Antioxidant activity of α- and γ-tocopherols in bulk oils and in oil-in-water emulsions

This study was aimed at evaluating the effectiveness of α- and γ-tocopherols in inhibiting the formation and decomposition of hydroperoxides in bulk corn oil stripped of natural tocopherols and in 10% oil-in-water emulsion systems. Whether α-tocopherol acted as an antioxidant or as a prooxidant depended on the test system, the concentration, the oxidation time, and the method used to determine lipid oxidation. On the basis of hydroperoxide formation, α-tocopherol showed maximum antioxidant activity at 100 ppm in bulk oil and at 250-500 ppm in emulsions, while γ-tocopherol showed maximum activity at 250-500 ppm in bulk oil but showed no significant difference in antioxidant activity between 250 and 1000 ppm in emulsions. α-Tocopherol had a slight, initial prooxidant effect at 250 ppm or higher concentrations in bulk oil and at 500 ppm or higher concentrations in emulsions, whereas γ-tocopherol showed no prooxidant activity in either system. However, γ-tocopherol showed less antioxidant activity than α-tocopherol at 100 ppm but higher antioxidant activity at higher concentrations in both systems. In contrast to hydroperoxide formation, the ability of both α- and γ-tocopherols in inhibiting hexanal formation improved with increased concentration and oxidation time.