Model Studies on the Key Aroma Compounds Formed by an Oxidative Degradation of ω-3 Fatty Acids Initiated by either Copper(II) Ions or Lipoxygenase

Due to the high number of double bonds, omega-3-polyunsaturated fatty acids such as eicosapentaenoic aid (EPA) or docosahexaenoic acid (DHA) are prone to rapid oxidation, leading to the formation of intense taints often described as "fishy". To clarify the compounds responsible for such off-flavors, EPA, DHA, and alpha-linolenic acid (ALA) were oxidized singly either in the presence of copper ions or in the presence of lipoxygenase. The autoxidation of EPA and DHA led to a mixture of odorants eliciting an overall fishy odor quality, whereas neither the oxidation of ALA by copper ions nor that by lipoxygenase led to an unpleasant odor. Application of aroma extract dilution analysis (AEDA) on the volatiles generated by autoxidation of EPA revealed trans-4,5-epoxy-(E,Z)-2,7-decadienal, identified for the first time as fatty acid degradation product, (Z)-1,5-octadien-3-one, (Z)-3-hexenal, (Z,Z)-2,5-octadienal, (Z,Z)-3,6-nonadienal, and (E,E,Z)-2,4,6-nonatrienal with the highest flavor dilution (FD) factors. The autoxidation as well as the enzymatic oxidation of all three acids led to the same odorants, but with different FD factors depending on the acid and/or the type of oxidation applied. Thus, the results suggested that a defined ratio of a few odorants is needed to generate a fishy off-flavor.