Autoxidation of polyunsaturated triacylglycerols. III. Synthetic triacylglycerols containing linoleate and linolenate

Four triacylglycerols containing linoleate (L) and linolenate (Ln) in specific positions were synthesized to determine the effect of fatty acid position on their relative rates and products of autoxidation. Analyses by reversedphase high performance liquid chromatography (HPLC) showed that autoxidation of L- and Ln-containing triacylglycerols form monohydroperoxides and hydroperoxy epidioxides as the main products. The peroxyl radicals of internal 12- and 13-mono-hydroperoxides of Ln triacylglycerol components cyclized rapidly and their relative triacylglycerol position had no influence on their rates of cyclization. A good linear relation was obtained between total HPLC peak areas (detected at 235 nm) of the main oxidation products and peroxide values. Reversed phase HPLC analyses thus provide a useful method to estimate oxidation of polyunsaturated triacylglycerols. The ratios of Ln to L mono-hydroperoxides were twice the ratios of Ln to L in the triacylglycerol substrates. Ln triacylglycerol components, therefore, oxidized twice as much as the L components. At 40°C, LnLnL oxidized slightly faster than LnLLn with respective induction periods of 45 and 47 hr. LLnL oxidized faster than LLLn with respective induction periods of 56 and 60 hr. Dilinolenoyl-linoleoylglycerols are, therefore, slightly less stable to oxidation when Ln is in the 1,2- than the 1,3-triacylglycerol position. Dilinoleoyl-linolenoylglycerols are less stable when L is in the 1,3- than the 1,2-triacylglycerol position. Presented in part at the 80th Annual American Oil Chemists' Society Meeting, Cincinnati, Ohio, May 3–6, 1989