IMPROVING LIPID OXIDATION MEASUREMENTS: INVESTIGATING A 1H NMR ALTERNATIVE TO AND EVALUATING THE ASSUMPTIONS OF STANDARD CHEMICAL METHODS

Abstract

Commonly used measures of lipid oxidation, such as the peroxide value (PV), conjugated diene value (CDV), and p-anisidine value (pAV) tests, are labourious and reagent-intensive. The objective of this thesis was to improve upon these methods. To assess the suitability of proton nuclear magnetic resonance (1H NMR) as an alternative method of measuring lipid oxidation, correlations between 1H NMR spectra and the PV, CDV, and pAV were evaluated by determining these parameters in soybean and canola oils oxidized to different extents. At high levels of oxidation, relationships were found between the measured PV and CDV and the 1H NMR signal responses of hydroperoxidic and conjugated dienic signals, respectively. A relationship was observed between the measured pAV and aldehydic signal responses only over a limited range. All relationships were hindered by poor sensitivity of the 1H NMR method. In the case of the pAV, sensitivity issues were compounded by deviation of the test from linearity at high levels of oxidation. These findings suggest that without additional time spent on sample preparation, instrument optimizations, or other sensitivity enhancements, 1H NMR cannot be applied to the quantitative assessment of oxidation in edible plant oils at low levels of oxidation.

The pAV test can also be inaccurate due to contributions from aldehydic flavour compounds. For this reason, the Global Organization for EPA and DHA Omega-3s (GOED) recommends a modified method to measure the pAV of flavoured marine oils that is predicated on the assumption that the flavour compounds themselves do not oxidize or degrade as the oil oxidizes. To test the null hypothesis that flavour oxidation and/or degradation is not occurring, I performed stability studies to compare the pAV of fish oil samples to which flavour had been added before and after oxidation, respectively. The pAV and the 1H NMR signal responses of the aldehydic flavour signals in these oils were evaluated to determine the extent of flavour degradation over the course of oxidation. For several sampling points in both chocolate-vanilla and lemon flavoured oils, the pAV of oils to which flavour was added before oxidation were significantly lower than those of oils to which flavour was added after oxidation, suggesting that these flavours degrade during oxidation. These findings suggest that the GOED recommendation may not consistently provide accurate estimates of the pAV in all flavoured marine oils.