Study by 1H NMR spectroscopy of the evolution of extra virgin olive oil composition submitted to frying temperature in an industrial fryer for a prolonged period of time

Extra virgin olive oil was heated at 190 °C in a discontinuous industrial fryer for 40 h. The evolution during heating of the acyl group proportions and of the iodine value was monitored by 1H nuclear magnetic resonance. Monounsaturated acyl groups degraded at faster rates than linoleic and linolenic groups. The formation of primary oxidation compounds was not observed with this technique, which contrasts with the early detection of different types of aldehydes, such as alkanals, (E)-2-alkenals, (E,E)-2,4-alkadienals, and 4-oxoalkanals, whose concentrations follow a curved path in most cases. In addition, the formation of (E) and (Z)-9,10-epoxystearyl groups was also observed and the evolution of their concentrations throughout the heating time determined. Equations that relate the above mentioned composition parameters to the heating time were obtained, all of them with high correlation coefficients and predictive value. Major differences were found between the behaviour of this extra virgin olive oil and sunflower oil submitted to identical conditions. The safety level, in terms of aldehydes contained, is higher in the extra virgin olive oil than in the sunflower; in fact, cytotoxic and genotoxic 4-hydroxy-(E)-2-alkenals were not detected in extra virgin olive oil, unlike in sunflower oil. The same percentage in weight of polar compounds in these oils involves great differences in composition and in safety level.