Antioxidative activities of volatile extracts from green tea, oolong tea, and black tea.
Antioxidative activities of volatile extracts from six teas (one green tea, one oolong tea, one roasted green tea, and three black teas) were investigated using an aldehyde/carboxylic acid assay and a conjugated diene assay. The samples were tested at levels of 20, 50, 100, and 200 micrograms/mL of dichloromethane. The results obtained from the two assays were consistent. All extracts except roasted green tea exhibited dose-dependent inhibitory activity in the aldehyde/carboxylic acid assay. A volatile extract from green tea exhibited the most potent activity in both assays among the six extracts. It inhibited hexanal oxidation by almost 100% over 40 days at the level of 200 micrograms/mL. The extract from oolong tea inhibited hexanal oxidation by 50% in 15 days. In the case of the extract from roasted green tea, the lowest antioxidative activity was obtained at the level of 200 micrograms/mL, suggesting that the extract from roasted green tea contained some pro-oxidants. The extracts from the three black teas showed slight anti- or proactivities in both assays. The major volatile constituents of green tea and roasted green tea extracts, which exhibited significant antioxidative activities, were analyzed using gas chromatography-mass spectrometry. The major volatile chemicals with possible antioxidative activity identified were alkyl compounds with double bond(s), such as 3,7-dimethyl-1,6-octadien-3-ol (8.04 mg/kg), in the extract from green tea and heterocyclic compounds, such as furfural (7.67 mg/kg), in the extract from roasted green tea. Benzyl alcohol, which was proved to be an antioxidant, was identified both in a green tea extract (4.67 mg/kg) and in a roasted tea extract (1.35 mg/kg).