Evaluation of Antioxidant Extraction by Microwave Treatment from Japanese Indigo Plant

Abstract

Tadeai (Awa-ai) is an indigo plant that is produced in large quantities in Tokushima prefecture, Japan. The leaves of Tadeai are used for dyeing (Ai-zome), but there is no use for the other parts, i.e. flowers, stems, leaves, and roots, most of which are discarded. In this study, we search for new ways to use Tadeai including parts other than the leaves and examined the antioxidant activity of water soluble extract from Tadeai. To extract antioxidants efficiently, processing was undertaken using a microwave irradiation device. The evaluation of Tadeai extract and its antioxidant activity was performed by measuring the amount of phenolic compounds by the Folin–Ciocalteu method and the DPPH radical scavenging activity test. Compared to the sample that was not subjected to microwave treatment, the amount of phenolic compounds was observed to be double, and the antioxidant activity eight times higher. The microwave treatment conditions that showed the best results are 200 ℃ and 1 min, and the evaluation indicated that the part that exhibits the best results is the root. Furthermore, the differences among Tadeai varieties were also examined. The best result was for the “Kojoko Akahana;” the root of the “Kojoko Akahana” exhibited the highest antioxidant effect. Its amount of phenolic compounds and EC50 value were 44.1 mg-catechin equiv./g-dry sample and 0.018 g/L, respectively. In this study, it was shown that the antioxidant activity of the parts other than the leaves is high, so it is expected that Tadeai waste will be effectively utilized as a raw material for producing antioxidants in the future.