Antioxidant properties of tocopherols in fish oils

alpha-Tocopherol (vitamin E) is generally the only tocopherol naturally present in oils from marine fish, and the tocopherol concentrations in refined fish oils are substantially lower than in most vegetable oils. To reduce the susceptibility of fish oils to oxidation and adverse flavour formation, further antioxidant protection is needed. In this work, the properties of alpha-Tocopherol (alphaTOH), gamma-tocopherol (gammaTOH) and delta-tocopherol (deltaTOH) in purified fish oil triacylglycerols (TAG) were studied over a wide concentration range and at an oil temperature close to ambient and in the dark. Purification of the substrate oils (menhaden oil and anchovy oil) by column chromatography removed the majority of the antioxidant and prooxidant molecules present and accordingly allowed better control over factors known to influence the autoxidation process.; The work presented demonstrates that the tocopherols affect many stages of the autoxidation process in purified fish oils, including the formation of hydroperoxides, the decomposition of hydroperoxides, the formation of nonvolatile secondary oxidation products, as well as the composition of volatile secondary oxidation products. In all the oxidation tests addressing these different aspects of lipid autoxidation, as well as in a study of the effect of the tocopherols on the oxidative degradation of astaxanthin, the results obtained could be related to the relative hydrogen-donating ability of the tocopherols, which decreases in the order of alphaTOH > gammaTOH > deltaTOH.; The number of methyl groups on the chroman ring of the tocopherols had a remarkable effect on their antioxidant properties in fish oil. The relative ability of the tocopherols to retard the formation of hydroperoxides in purified fish oil TAG decreased in the order of alphaTOH > gammaTOH > deltaTOH at a low initial level of addition (100 ppm), but a reverse order of activity was found when the initial tocopherol concentration was 500 ppm and above. This dependence of relative antioxidant activity on tocopherol concentration was caused by the existence of concentrations for maximum antioxidant activity for alphaTOH and for gammaTOH. The removal of minor oil components reduced the rate of TAG autoxidation, but did not appear to affect the alphaTOH concentration for inversion of activity. Contrary to their effect on primary oxidation, all three tocopherols retarded the formation of volatile secondary oxidation products in a concentration-dependent manner. Principal component analysis suggested that high tocopherol hydrogen-donating power directs the formation of the more flavour-potent carbonyls, and, moreover, that the C3-aldehydes together (propanal + 2-propenal) are a more suitable marker of secondary oxidation of n-3 fatty acids than propanal alone. An important finding in this work was that the addition of ascorbyl palmitate to fish oil with alphaTOH reduced the initial rate of autoxidation, thereby eliminating the alphaTOH-mediated accumulation of hydroperoxides, only when the alphaTOH level was above that for maximum activity. Ascorbyl palmitate is known to regenerate alphaTOH from its tocopheroxyl radical. This emphasizes the importance of the alphaTO·, as opposed to alphaTOH itself, in side-reactions inducing hydroperoxide formation in fish oil.