Evaluation of the Influence of Thermal Oxidation on the Phenolic Composition and on the Antioxidant Activity of Extra-Virgin Olive Oils

A comparison between the results obtained by using HPLC-UV, HPLC-MS, and CE-UV for characterizing the deterioration of extra-virgin olive oil during heating (180 degrees C) was investigated, taking into account phenolic compounds. The concentration of several compounds belonging to four families of phenols (simple phenols, lignans, complex phenols, and phenolic acids) was determined in the samples after the thermal treatment by all three techniques. Hydroxytyrosol, elenolic acid, decarboxymethyl oleuropein aglycon, and oleuropein aglycon reduced their concentration with the thermal treatment more quickly than other phenolic compounds present in olive oil. HYTY-Ac and Lig Agl were demonstrated to be quite resistant to this kind of treatment, and the behavior of lignans could be outstanding, as they belong to the family most resistant to thermal treatment. Several "unknown" compounds were determined in the phenolic profiles of the oils after the thermal treatment, and their presence was confirmed in refined olive oils. The oxidative stability index (OSI time) was reduced from 25 to 5 h after 3 h of heating, whereas the peroxide value showed a minimum after 1 h of heating.