Effect of microwave power, air velocity and temperature on the final drying of osmotically dehydrated bananas

This paper reports on a study of the final stage of microwave-hot air drying of osmotically dehydrated bananas, focusing on the effects of microwave power, air temperature and air velocity on drying kinetics and product quality, evaluated in terms of colour, apparent volume and porosity. The drying process was divided into three periods: phase I (760 W; 2 kgmoisture/kgdry matter); phase II (380 W; 0.67 kgmoisture/kgdry matter); and phase III (0 W, 76 W, 150 W or 230 W up to the final sample moisture of 0.17 kgwater/kgd m). Three conditions for the hot air were tested: 50 °C and 3.3 m/s; 70 °C and 3.3 m/s; 70 °C and 5.7 m/s. The results show that increasing the microwave power in phase III increased the drying rate, thus making the drying time shorter. However, higher microwave power also caused temperature runaway leading to charring on the dried product. Air flow cools the product surface and improves product quality by reducing charring.