Lipids

Lipid Oxidation

Lipid autoxidation is a free-radical chain reaction involving hydroperoxides. The mechanism has been extensively investigated since the first systematic study by Bateman and Bolland in the 1940s. In 1970, Rawls and van Santen proposed a possible role for singlet oxygen in the initiation of fatty acid autoxidation (31). In the presence of a suitable sensitizer, lipid oxidation proceeds via the "ene" reaction in which the dioxygen molecule is added to the olefinic carbon with a subsequent shift in the position of the double bond, and both conjugated and nonconjugated hydroperoxides are formed. In contrast, the classical free-radical mechanism produces only the conjugated hydroperoxides. The distribution of hydroperoxides are different from that found in the free-radical reaction.

In linoleate autoxidation, the product is composed predominantly of the 9- and 13-hydroperoxides, because the stability of the conjugated diene system favors the oxygen attack at the end position. Experimentally, however, four hydroperoxides are obtained: two with trans, cis and the other two with trans, trans stereochemistry (13-hydro-peroxy-9-cis, ll-ira/is-octadecadienoic; 13-hydroxyperoxy-9-trans, 11-iraws-octadecadienoic; 9-hydroxyperoxy-10-trans, 12-c?s-octadecadienoic, and 9-hydroperoxy-lO-iraws, 12-iraras-octadecadienoic). A unified mechanism has been proposed to account for the formation of these products (32) (Fig. 6). In this scheme the peroxy radicals initially formed have the trans, cis configuration and exist in two conformational isomers. The normal pathway involves the abstraction of hydrogen from another linoleate by either isomer to yield the trans, cis hydroperoxides. The alternative pathway involves the loss of oxygen from the conformers to give either the original pentadienyl radical or a new carbon radical in which the stereochemistry of the partial double bond is inverted. Oxygen addition to this new pentadienyl radical yields a trans, trans-diene peroxy radical which ultimately gives the trans, iraras-conjugated hydroperoxide.

Because autoxidation involves free radicals, radical scavengers should effectively terminate the chain reaction. Most antioxidants are substituted phenolic compounds that act by transferring hydrogen to lipid peroxy radicals. The resulting aryloxy radical of the antioxidant is stable and unreactive in oxidative reactions. The resonance stability of the aryloxy radical depends on the substitution groups. Electron-releasing groups decrease the transition energy for the formation of the aryloxy radical. Bulky sub-stituents stabilize the aryloxy radical but also create steric hindrance making the antioxidant less accessible to the lipid peroxy radical (33).

Physical Chemistry of Lipids

Another area of great interest to food chemists concerns the physical chemistry of lipids—polymorphism, crystal

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