Kinetics of antioxidant action of α- and γ-tocopherols in sunflower and soybean triacylglycerols

A kinetic analysis was performed to evaluate the antioxidant behavior of α- and γ-tocopherols (5-2000 ppm) in purified triacylglycerols obtained from sunflower oil (TGSO) and soybean oil (TGSBO) at 100°C. Different kinetic parameters were determined, viz. the stabilization factor as a measure of effectiveness, the oxidation rate ratio as a measure of strength, and the antioxidant activity which combines the other two parameters. In the low concentration range (up to 400 ppm in TGSBO and up to 700 ppm in TGSO) α-tocopherol was a more active antioxidant than γ-tocopherol whereas the latter was more active at higher concentrations. It has been found that the different activity of the tocopherols is not due to their participation in chain initiation reactions, but that the loss of antioxidant activity at high tocopherol concentrations is due to their consumption in side reactions. The rates of these reactions are higher in TGSBO than in TGSO. Both α-tocopherol itself and its radicals participated more readily in side reactions than γ-tocopherol and its radicals. Both α- and γ-tocopherol reduce lipid hydroperoxides, thus generating alkoxyl radicals which are able to amplify the rate of lipid oxidation by participating in chain propagation reactions.