Antioxidant activities of α- and γ-tocopherols in the oxidation of rapeseed oil triacylglycerols

Antioxidant properties of 5 to 500 μg/g levels of α- and γ-tocopherols, in the oxidation of rapeseed oil triacylglycerols (RO TAG), were studied at 40 °C in the dark. Each tocopherol alone and in a mixture was studied for its stability in oxidizing RO TAG. Also the effects of tocopherols on the formation of primary and secondary oxidation products of RO TAG were investigated. Both tocopherols significantly retarded the oxidation of RO TAG. At low levels (≤50 μg/g), α-tocopherol was more stable and was a more effective antioxidant than γ-tocopherol. At higher α-tocopherol levels (>100 μg/g), there was a relative increase in hydroperoxide formation parallel to consumption of α-tocopherol, which was not found with γ-tocopherol. Therefore, γ-tocopherol was a more effective antioxidant than α-tocopherol at levels above 100 μg/g. As long as there were tocopherols present, the hydroperoxides were quite stable and no volatile aldehydes were formed. In a mixture, α-tocopherol protected γ-tocopherol from being oxidized at the addition levels of 5+5 and 10+10 μg/g but no synergism between the tocopherols was found. α-Tocopherol was less stable in the 500+500 μg/g mixture than when added alone to the RO TAG. No prooxidant activity of either tocopherol or their mixture was found.