Effects and prooxidant mechanisms of oxidized alpha-tocopherol on the oxidative stability of soybean oil.

Alpha-tocopherol was oxidized in methanol containing methylene blue for 30 h under light. The effects of 0, 250, 500, 1000, and 1500 ppm of oxidized alpha-tocopherol on the oxidative stability of purified soybean oil in the dark at 55 degrees C were studied by measuring the peroxide values and headspace oxygen contents in sample bottles. As the concentrations of oxidized alpha-tocopherol increased, the peroxide values increased and the headspace oxygen contents decreased during the 6 d of storage. The oxidized alpha-tocopherol compounds acted as prooxidant on the peroxide values and headspace oxygen contents of purified soybean oil. Tukey's test showed that oxidized alpha-tocopherol had a significant effect on the peroxide value and headspace oxygen disappearance of oil at P < 0.05. The prooxidant mechanisms of oxidized alpha-tocopherol may be due to alpha-tocopherol peroxy radical, alpha-tocopherol oxy radical, hydroxy radical, and singlet oxygen formed from the alpha-tocopherol. The oxidized alpha-tocopherol containing polar and nonpolar groups in the same molecule may reduce the surface tension of oil to increase the transfer of headspace oxygen to oil and accelerate the oil oxidation.