Microwave Drying of Moringa oleifera (Lam.) Leaves: Drying Characteristics and Quality Aspects

Moringa oleifera (L.) leaves contain significant amounts of bioactive compounds with high antioxidant activities. The aim of this study was to determine drying characteristics using microwave drying (MWD) at different MW powers from 150 to 900 W. Quality aspects in terms of total phenolic content (TPC), DPPH radical scavenging activity, color, rehydration ratio as well as HPLC measurement of quercetin and kaempferol. The drying data were fitted to four drying models. It was found that three parameter model gave the best fit. The drying constant was related to the MW power using an Arrhenius model. Effective moisture diffusivity (Deff) increased with MW power. The dominant antioxidants were measured in terms of TPC, DPPH radical scavenging activity, content of quercetin and kaempferol. All antioxidants were increased with increasing MW powers for both whole-leaf and half-leaf. The dried whole-leaf at 900W yielded the highest TPC, DPPH radical scavenging activity, content of quercetin and the lowest total color difference (E*). The dried whole-leaf using MWD at 900W were compared with the conventional drying (tray drying at 60C, 70 min), sun drying (SD) and infrared drying (IRD) in terms of TPC, DPPH radical scavenging activity, contents of quercetin and kaempferol. It was found that the dried whole-leaf using MWD at 900W had the highest TPC, DPPH radical scavenging activity and quercetin content and could increase the retention of TPC (43.28%), DPPH radical scavenging activity (25.64%) and quercetin (64.10%). Therefore, the commercial processing of M. oleifera leaves could be improved by using MWD, as the drying time was considerably reduced and the dried M. oleifera leaves had a higher TPC, DPPH radical scavenging activity and quercetin.