Aqueous Oxidation of Ethyl Linoleate, Ethyl Linolenate, and Ethyl Docosahexaenoate

Abstract

The oxidative stability of ethyl linoleate (LA), ethyl linolenate (LN), and ethyl docosahexaenoate (DHA) in an aqueous solution was compared by measuring the decrease in unoxidized substrate and the formation of total hydroperoxides, conjugated dienes, and monohydroperoxides (MHPs). The highest stability was shown by DHA, this being followed by LN and LA in both cases when using Fe²⁺-ascorbic acid and 2, 2'-azobis(2, 4-dimethylvaleronitrile) (AMVN) as a initiator, while the reverse order of oxidative stability was obtained when the esters were oxidized in chloroform or ethanol with AMVN. The stability of MHPs in an aqueous solution also increased with increasing degree of unsaturation. HPLC analyses showed little difference in the positional distribution of MHP isomers between aqueous oxidation and autoxidation in the air. This result suggests no selective abstraction of hydrogen atoms from the bisallylic positions of polyunsaturated fatty acid in an aqueous phase. The product ratio of hydoroperoxy epidioxides to MHPs in the aqueous oxidation of LN was lower than that of autoxidized LN in the air, showing the rapid decomposition of epidioxides in an aqueous solution.