EFFECTS OF INDIVIDUAL TOCOPHEROLS AND TOCOPHEROL MIXTURES ON THE OXIDATIVE STABILITY OF CORN-OIL TRIGLYCERIDES

This study was aimed at evaluating the effectiveness of individual tocopherols and their mixtures in inhibiting the formation and decomposition of hydroperoxides in bulk corn oil stripped of natural tocopherols. delta-Tocopherol showed an antioxidant activity in the inhibition of both formation and decomposition of hydroperoxides at 2000 ppm or below, and its effectiveness increased with concentration. gamma-Tocopherol promoted the formation of hydroperoxides but strongly inhibited their decomposition at 5000 ppm. On the basis of hydroperoxide formation, the concentrations for maximum antioxidant activity of alpha- and gamma-tocopherol mixtures (1:1) and natural soybean tocopherol mixtures (alpha:gamma:delta = 13:64:21) were 250 and 500 ppm, respectively. Prooxidant activity was also observed at a lower concentration with the alpha- + gamma-tocopherol mixture (500 ppm) than with the soybean tocopherol mixture (1000 ppm). However, the inhibition by tocopherol mixtures increased with oxidation time. In contrast to hydroperoxide formation, tocopherol mixtures inhibited hydroperoxide decomposition more effectively at higher concentrations than at lower concentrations. Whether tocopherol mixtures acted as antioxidants or prooxidants depended on the concentration of a-tocopherol in the mixtures. The optimum concentration of tocopherol mixtures to increase the oxidative stability of bulk oils is determined by inhibition of hydroperoxide formation but not by inhibition of hydroperoxide decomposition.