Antioxidant activity of the phenolic fraction of olive oil as evaluated by different methods: it’s relationship with oil polyphenols content and oxidative stability

Antioxidant activity of the phenolic fraction of extra virgin olive oil (EVOO) was measured by means of a chemical (DPPH·) and an electrochemical method. Radical scavenging activity (RSA) expressed as the rate of DPPH· bleaching was calculated using a ‘half-life’ approach and an empirical kinetic approach. The results calculated by the kinetic approach were better correlated to total polyphenols (TPP) content and oxidative stability of oils as measured by Rancimat.The electrochemical determination was conduced at three oxidation potentials on the basis of cyclic voltammetry experiments. The current values were registered and expressed as µg/ml quercetin equivalent (QE) at potentials 0 and 125 mV vs Ag/AgCl (QE0 and QE125), and µg/ml gallic acid equivalent (GAE) at 250 mV vs Ag/AgCl (GAE250) using the correspondent calibration curves. The correlations of QE0, QE125 and GAE250 with RSA and TPP were investigated. TPP was better correlated with GAE250, whereas, as expected, the RSA correlated best with QE0 which represents the most readily oxidizable compounds; therefore QE0 was used as a measure of AOP. Both methods were tested in predicting the oxidative stability of 22 EVOO samples and resulted significantly correlated with the induction period as measured by Rancimat.[...]