Evaluation of oxidative stability of vegetable oils by monitoring the tendency to radical formation. A comparison of electron spin resonance spectroscopy with the Rancimat method and differential scanning calorimetry

Electron spin resonance spectroscopy (ESR), based upon the spin trapping technique, was applied at 60 °C for evaluating the oxidative stability of 11 vegetable oil samples, including sunflower oil, rapeseed oil and their mixtures. The ESR results were compared with the oxidative stability values provided by the Rancimat method and differential scanning calorimetry (DSC) at 100 °C. High linear correlations were found between ESR and Rancimat (r=0.963), and between ESR and DSC (r=0.979), suggesting that ESR can predict the oxidative stabilities provided by the Rancimat method, as well as by DSC, within a wide range of stability values. In order to examine the capability of each method to evaluate antioxidant activity, different concentrations of α-tocopherol were tested in purified sunflower oil and purified rapeseed oil. Minor differences among the three methods were found when low concentrations of α-tocopherol were used, whereas similar responses were obtained at the highest antioxidant concentration assayed. In comparison with the Rancimat method and DSC, the ESR method is concluded to be useful as a method employing milder conditions and shorter time, and which can be applied for the evaluation of oxidative stability of oils containing volatile antioxidants and other lipid systems containing water.