Infrared drying of apple slices

Abstract

Laboratory dryer was designed in such a way that drying could be done either with infrared energy or by convection. It was equipped with near-infrared radiators with peak wavelength at 1200 nm. The energy efficiency of the infrared dryer was between 35% and 45%. Apple slices were dried with infrared energy and by convection under equivalent conditions. Kinetics of infrared drying was dependent on the distance between emitters and the heat-irradiated surface and air velocity. Drying kinetics was inversely proportional to both the distance and the air velocity. It was found that both surfaces of apple slice participate in water evaporation. However, the heat-irradiated surface evaporates much more water than that not heated by infrared energy until 80% of water is removed from the material. At the final stages of drying, there is no difference between upper and bottom surfaces of the apple slice as far as the flux of evaporated water is concerned. Comparison of infrared drying with convective drying done at equivalent parameters showed that time of the process can be shortened by up to 50% when heating is done with infrared energy.