Effects of α-, γ-, and δ-Tocopherols on the Autoxidation of Purified Rapeseed Oil Triacylglycerols in a System Containing Low Oxygen

Controversial data on the antioxidant effects of tocopherols have already been shown in different test systems, yet δ-tocopherol was hardly considered. This study was designed to assess the effects and degradation of α-, γ-, and δ-tocopherol in four concentrations from between 0.01 and 0.25% on the oxidation of purified rapeseed oil trigacylglycerols (RO-TAG) at 40°C in the dark in a low oxygen containing system for 11 weeks. Oxidation experiments were performed weekly by assessing primary (peroxide value, PV; conjugated dienes, CD) and secondary (p-anisidine reactive products, p-AV; hexanal) oxidation products, the degree of unsaturation with the iodine value (IV), and the stability of tocopherols. Test approaches were performed with and without the addition of 0.01% α,α′ -azoisobutyronitrile (AIBN), which is a known radical initiator. α- and γ-Tocopherols increased the rate of lipid oxidation, which was more pronounced in the presence of AIBN. Only the lowest amount of 0.01% γ-tocopherol was comparable to the control sample in the test without AIBN. The most effective was shown to be δ-tocopherol, which did not elevate lipid oxidation except the PV in the AIBN test, but they did not delay it either. δ-Tocopherol was the most stable followed by γ- and α-tocopherol. For α- and γ-tocopherol, but not for δ-tocopherol, strong correlations were found between the tocopherol degradation and the extent of oxidation. Results suggest that (i) at concentrations higher than 0.05%, tocopherols are less efficient and turn their mode of action or participate in side reactions in RO-TAG and (ii) δ-tocopherol was shown to be the most stable and effective under these low oxygen conditions.