Prooxidant effect of α-tocopherol on soybean oil. Global monitoring of its oxidation process under accelerated storage conditions by 1H nuclear magnetic resonance

The effect of adding alpha-tocopherol in proportions ranging from 0.002 to 5% in weight on the oxidative stability of soybean oil was studied. For the first time, the oxidation process under accelerated storage conditions including evolution of the molar percentages of the several types of oil acyl groups, and formation and evolution of various kinds of oxidation products comprising hydroperoxides, hydroxy-dienes and other alcohols, epoxides, aldehydes and keto-dienes, was followed by H-1 nuclear magnetic resonance. It is proved that, except in the lowest proportion, alpha-tocopherol not only exerts a prooxidant effect on soybean oil but also modifies its oxidation pathway, affecting the oxidation products generation rate, their nature, relative proportions and concentrations. It is noticeable that the highest alpha-tocopherol concentrations induce the generation of some toxic compounds at earlier stages of the thermoxidation process and sometimes in higher concentration, such as certain oxygenated a, beta-unsaturated aldehydes and monoepoxides derived from linoleic groups.