Association between tocopherol isoform composition and lipid oxidation in selected multiple edible oils

The tocopherol (Toc) isoforms in 14 edible oils were determined and related to lipid oxidation of the oils. Oxidative stability of the vegetable oils was assessed using Rancimat analysis. The oxidation of these oils at specific lipid oxidation phases was determined by measuring the primary oxidation products generated during propagation phase (conjugated dienes and trienes), and lipid peroxides (iodometric assay), together with secondary oxidation products (TBARS assay). The naturally occurring α-Toc level of oils was correlated (R = 0.696; P < 0.05) with conjugated diene level in vegetable oils, indicating a potential pro-oxidant effect. This relationship was not observed for other lipid oxidation measures, or with other Toc isoforms. In addition, there was a common tendency for significant losses of α-Toc which paralleled the formation of conjugated dienes (R = 928, P < 0.05) in heated vegetable oils; again confirming the potential for α-Toc content to elicit pro-oxidant activity. The fact that γ-Toc was the dominant form of Toc isoform in many different vegetable oil sources tested herein, suggests that the pro-oxidant activity of α-Toc can be attributed to its high chemical reactivity as a free radical scavenger, rather than the abundance of α-Toc, in comparison to γ-Toc or δ-Toc in the oil. This observation was supported by the consistently greater degradation of α-Toc in heated vegetable oils compared to the non-α-Toc isoform counterparts. Lastly, the observed pro-oxidant activity of α-Toc was found to be specific to the formation of conjugated dienes, thus indicating that this measure of lipid oxidation assessment needs to be included when evaluating the roles of mixed Toc in different plant oil systems.