Antioxidant activities of volatile components isolated from Eucalyptus species

Extracts of three species of eucalyptus leaves (Eucalyptus polyanthemos Schauer, E globulus Labill and E perriniana) and their major volatile components were assessed for antioxidant activity in two different assays. The inhibitory effect of each extract and its components towards aldehyde/carboxylic acid conversion was measured over a period of 30 days. Their inhibitory effects towards malonaldehyde formation from lipid by oxidation with Fenton's reagent were also measured; the extract from E polyanthemos inhibited the oxidation of hexanal completely for 30 days at a level of 200 and 500 μg ml^-1. It also inhibited malonaldehyde (MA) formation in cod liver oil by 86% at the 160 μg ml^-1 level. At the 500 μg ml^-1 level, thymol, 1,8-cineole, benzyl alcohol and terpinen-4-ol identified in the extract from E polyanthemos reduced the extent of hexanal oxidation over a period of 30 days by 100, 96, 82 and 75% respectively. In the lipid/MA assay, thymol, benzyl alcohol, terpinen-4-ol and 1,8-cineole inhibited MA formation by 80, 63, 58 and 26% respectively at the level of 160 μg ml^-1. Thymol exhibited potent antioxidant activity comparable to that of the known natural antioxidant α-tocopherol.