Vol.:(0123456789) 1 3 Journal of Food Measurement and Characterization https://doi.org/10.1007/s11694-019-00168-7 ORIGINAL PAPER Microwave assisted convective drying of bitter gourd: drying kinetics and efect on ascorbic acid, total phenolics and antioxidant activity Insha Zahoor 1  · Mohammad Ali Khan 1 Received: 25 January 2019 / Accepted: 21 May 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 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 favonoid 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 activ- ity than higher temperatures. In MACD, the total phenolic content and total favonoid 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. Keywords Bitter gourd · Ascorbic acid · Total phenolic content · Antioxidant activity · Drying kinetics Introduction Bitter gourd (Momordica charantia L.) which belongs to the family Cucurbitaceous is widely grown in India, China, Malaysia, Taiwan, Indonesia, Vietnam and Africa [1]. Bitter gourd has received growing interest because of the presence of important antioxidant compounds such as phenolic acids, flavonoids, ascorbic acid and carotenoids which impart various health benefts [2]. In terms of food application, fresh bitter gourd can be traditionally used as a vegetable for direct cooking. The dried bitter gourd chips, dried bit- ter gourd powder and bitter gourd pickles are some of the value-added products obtained from the bitter gourd [3]. Like other perishable crops, fresh bitter gourd also deterio- rates rapidly after harvest due to its excess moisture content [4]. Therefore, to widen its shelf life and to expand the pos- sibility of ofseason availability of bitter gourd, immediate preservation becomes a vital requirement. Drying is recognized as one of the ancient food process- ing techniques, and today it is broadly used in food indus- tries for fruit and vegetable preservation. The basic concept involved in drying is the lowering of moisture content up to a defnite threshold value that prevents deterioration and subsequent decay within a defnite period. The increasing reluctance of consumers to the foods, preserved by the use of chemicals and the growing demand of high quality fast- dried food products has led to the fast expansion of the mar- ket for the additive-free dried alternatives [5]. Hot air drying is a common food preservation method that results in dried foodstufs that have an enlarged shelf life. However, it sufers from a major drawback of production of low quality food due to prolonged drying time and elevated temperatures used [6]. Other methods such as microwave drying, a relatively new form of drying, could be considered faster and more efcient than conventional air drying. How- ever, microwave drying application alone results in some serious drawbacks such as overheating and charring of mate- rial because of non-uniform heating, improper microwave energy conversion to the heat at reduced amount of water and restricted microwave entry through the food product [7]. Moreover, because of the costly afairs of microwave dry- ing of food products, it cannot compete with hot air drying. * Insha Zahoor insha.zahor@gmail.com 1 Department of Post-Harvest Engineering & Technology, Faculty of Agricultural Sciences, Aligarh Muslim University, Aligarh 202002, India