Analysis of autoxidized fats by gas chromatography-mass spectrometry: VI. Methyl 9,15- and 12,15-octadecadienoate

Abstract

The gas chromatography-mass spectrometry (GC-MS) method developed in preceding papers was extended to the structural analysis of autoxidation products of methylcis-9,cis-15-octadecadienoate and of an 87% concentrate ofcis-12,cis-15-octadecadienoate. Eight isomeric hydroxydienes with one allylic and one isolated double bond were identified from oxidized 9,15-diene and 2 conjugated hydroxydienes from oxidized 12,15-diene, after reduction of the hydroperoxides. The proportions of 16-(15%) and 17-hydroperoxides (22%) from 9,15-diene were significantly higher than that of the other isomers (8-12% each: 8-, 9-, 10-, 11-, 14- and 15-OOH). Similarly, the amount of 16-hydroperoxide from 12,15-diene was larger (42%) than the 12-hydroperoxide (31%). Substantial amounts of dihydroxy esters with one OH substituent on carbons-8,-9,-10 or-11 and the other OH on carbons-14,-16 or-17, were identified after hydrogenation in highly oxidized 9,15-diene. The implications of these hydroperoxide analyses are discussed in relationship to the precursors of flavor deterioration of oils and partially hydrogenated oils containing an ω-3 double bond.