New ¹H NMR-Based Technique To Determine Epoxide Concentrations in Oxidized Oil.

A new method to determine epoxide concentrations in oxidized oils was developed and validated using (1)H NMR. Epoxides derived from lipid oxidation gave signals between 2.90 and 3.24 ppm, well separated from the signals of other lipid oxidation products. To calibrate, soybean oils with a range of epoxide concentrations were synthesized and analyzed using (1)H NMR by taking the sn-1,3 glycerol protons (4.18, 4.33 ppm) as internal references. The (1)H NMR signals were compared to the epoxide content determined by titration with hydrogen bromide (HBr)-acetic acid solution. As expected, the signal response increased with concentration linearly (R(2) = 99.96%), and validation of the method gave results comparable to those of the HBr method. A study of the oxidative stability of soybean oil was performed by applying this method to monitor epoxides during thermal lipid oxidation. The epoxide content increased over time and showed a different trend compared to peroxide value (PV). A phenomenological model was suggested to model epoxides derived from lipid oxidation.