Customized drying treatments increased the extraction of phytochemicals and antioxidant activity from economically viable medicinal plants

Owing to the complexity of phytochemicals in different medicinal plant species, further optimisation study is needed to identify the ideal drying technique that maximize the extraction of plant phytochemicals with high commercial values. This study evaluated the effect of hot air drying, microwave drying, microwave-assisted hot air hybrid drying, and freeze drying on the extraction of phytochemicals and antioxidant activities from Aloe vera (L.) Burman rinds, Centella asiatica (L.) Urban stem and leaves, Cymbopogon citratus (DC.) Stapf stalk, and Psophocarpus tetragonolobus (L.) DC. pods. The moisture content and browning index of plant samples were compared among the different drying treatments. Seven in vitro mechanism based colorimetric assays were utilised to compare the total tannin, flavonoids, phenolic, ascorbic acid, ferric reducing antioxidant power, and radical scavenging potential between the fresh and dried plant extracts. Microwave drying utilised the shortest drying duration, followed by microwave-assisted hot air hybrid drying, hot air drying, and freeze drying. The browning index of medicinal plants was greatly dependant on drying methods, duration, and plant textures. All drying treatments significantly (p < 0.05) increased the extraction of antioxidant activity, phenolic, and ascorbic acid from all plants except A. vera. Since phenolic and ascorbic acid were positively correlated with reducing power (r = 0.537 to 0.680) and inversely correlated with the IC50 of radical scavenging activity (r = -0.507 to -0.666), both phytochemicals were considered good reducing agents and radical scavengers. Principle component analysis revealed that plant species and drying methods induced 68 % and 10 % of activity variation, respectively. This study recommended microwave drying and freeze drying for C. asiatica and A. vera, respectively while microwave-assisted hot air hybrid drying was preferred for both C. citratus and P. tetragonolobus. The proposed antioxidant index could act as a novel indicator to identify the ideal drying method for different medicinal plants.