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 alpha-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 alpha-Toc which paralleled the formation of conjugated dienes (R=928, P<0.05) in heated vegetable oils; again confirming the potential for alpha-Toc content to elicit pro-oxidant activity. The fact that gamma-Toc was the dominant form of Toc isoform in many different vegetable oil sources tested herein, suggests that the pro-oxidant activity of alpha-Toc can be attributed to its high chemical reactivity as a free radical scavenger, rather than the abundance of alpha-Toc, in comparison to gamma-Toc or delta-Toc in the oil. This observation was supported by the consistently greater degradation of alpha-Toc in heated vegetable oils compared to the non-alpha-Toc isoform counterparts. Lastly, the observed pro-oxidant activity of alpha-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. (C) 2013 Elsevier Ltd. All rights reserved.