USE OF EPR AND ENDOR SPECTROSCOPY IN CONJUNCTION WITH THE SPIN-TRAPPING TECHNIQUE TO STUDY THE HIGH-TEMPERATURE OXIDATIVE-DEGRADATION OF FATTY-ACID METHYL-ESTERS

Free radicals produced during the autoxidation of unsaturated edible o ils are extremely short-lived, but are able to react with spin traps t o produce adducts with sufficient stability for spectroscopic characte risation at (near) cooking temperatures (353-443 K), EPR spectra have shown that the model esters methyl oleate, linoleate and linolenate ea ch formed three distinct radical adducts with N-tert-butyl-alpha-pheny lnitrone (PBN), These adducts have been further characterised by obtai ning spectra under conditions of limited oxygen availability and in th e presence of alpha-tocopherol; two of these adducts corresponded to p eroxyl and alkyl radical adducts of PBN, whereas the other was an alky l adduct of 2-methyl-2-nitrosopropane (MNP), which was formed as a res ult of decomposition of the PBN peroxyl radical adduct, The origins of the various H-1 hyperfine splittings have been determined by using se lectively and fully deuterated PBN and selectively deuterated oleate a nd some of their magnitudes have been confirmed by ENDOR spectroscopy, The results obtained clearly confirm the high temperature oxidation o f fatty acid esters to proceed via a different mechanism from that obs erved at low temperature, and point to significant differences in oxid ation mechanisms of monounsaturated fatty acid esters relative to poly unsaturated.