DETERMINATION OF EPOXIDES AND ALCOHOLS IN EDIBLE OILS

Abstract

Lipid oxidation refers to the oxidative degradation of lipids, leading to toxicity and rancid flavours of lipid-containing foods. This process is often measured by determining hydroperoxides and their breakdown products, such as short-chain volatiles and unsaturated carbonyls. The formation of epoxides and alcohols during lipid oxidation was described decades ago but only a few studies have reported their concentrations in oils and foods. As are all lipid oxidation products, epoxides and alcohols are present at very low concentrations so their quantification requires suitable sensitivity and selectivity of the analytical method employed. In this thesis, a 1H nuclear magnetic resonance (NMR) method and a gas chromatography (GC) method are presented to quantify epoxides and alcohols in edible oils. The 1H NMR chemical shifts of epoxides were examined by epoxidation of triacylglycerol standards and oils. For quantification, the signals of glycerol protons were taken as the internal standard. The method was successfully applied to quantify epoxides formed in soybean oil oxidized at 100 oC. The experimental procedures are simple and rapid and can be easily applied to other types of vegetable oils. The GC method was developed to monitor epoxy and hydroxy fatty acids in vegetable oils, determining both saturated and unsaturated structures. The sample preparation employed transmethylation, solid-phase extraction (SPE), and trimethylsilyl derivatization. GC-mass spectrometry (GC-MS) enabled identification of the oxidation products, while quantification was carried out with GC-flame ionization detection (GC-FID) and GC-MS with selected ion monitoring. The new method was used to characterize and quantify individual epoxy and hydroxy fatty acids in sunflower and canola oils oxidized at 40 oC. The sensitivity of the GC method is superior to the 1H NMR technique but is more time-consuming, mainly due to the SPE step. The 1H NMR method is convenient but is limited by its sensitivity so it does not allow determination of epoxides in fresh or mildly oxidized oils. Overall, the 1H NMR method is suitable for rapid analytical purposes while the GC method is appropriate for stability studies under mild oxidation conditions. Future work can apply these methods to monitor epoxides and alcohols to study competitive oxidation pathways.