Monitoring compositional changes in sunflower oil-derived deep-frying media by 1H Nuclear Magnetic Resonance

The evolution of the composition of three sunflower oils submitted to frying temperatures over a prolonged period of time was carried out by means of H-1 nuclear magnetic resonance. Using the same technique, the evolution of the composition of three sunflower oils derived frying media, formed throughout 14 deep-frying experiments of three different foods, was also studied. The foods involved in the deep-frying processes differ in both amount and nature of their lipids. The occurrence of migration of food lipids, thermo-oxidation, and hydrolytic processes in the sunflower oil frying media can be studied simultaneously by this technique. Changes in the evolution of the molar percentage of the different kinds of acyl groups, as well as of the concentration and nature of the aldehydes and alcohols formed were addressed. It is noteworthy that the main aldehydes detected, in both sunflower oils and sunflower oil frying media, were the reactive (E,E)-2,4-alkadienals, likewise the genotoxic and cytotoxic 4-hydroxy-(E)-2-alkenals and 4,5-epoxy-2-alkenals were also present. In addition, the extent of hydrolytic processes, and, both, the evolution of iodine value and percentage in weight of polar compounds, were also studied. Practical applications: The used technique provides, in only one run and without chemical modification of the sample, a great deal of information related to the quality and safety of the sunflower frying media used in repeated deep frying episodes. The fact that during frying the sunflower oil generates toxic aldehydes with concentrations in line with the heating time and number of fryings should be taken into account at both industrial and culinary levels. It has been shown that the migration of food lipids during frying influences importantly on parameters such as iodine value and percentage in weight of polar compounds, which can mask the degradation level reached by the oil. This study provides useful information about the behavior in deep frying of sunflower, soybean, and extra virgin olive oils, which can be taken into account for the selection of the most suitable oil for frying each kind of food.