Comparison of non‐volatile degradation products formed from different vegetable oils during deep frying of French fries

Summary Deterioration of frying oil greatly affects its nutritional quality. This study sought to establish how oil type affected the evolution of non‐volatile degradation products in frying oils. Frying experiments were applied with three different commercial oils (palm oil, rapeseed oil, soybean oil) known to be used in French fries, and the formed degradation products in oils were determined by classical assays, Fourier transform infrared spectroscopy (FTIR) and 1 H nuclear magnetic resonance (NMR). The FTIR demonstrated that the thermal stress may result in gradual alteration of cis fatty acids to trans fatty acids. The acyl groups, oxygenated compounds and 1,2‐diglycerides in samples were determined by 1 H NMR. Soybean and rapeseed oils contained more oxygenated compounds than palm oil owing to higher polyunsaturated acyl group content. However, a contrary result was shown for 1,2‐diglycerides. Of note, the major aldehydes formed were n‐alkanals, (E,E)‐2,4‐alkadienals and (E)‐2‐alkenals; 4‐oxo‐alkanals were not detected in palm oil. Correlation analysis indicated that these aldehydes were positively correlated with the polymerisation and oxidation of frying oils ( P < 0.01). The research provided new data for evaluating the suitability of these oils for food preparations.