Simultaneous control of the evolution of the percentage in weight of polar compounds, iodine value, acyl groups proportions and aldehydes concentrations in sunflower oil submitted to frying temperature in an industrial fryer

Sunflower oil was heated at 190 °C in a discontinuous industrial fryer for 32 h. The simultaneous monitoring during heating of the Iodine Value, acyl groups proportions and aldehydes concentrations was carried out by 1H Nuclear Magnetic Resonance; at the same time the percentage in weight of Polar Compounds was also controlled by an instrumental test. The evolution with heating time of Iodine value, percentage in weight of polar compound and acyl groups proportions fit well to linear equations. The formation of primary oxidation compounds was not observed in contrast with the early detection of different types of aldehydes such as alkanals, (E)-2-alkenals, (E,E)-2,4-alkadienals, (Z,E)-2,4-alkadienals, 4-oxoalkanals and the genotoxic and citotoxic 4-hydroxy-(E)-2-alkenals; their evolution with heating time fits well to curved lines in most cases. The results obtained were compared with those previously obtained in a domestic fryer, resulting in clearly higher degradation rate of acyl groups in the domestic fryer than in the industrial one; however, the differences in aldehydes concentration were less remarkable due to the higher retention degree of aldehydes in the industrial version. Aldehydes retention was shown to be directly related with oil volume and inversely to the oil–air surface and aldehyde volatilities. The presence of the genotoxic and citotoxic 4-hydroxy-(E)-2-alkenals from the beginning of the sunflower oil heating is cause of concern for the human health. The concentration of aldehydes when the oil reaches 25% in weight of polar compounds is near its maximum values. These results suggest the need to analyze in depth the safety of some oils when they are submitted to heating at frying temperature.