Chemistry of free radical and singlet oxidation of lipids

Abstract

The reaction of oxygen with unsaturated lipids produces a wide range of compounds that have attracted considerable interest and research. In foods, lipid oxidation products are responsible for the development of rancidity by the production oflow molecular weight fission compounds that impart undesirable flavors. Potentially toxic and harmful cyclic and higher molecular weight materials are formed by thermal oxidation at ·the elevated temperatures of cooking and frying fats . .In biological systems, enzymatic oxidation prodilces materials that are structurally related to those formed by non-enzymatic' autoxidation. Much evidence points to the possible involvement of these lipid oxidation products in cancer. strokes, atheriosclerosis, inflammation. asthma. arthritis and the aging process. Many of these physiological effects of lipid oxidation have been well covered in other review articles. ~7.49. 77.105 Another important process' for the oxidation of unsaturated lipids involves activated species of oxygen. Singlet oxygen. produced by photo-oxidation in the presence of sensitizers such as chlorophylL is an important reactant that has received much attention. Singlet oxidation provides a powerful tool in synthetic organic chemistry. The role that singlet oxygen and related species play in the hydroperoxidation of unsaturated lipids is now under intensive investigation. ~3.49. 71 In the last decade much progress has been made in the chemistry of primary and secondary products of lipid oxidation. mainly as a result of advances in separation techniques and analytical methodology. This paper reviews structural studies of primary and secondary products of lipid oxidation. Free radical mechanisms for their formation are compared with that for singlet oxidation. Our current interests lie in further elucidating the structures of complex volatile and non-volatile decomposition products and in the mechanistic pathways for their formation. These decomposition products have a profound impact on the flavor and safety of lipid-containing foods. and they are implicated in many in L'it'o biological processes. 'J.llh