Quantitative evaluation of the aldehydes formed in heated vegetable oils using high resolution proton-NMR spectroscopy

High-resolution proton nuclear magnetic resonance (1H-NMR) spectroscopy (400-600 MHz) was applied to the quantitative analysis of hydroperoxide decomposition products in thermally oxidized oils. Different oils (olive, sunflower and soybean) were heated in a thermostatic bath (180°C for 360 min). Proton NMR quantitative aldehyde analysis (n-alkanals, trans-2-alkenals, 4-hydroxy-trans-2-alkenals, alka-2,4-dienals) of fifteen oil samples (0, 60, 120, 240 and 360 minute heating) was compared with that of total polar compounds through silica column chromatography and of short-chain fatty acids. A good linear relationship (R2=0.94) was found between the unsaturated aldehyde level and the amount of methyl-octanoate as well as between total aldehyde determined by 1H-NMR and total polar compounds (R2=0.92). The same NMR sensitivity obtained at 400 MHz through 3000 scans was reached with only 64 scans for a total acquisition time of 2 min at 600 MHz. The potential use of 1H-NMR as a direct tool to monitor the quality and oxidative degradation in heated and fried oils is discussed.