γ- and δ-tocopherols are more effective than α-tocopherol on the autoxidation of a 10% rapeseed oil triacylglycerol-in-water emulsion with and without a radical initiator

The antioxidative effects of γ- and mainly δ-tocopherol in a multiphase system were hardly considered up to now. The aim of this study was i) to assess the effects and ii) to follow the degradation of α-, γ- and δ-tocopherol in concentrations of 0.01%, 0.05%, 0.1% and 0.25% during the oxidation of a 10% purified rapeseed oil triacylglycerol-in-water emulsion at 40°C in the dark for 15 wk in a system containing a low oxygen concentration. Oxidation experiments were performed weekly by assessing the formation of hydroperoxides and hexanal, and the stability of the tocopherols was determined using high-performance liquid chromatography. Storage tests were conducted with and without the addition of 0.01% α,α′- azoisobutyronitrile (AIBN), which is a known radical initiator. α-Tocopherol increased the formation of hydroperoxides in both tests as well as the generation of hexanal when the radical initiator was added; furthermore it was the least stable. γ-Tocopherol delayed the formation of hexanal and prolonged the stability of the emulsion in a dose-dependant manner. δ-Tocopherol was the most stable and also the most effective in delaying lipid oxidation in the emulsions. Each concentration that was tested reduced the rate of hydroperoxide and especially hexanal formation. Hexanal was only formed to a slight extent after 15 wk of oxidation in the test with AIBN and the lowest dose of 0.01 % δ-tocopherol. For all tocopherols, strong correlations were found between tocopherol stability and the extent of oxidation. Results suggest that i) mainly δ-tocopherol, but also γ-tocopherol even less pronounced, are very good antioxidants in order to stabilize and prolong the shelf life of oil-in-water emulsions, ii) the antioxidative effects were intensified with increasing amounts.