Antioxidant and antityrosinase activities of rhizome extracts from six Zingiber species in Thailand

Zingiber comprises of 120–140 species are distributed in tropical and subtropical Asia and is one of the most widespread genera of the Zingiberaceae. Thailand harbors about 57 species of the genus. Some species, such as Zingiber officinale and Z. montanum, are widely used throughout the world in food, medicine and cosmetics. However, there are few scientific references on the ethnobotany and pharmacology of Zingiber species in Thailand. In this study, the antioxidant properties of rhizome extracts, total phenolic content and antityrosinase activities from six Zingiber species (Z. bradleyanum, Z. densissimum, Z. larsenii, Z. monglaense, Z. sirindhorniae and Z. smilesianum) were investigated. The results showed that ethyl acetate extracts of all samples and ethanol extract of Z. sirindhorniae exhibited strong antioxidant activities (assessed by DPPH radical scavenging assay) with the IC₅₀ values in the ranged of 0.11–0.77 mg/mL. These extracts also showed quantitative antioxidant activities according to the ABTS^.+ radical scavenging assay (68.66–251.37 mg TEAC/g extract) and the total phenolic content (5.89–81.53 mgGAE/g extract). The tyrosinase inhibitory activity of hexane extracts of Z. densissimum, Z. sirindhorniae and Z. smilesianum were 0.45, 1.18 and 4.31 mg/mL, respectively. The results indicated that these six Zingiber species have potential to be a good source of native antioxidants and antityrosinase agents that could be further applied in the pharmaceutical and cosmetic industries.