Monitoring the oxidation of unsaturated oils and formation of oxygenated aldehydes by proton NMR

The oxidation process of rapeseed, walnut and linseed oils, rich sources of linolenic acyl groups, was studied by means of 1H nuclear magnetic resonance spectroscopy. The visible changes in some spectrum signals, especially in those of methylic, allylic and bis‐allylic hydrogen atoms, provide information about the degradation of the different acyl groups throughout the oxidation process. The rate of the changes produced in the three oils was compared, not only in relation to the rate of degradation of the different acyl groups, but also to both the formation and degradation of primary as well as to the formation of secondary oxidation products. Among the secondary oxidation products, the study was focused on the formation of aldehydes, their nature and evolution throughout the oxidation process of these three oils. The formation of n‐alkanals, trans‐2‐alkenals, trans,trans‐2,4‐alkadienals, as well as of the well known geno‐ and cytotoxic 4,5‐epoxy‐, 4‐hydroperoxy‐ and 4‐hydroxy‐trans‐2‐alkenals, was proved to take place.