6 mm with air temperature 40 o C and air velocity 1.0 m/s were studied. Drying curves as affected by infrared power, thickness of samples and the microwave pretreatment, experimental data were fitted to some drying models. Specific energy consumption, dryer thermal efficiency, shrinkage of sample thickness and rehydration ratio were also studied. It is clear that the lower thickness sample and the higher infrared power the lower moisture ratio and total drying time at all experiments without microwave and with microwave and it was found that the least drying time was 30 min. at infrared power 250W and sample thickness 2mm with microwave pretreatment. The minimum value of specific energy consumption 1.24 (kw.h/kgwater removed) was recorded at infrared power 250 (W) and thickness of sample 2 (mm) for with assisted the microwave. The highest value of thermal efficiency 56.50 (%) was recorded at infrared power 250 (W) and thickness of sample 2 (mm) for with microwave assisted treatment.