Abstract: The pattern of changes in free radicals during the oxidation of unsaturated fatty acids including oleic, linoleic, and linolenic acid at room temperature was investigated by electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). The impact of exogenous substances on it was also evaluated. The results showed that the EPR spectra of oleic acid were dominated by alkyl radicals (R·) during oxidation from 0 to 60 min and then by alkyl peroxyl radicals (ROO·) and alkoxyl radicals (RO·). The radicals formed during the oxidation of linoleic and linolenic acids were mainly R·. During the oxidation process, the fatty acids (RH) were induced to form R· and H·, and then H· combined with ROO· to generate hydroperoxides, which were partly decomposed. The oxidation and degradation of linolenic acid were higher than those of linoleic acid and oleic acid. Addition of either tocopherol (VE) or tert-butylhydroquinone (TBHQ) inhibited the formation of free radicals such as RO· and ROO·. The EPR spectra of fatty acids added with 2,2’-azobis(2,4-dimethyl)valeronitrile (ADVN) were dominated by R· during oxidation, and inorganic carbon-centered free radicals were also introduced. The total number of spins in oleic and linolenic acid added with chlorophyll significantly increased at 150 and 90 min, respectively, and was fluctuated at other times. The higher the degree of unsaturation of fatty acids, the greater the amount of R· radicals produced during oxidation. Addition of VE and TBHQ could provide H·, which combined with lipid free radicals to inhibit the process of chain reaction and particularly inhibit the generation of ROO· and RO·. ADVN and chlorophyll could have a free radical-initiating effect, thus promoting the chain reaction.

Key words: unsaturated fatty acids; room temperature oxidation; free radicals; exogenous substances; electron paramagnetic resonance