Epoxides are major products in oxidation of methyl oleate and linoleate and their triacylglycerols

Abstract

Lipid autoxidation poses a significant problem for stabilizing sensory quality, nutritional value, and chemical safety of lipid containing foods. Peroxide value and volatile carbonyls are the most measured markers to assess oxidation, but products such as epoxides and alcohols are now being identified and quantified in foods. The mechanisms and conditions under which other products form are poorly understood, particularly how lipid structure and oxidation conditions affect reaction pathways, products, and rates. This research paper aims to show how number of double bonds affects oxidation product formation, how oxidation temperature affects reaction rates and product formation and decomposition, and how open or closed packaging systems affect reaction rates.Methyl oleate and methyl linoleate were autoxidized neat and incubated at 25, 40, or 60 °C for various incubation periods. Class assays for quantification of oxidation products included the following: conjugated dienes by UV absorbance at 233 nm; peroxide value by reaction with TPP; epoxide value by reaction with diethydithiocarbamate; soluble carbonyls by reaction with 2,4-dinitrophenylhydrazine. Direct separation of oxidation products was performed using NP-HPLC with detection and quantitation by UV and Corona Charged Aerosol (CAD) detection. Alternate autoxidation products competed with the formation of hydroperoxides in both autoxidizing oleate and linoleate. Epoxides were found to be the dominant product in oleate autoxidized at 25 C, reaching a maximum value of 69.2 mmol/mol lipid, while peroxide value reached a maximum of 23.71 mmol/mol lipid. Epoxides were also found to be a major product in linoleate autoxidation reaching a maximum value of 357.5 mmol/mol lipid at 25 C, while peroxide value reached a maximum of 357.1 mmol/mol lipid. For both oxidized oleate and linoleate, rate of epoxide formation competed with that of hydroperoxides, demonstrating that alternate reactions are active and important in directing the reaction rates and product distributions of autoxidizing lipids.