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

Abstract

A new method to determine epoxide concentrations in oxidized oils was developed and validated using ¹H 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 ¹H NMR by taking the <i>sn</i>-1,3 glycerol protons (4.18, 4.33 ppm) as internal references. The ¹H 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 (<i>R</i>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