Evidence for multiple oxidation pathways from non-volatile products of methyl linoleate

Oxidation of unsaturated lipids is the most active chemical reaction leading to deterioration of food quality and shortening of shelf life. Lipid oxidation is known to be a free radical chain reaction driven by lipid peroxyl and alkoxyl radicals abstracting hydrogen atoms from neighboring molecules to form stable hydroperoxide intermediates and transfer a radical to a new molecule. In this process, products accumulate only after hydroperoxides decompose. However, this simplistic reaction fails to account for observed kinetics and compounds generated. Recently, a more complex reaction scheme for lipid oxidation was proposed which integrates traditional hydrogen abstraction with alternate reactions of peroxyl and alkoxyl radicals -- internal rearrangement, double bond addition, scission, dismutation. The alternate reactions run simultaneously and in competition with hydrogen abstraction, and alter the overall picture of lipid oxidation under different conditions. This dissertation research seeks to provide experimental proof of principle that these alternate pathways exist in lipid oxidation and have important consequences to timing and types of products developed. Lipid oxidation was studied in pure methyl linoleate incubated for 20 days under a range of conditions to investigate early reactions. Four types of major oxidation non-volatile products (conjugated dienes, hydroperoxides, epoxides, and carbonyls) were measured to determine product distributions and impact of reaction conditions on pathways. Two observations supported activity of reaction pathways other than hydrogen abstraction. First, epoxides were the dominant product under all conditions, with levels higher than hydroperoxides and in some cases almost as high as conjugated dienes. Second, all products began accumulating immediately at start of oxidation, although at different rates. Carbonyls were produced at much lower levels than other products. Coordinated analyses of volatile products suggested that these unusual patterns could be explained by competitive activity of two alternate reactions: 1) peroxyl radicals add to double bonds, forming a dimer that decomposes to an epoxide and an alkoxyl radical which can either form more epoxides or undergo scission to carbonyl products; 2) alkoxyl radicals add to an adjacent double bond to form epoxides and transfer the free radical down chain to a new site. Environmental conditions mostly affected initial oxidation rates.Ph.D.Includes bibliographical referencesby Jia Xi