Effects of drying conditions in low-temperature microwave-assisted drying on bioactive compounds and antioxidant activity of dehydrated bitter melon ( L.).

Bitter melon ( L.) is a fruit that brings health benefits to consumers because the fruit is rich in bioactive compounds. In this work, a combination of low-temperature convective drying and microwave radiation was used to dehydrate sliced bitter melon. One-factor-at-a-time design was performed to evaluate the influence of microwave power density (1.5, 3.0, 4.5 W/g), drying temperatures (20, 25, and 30°C), and air velocity (1.0, 1.2 and 1.4 m/s) on the change of moisture content, nutrient levels (vitamin C and total phenolics), and the antioxidant activities (DPPH and FRAP assays) of the bitter melon. The obtained results showed that all investigated factors affected the rate of moisture removal. Microwave power density output and air-drying temperature strongly participated in the retention of nutrients. In this study, the drying process was driven by both heat and mass transfer processes, so the increase of air velocity prolonged the drying time causing more loss of nutrient levels and antioxidant activities. It was found that DPPH free radical scavenging ability directly correlated with total phenolic content, but the ferric-reducing antioxidant power was related to the presence of reductants including phenolic compounds, vitamin C, and other phytochemicals in bitter melons. This work determined that microwave power density and the air-drying temperature are the main two factors that should be used for further investigations.