Microwave assisted convective drying of bitter gourd: drying kinetics and effect on ascorbic acid, total phenolics and antioxidant activity

Abstract

Bitter gourds were dried by convective drying (CD) at 40, 50 and 60 °C and by microwave assisted convective drying (MACD) at 40, 50 and 60 °C combined with microwave power at 320, 400, and 480 W. In MACD, temperature and microwave power were applied synchronously. Quality of dried products in terms of ascorbic acid content, total phenolic content, total flavonoid content, vitamin A, DPPH radical scavenging activity, rehydration ratio, and total color change were compared. The drying rate increased with an increase in air temperature and microwave power. The MACD technique shortened the drying time compared to the CD. Prolonged drying time caused more deterioration of ascorbic acid and antioxidant activity than higher temperatures. In MACD, the total phenolic content and total flavonoid content did not degrade at elevated temperatures due to faster and homogenous drying. Moreover, the application of microwave power resulted in less reduction of Vitamin A and better retention of the surface color of the bitter gourd. Considering the drying time and quality attributes, microwave assisted convective drying would be a very promising alternative drying technique for bitter gourd.