Characterisation of aldehydic acids in used and unused frying oils

Aldehydic acids, expected to be present in auto-oxidised lipids as a result of decomposition of hydroperoxides by p-scission reactions, were analysed in unused and in frying oils used for two days. The oils studied were partially hydrogenated rapeseed/palm oil mixture, conventional sunflower oil and high oleic acid sunflower oil. The method involved saponification of lipids, elimination of the unsaponifiable matter, transmethylation and further fractionation of the methyl esters into polar and non-polar fractions by silica column chromatography. Both fractions were analysed by high-performance size-exclusion chromatography to determine the levels of non-polar dimers, polar dimers and oxidised monomers. The polar fraction was also analysed by gas chromatography and gas chromatography-mass spectrometry. Using this approach, the aldehydic acid methyl esters: methyl 8-oxooctanoate, methyl 9-oxononoate, methyl 10-oxo-8-decenoate, methyl 1-oxo-9-undecenoate and methyl 12-oxo-9-dodecenoate were characterised in used as well as unused frying oils.