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

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Abstract
Controversial data on the antioxidant effects of tocopherols have already been shown in different test systems, yet delta-tocopherol was hardly considered. This study was designed to assess the effects and degradation of alpha-, gamma-, and delta-tocopherol in four concentrations from between 0.01 and 0.25% on the oxidation of purified rapeseed oil trigacylglycerols (RO-TAG) at 40 degrees 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% alpha,alpha'-azoisobutyronitrile (AIBN), which is a known radical initiator. alpha- and gamma-Tocopherols increased the rate of lipid oxidation, which was more pronounced in the presence of AIBN. Only the lowest amount of 0.01% gamma-tocopherol was comparable to the control sample in the test without AIBN. The most effective was shown to be delta-tocopherol, which did not elevate lipid oxidation except the PV in the AIBN test, but they did not delay it either. delta-Tocopherol was the most stable followed by gamma- and alpha-tocopherol. For alpha- and gamma-tocopherol, but not for delta-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). delta-tocopherol was shown to be the most stable and effective under these low oxygen conditions.