Assessing the effects of thermo-oxidation and naturally-occurring trace metal ions on the peroxidative susceptibilities of unrefined and refined sunflower oils: An 1H NMR-coupled chemometrics study

The peroxidation of unsaturated fatty acids (UFAs) in culinary frying oils leads to the formation of lipid oxidation products (LOPs). LOPs such as α,β-unsaturated aldehydes (α,β-UAs) are stable species and therefore their concentrations in oils are required to be regulated and suppressed. A 14.1 T nuclear magnetic resonance (NMR) spectrometer was employed to evaluate the susceptibilities of unrefined (virgin) East African and refined UK sunflower oils (UEASOs and RUKSOs, respectively) to thermo-oxidation according to standard frying practices at 180 °C. The effect of trace metal ions on the susceptibility of the sunflower oils to peroxidation was also studied. ¹H NMR analysis demonstrated that RUKSOs had significantly higher and lower contents of PUFAs and MUFAs, respectively, than UEASOs, although those of the latter were much more variable. (E)-2-alkenals, (E,E)-2,4-alkadienals, and n-alkanals were the most abundant aldehydes generated when exposed to thermal stressing episodes, and that UFAs in RUKSOs were more prone to peroxidation than that in UEASOs. For UEASOs, all legislated trace metal ion contaminants were found to not exceed the European Food Safety Authority's (EFSA) limits. Irrespective of the number of UEASO sample composition anomalies, which appear to arise from differential oil FA and product antioxidant, and possibly catalytic trace metal ion compositions, which were revealed herein as highly significant first-order heating time-point × oil classification analysis-of-variance (ANOVA) interaction effects, statistical analysis of LOPs repeatedly and consistently demonstrated that the UEASOs were significantly more resistant to thermo-oxidation at 180 °C. Furthermore, trace manganese and chromium ions may promote the thermo-induced peroxidation of UEASO UFAs, whereas trace lead ions may strongly suppress this process. The FA compositions, notably [MUFA]:[PUFA] ratios, were distinguishable based on their geographical sites of UEASOs production. This report necessitates the requirement to preferably use unrefined oils rather than refined ones for cooking and frying purposes and highlights the importance of regulating the compositions of FAs, antioxidants, and trace metal ions therein. It also supports the application of both univariate and multivariate (MV) chemometrics techniques to explore and detect unusual patterns of aldehydic LOPs in thermally-stressed culinary oils.