Effects of antioxidants on the stability of triacylglycerols and methyl esters of fatty acids of sunflower oil

The autoxidation of kinetically pure triacylglycerols and methyl esters of sunflower oil (TGSO and MESO) in the presence of four different concentrations of 3,4-dihydroxybenzoic, ferulic, sinapic and caffeic acids, and in the absence of metal ions, at 100 °C was studied. It was established that the effectiveness and strength of the phenolic acids was greater in TGSO than in MESO. The interpretation of the results by kinetic and computational methods allowed the mechanism of action of the antioxidants to be elucidated. In both lipid substrates the molecules of the phenolic acids participate in one side reaction. The rate constants of this reaction in TGSO and MESO are practically the same, which shows that the binding of the fatty acids to the triacylglycerol structure of the sunflower oil does not change the mechanism of phenolic acid consumption in side reactions. The phenolic acids participate in reactions of chain initiation, which have a three to six times higher rate in MESO than in TGSO. The radicals of 3,4-dihydroxybenzoic, sinapic and caffeic acids in MESO, and of 3,4-dihydroxybenzoic and caffeic acids in TGSO, take part in one reaction of chain propagation (with the lipid substrate). The radicals of ferulic acid in MESO, and of ferulic and sinapic acids in TGSO, participate in more than one reaction of chain propagation. The effectiveness and strength of the phenolic acids are higher in TGSO than in MESO because, during the oxidation of TGSO, the contribution of the inhibitor radicals and molecules to chain initiation and propagation is smaller than in the case of MESO oxidation.