Electron Paramagnetic Resonance Spin Trapping of Sunflower and Olive Oils Subjected to Thermal Treatment: Optimization of Experimental and Fitting Parameters

Abstract

Sunflower oil (SO) and extra virgin olive oil (EVOO) were heated at 90 °C in the presence of PBN. The radical species formed during thermal treatment, trapped by PBN, were revealed by electron paramagnetic resonance (EPR) spectroscopy, a widely used method to study the oils’ oxidation. The effect of the experimental parameters on the intensity of PBN adduct was analyzed with the aim to standardize the spin trapping protocol for oils. A modification of the Boltzmann sigmoidal equation was proposed to fit the experimental points representing the changes of the EPR intensity of the PBN adduct versus time. The fitting parameters allowed for distinguishing between SO and EVOO and to obtain more reliable induction period (IP) values. The fitting parameters and the shape of the curve depend on the diameter of the sample holder. The IP and the time at which maximum intensity is reached, t(Imax) in thin capillary tubes (IP 35.92 min, t(Imax) 186 min) were shifted at longer times in comparison with flat cell (IP 69.54 min, t(Imax) 106 min). The peroxide values (PV) were measured in SO and EVOO samples with and without PBN at specific points of the curve and related to the intensity of the EPR signals. PBN inhibits the propagation of the chain reaction, and the extent of inhibition is lower in EVOO than in SO possibly due to the effect of phenolic compounds that in SO are lacking. The phenolic compounds are also responsible for the lower PBN adduct intensities observed in EVOO than in SO. This study further highlights the power of EPR spectroscopy in the evaluation of oil oxidation and provides a guide for EPR experimental and fitting parameters for oils.