Mechanisms Of Autoxidation

Autoxidation is a free-radical chain reaction that includes initiation, propagation, and termination steps:

Initiation RH -> R- + H-Propagation R- + 02 ROO-

ROO- + RH -» ROOH + R-Termination ROO- + R— ROOR

Unsaturated fatty acids undergo oxidation by the above free-radical chain reaction to form hydroperoxides. The hydroperoxides are decomposed to form off-flavor compounds, which are called secondary oxidative products (2,3). Though the reaction of singlet-state fatty acids with triplet-state oxygen is thermodynamically difficult, fatty acid free radicals can react with triplet-state oxygen molecules fairly easily. Heat, metal catalysts, and both ultraviolet and visible light can accelerate the formation of free radicals of unsaturated fats during the initiation step of autoxidation. Typically, a hydrogen atom is removed from the methylene group alpha to the double bond of the fatty acid to form a fatty acid free radical, which has a short lifetime due to its tendency to achieve a stable electron pair. The alkyl radical reacts with oxygen to form a peroxy radical. This peroxy radical then reacts with a hydrogen atom that was abstracted from another unsaturated fatty acid to form a hydroperoxide. The formation of hydroperoxides in this manner is usually accompanied by a shift in the position of the double bonds, due to resonance stabilization of the fatty acid free radical and resulting formation of isomeric hydroperoxides. A mixture of four isomeric hydroperoxides that have -OOH groups at the 8, 9, 10, and 11 carbon positions are produced in the autoxidation of oleic acid. Hydroperoxides themselves are generally tasteless, colorless, and odorless; however, they are relatively unstable and therefore readily decompose to form hydrocarbons, alcohols, esters, acids, aldehydes, and ketones, each having differing molecular weights and flavor thresholds. The principal aldehyde compounds formed from the decomposition of hydroperoxides at the 8, 9, 10, or 11 carbon atom positions are 2-undecenal, 2-decenal, nonanal, and octanal, respectively. Oxidized linoleic acid produces hydroperoxides at either the 9 or 13 position, and a new conjugated double bond system is formed at either the 10 or 11 position (4). The autoxidation of linolenic acid typically results in the formation of hydroperoxides at either the 9, 12, 13, or 16 position on the hydrocarbon.

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