Effects of tocopherols and tocotrienols on the inhibition of autoxidation of conjugated linoleic acid

The effect of eight vitamin E homologues, i.e. α-, β-, γ-, and δ-tocopherol and α-, β-, γ, and δ-tocotrienol, on the inhibition of autoxidation of conjugated linoleic acid (CLA) were investigated. The oxidation was carried out in the dark for 21 days at 50 °C and monitored by peroxide values (PV) and TBA values. The levels of the individual vitamin E homologues in CLA during storage were determined by HPLC. γ-Tocopherol exhibited the highest antioxidant activity among the homologues tested in this study when the antioxidant activities of the individual homologues in CLA were compared by PV. The order of antioxidant activity of eight homologues was γ-tocopherol > δ-tocopherol = δ-tocotrienol ≥ δ-tocotrienol > β-tocopherol = β-tocotrienol > α-tocopherol = α-tocotrienol. The degradation rates of α-tocopherol and α-tocotrienol were faster than those of the other homologues, whereas δ-tocopherol had the highest stability in CLA during storage. All homologues exhibited an antioxidant activity by inhibiting the formation of secondary oxidation products. It appears that α-tocotrienol and β-tocotrienol have significantly higher antioxidant activities for secondary oxidation in CLA than α-tocopherol and β-tocopherol. Meanwhile, the other homologues, namely γ-tocopherol, γ-tocotrienol, δ-tocopherol, and d-tocotrienol, exhibited similar antioxidant activity for secondary oxidation in CLA.