Epoxides in oxidation of methyl oleate

Abstract: Current development of monounsaturated oils is based partly on presumed greater stability than found in polyunsaturated oils. However, most data describing oleate oxidation has followed only hydroperoxides and perhaps volatile aldehydes, while scant information is available about epoxides that are highly reactive and potentially toxic. To provide more detailed information about products generated in alternate pathways, methyl oleate was oxidized in headspace vials at 20, 40, and 60 °C in dark for up to 60 days. Non-volatile products were monitored by chemical and HPLC analyses, while volatile products were quantitated by direct-injection GC with FID and MS detection.

GC and HPLC detected epoxides as major or dominant, products at all temperatures. Both internal and terminal epoxides appear to be present so multiple formation pathways are active. Early epoxides form via LOO^· addition (donor) to double bonds (receiver); subsequent scission of the -OO- link yields a donor LO^· and a receiver epoxide with an adjacent radical which then oxidizes. Dominant volatile aldehydes resulted from scission of receiver LO^· adjacent to receiver epoxides; aldehydes from scission of donor LO^· were present at lower concentrations, probably due to competing LO^· fates. Oleate reaction sites were not all equal. Products from epoxide formation via C9-OO^· addition to double bonds at C11 and then at C9 appeared first, followed by C11-OO^· additions. Reactions of C8 and C10 peroxyl and alkoxyl radicals gave only low levels of identified volatiles, and then after longer incubation times and at higher temperatures. Whether these sites generate less volatile hydroperoxyepoxides is under investigation.

Incubation time and temperature increase concentrations and numbers of volatile products, but at the same, notably shift the balance between C8, C9, C10, and C11 radical reactions and their resulting product distributions. These product variations must be considered in shelf-life studies and stability/food safety strategies for foods.