MICROWAVE DRYING OF GRAPES IN A SINGLE MODE CAVITY AT 2450 MHz - Ⅱ: QUALITY AND ENERGY ASPECTS

摘要

Drying studies were conducted to optimize process parameters (air temperature T, microwave power density P and air velocity u) for quality and energy in microwave drying of grapes using a single mode cavity applicator at 2450 MHz. The quality was assessed by several attributes like colour, damage, darkness, crystallized sugar, stickiness and non-uniformity. Colour and damage attributes provided more precise predictability when compared to the other quality attributes. Microwave dried raisins were lighter in colour and hence were superior to hot air dried samples. Quality attributes were significantly dependent on T, P and u. Increase in air velocity resulted in better quality raisins whereas P and T had the opposite effect. Optimum selection of T, P and u was found to be critical to achieve an energy efficient process for a quality product. Response surface models in terms of temperature and microwave power were obtained for each of the quality attributes. Process optimization of the operating variables through response surface method was performed by imposing certain constraint levels on quality attributes and contours were developed. With the help of these contours it becomes possible to select suitable levels of operating parameters to achieve a desired quality in the product. The specific energy consumption of microwave drying was compared with that of convective drying. Convective drying was found to be highly energy intensive because of longer drying times as against shorter drying times and therefore lower specific energy consumption achieved in microwave drying conditions.