Photosensitized oxidation of methyl linolenate. Secondary products

Abstract

Abstract Previous studies of secondary oxidation products by high‐pressure liquid chromatography (HPLC) of autoxidized methyl oleate, linoleate and linolenate and photosensitized‐oxidized linoleate are extended to photosensitized‐oxidized linolenate. Photosensitized‐oxidized linolenate was fractionated by silicic acid chromatography with diethyl ether/hexane mixtures. Selected silicic acid chromatographic fractions were separated by polar phase HPLC and characterized by thin layer and gas liquid chromatography and by ultraviolet, infrared, nuclear magnetic resonance and mass spectrometry. Secondary products from the photosensitized oxidation mixtures (containing 8.2 to 29.0% monohydroperoxides) included keto‐ and epoxy‐dienes (0.4–1.6%), hydroperoxy epidioxides (0.8–4.9%), hydroperoxy bicyclic monoenes (0.1–0.3%), dihydroperoxides (1.0–5.6%), and hydroperoxy bisepidioxides (0.7–1.6%). Some of these secondary products are new and unique to photosensitized oxidation. Cyclization of the 10‐, 12‐, 13‐ and 15‐hydroperoxides of linolenate would account for their lower relative concentration than that found for the 9‐ and 16‐hydroperoxides. Dihydroperoxides may be derived from monohydroperoxides by singlet oxygenation or free radical oxidation. The hydroperoxy bis‐epidioxides may be formed by further serial cyclization of the hydroperoxy epidioxides from 10‐ and 15‐monohydroperoxides. Dihydroperoxides, hydroperoxy epidioxides and hydroperoxy bis‐epidioxides are suggested as important flavor precursors in oxidized fats.