Oxidation of corn oil at room temperature: Primary and secondary oxidation products and determination of their concentration in the oil liquid matrix from 1H nuclear magnetic resonance data

The oil liquid matrix of several corn oil samples that have been stored at room temperature in closed receptacles for different periods of time is studied by means of 1H nuclear magnetic resonance (1H NMR), in order to further knowledge about this type of edible oil oxidation. As expected, the degradation of linoleic acyl groups predominates. In samples at early oxidation stages the presence of hydroperoxides and of (Z,E) conjugated-dienic systems is demonstrated, the concentration of the first group being higher than that of the second. In addition to these compounds, the presence of hydroxy derivatives supporting (Z,E) conjugated-dienic systems, as well as of hydroperoxy derivatives supporting (E,E) conjugated-dienic systems, in samples at intermediate and advanced oxidation stages, is also shown. Corn oil samples at advanced stages of oxidation also contain aldehydes, among which there are alkanals, (E)-2-alkenals, (E,E)-2,4-alkadienals, 4-hydroxy-(E)-2-alkenals, 4-hydroperoxy-(E)-2-alkenals and 4,5-epoxy-(E)-2-alkenals. The concentrations of the different kinds of intermediate compounds above mentioned as well as of the different kinds of aldehydes present in the oil liquid matrix were determined. These latter compounds can be either free or joined to truncated structures of triglycerides. In addition, a principal component analysis between storage conditions and oxidation level of the samples was carried out.