Dimers formed in oxygenated methyl linoleate hydroperoxides

Abstract The formation of dimers was demonstrated in methyl linoleate hydroperoxides decomposed by bubbling with dry air at 30 C. The dimer fraction isolated by gel permeation chromatography was further fractionated by successive silicic acid column and high performance liquid chromatography. Major components were analyzed after derivatizations by gas chromatography‐mass spectrometry with several ionization methods, i.e., electron impact, chemical ionization and field desorption. After aeration for 90 min, 1.4% of the hydroperoxides were decomposed. However, almost all of the secondary products were dimers, while polar monomeric and low molecular products were negligible. After aeration for 390 min, both polar monomeric (5.0%) and low molecular (0.8%) compounds formed, but dimers and polymers (18.1%) were still the major products. These results show the importance of polymerization in the aerobic breakdown of hydroperoxides. The dimers isolated from hydroperoxides aerated 90 min could be separated into two fractions according to their polarities. The dimers identified usually were composed of octadecadienoate and oxygenated octadecenoate moieties crosslinked through either ether or peroxy linkages across C‐9 or C‐13 positions. The oxygen‐containing functional groups found in the dimers include hydroperoxy, hydroxy and oxo groups. The polar dimers had two of these groups per molecule, while the less polar dimers had one. The main constituents of dimers were linked through peroxy bridges and found to be similar to the dimers previously identified in autoxidized methyl linoleate. These dimers are suggested as important intermediates in linoleate oxidation and as precursors of flavor deterioration.