Microwave assisted air drying of osmotically treated pineapple with variable power programmes

Abstract

Variable power programmes for microwave assisted air drying of pineapple were studied. The pineapple pieces were pre-treated by osmotic dehydration in a 55º Brix sucrose solution at 40ºC for 90 minutes. Variable power output programmes were designed and run with different inlet air temperatures between 30 and 70 degrees C. Results indicated that the use of variable microwave power combined with low air temperatures can result in a fast drying process without significant charring of pineapple pieces. High microwave powers need to be reduced quickly, faster than the decrease in water content would suggest, to minimize charring. In this study inlet air temperatures of 70 degrees C were found to be excessive when combined with microwave energy (5W/g), resulting in fast temperature increase. Microwave power was found to be most effective in the first hour to 1.5 hr of processing, and afterwards should be reduced to 0.1 W/(g initial product weight) in the final stages of drying to avoid charring. The best microwave programme tested lead to 20% water content with just 1% losses due to charring, but the results allow to conclude that charring could be completely reduced by switching off microwave energy altogether after 1.5 hours and then finish off drying with higher air temperatures. The use of low air temperatures (30 – 50 degrees C) is advantageous with microwave energy in the first stages of drying as it limits the peaks of specific energy absorption, but it slows down drying towards the end probably because of a too low point of equilibrium (saturation humidity of air). Microwave energy did not significantly influence the drying process towards the end, although drying rates showed a “memory effect”, that is, drying rates in processes with the same conditions after a given time depended on the conditions up to that point.