Production and antioxidant activity of phenolic compounds from indigo plant waste using pressurized microwave-assisted hydrothermal treatment followed by water extraction

Tadeai (Awa-ai, Persicaria tinctoria) is an indigo plant cultivated 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, 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 extract from Tadeai. To obtain a water extract having a high antioxidant activity efficiently, a pressurized microwave-assisted hydrothermal treatment followed by water extraction was used and examined. The amount of phenolic compounds contained in water extract from Tadeai and its antioxidant activity were evaluated by the Folin-Ciocalteu method and the DPPH radical scavenging activity test, respectively. The effective condition was a processing temperature of 200 degrees C for a processing time of 2 min, and the highest amount of phenolic compounds contained in water extract was obtained when using a root as a sample. Furthermore, the differences among Tadeai varieties were also examined. The best result of the antioxidant activities was from the root of the Kojoko Akahana. Its amount of phenolic compounds and EC50 value were 44.1 mg-catechin equiv./g-dry vegetable sample and 0.018 g/L, respectively. In this study, it was shown that the antioxidant activity of water extract from 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 by using the pressurized microwave-assisted hydrothermal treatment followed by water extraction in the future.

Automated summary

This study found that parts of the Tadeai plant, such as the roots, have high antioxidant activity. A pressurized microwave-assisted hydrothermal treatment followed by water extraction was used to efficiently obtain a water extract with a high amount of phenolic compounds. The antioxidant activity of the water extract was evaluated and showed promising results, suggesting the potential use of Tadeai waste as a raw material for producing antioxidants.