Total composition of sterols (4-desmethyl-sterols) in extravirgin olive oils obtained with different extraction technologies and their influence on the oil oxidative stability

The oxidative stability of vegetable oils mainly depends on the degree of unsaturation and the content of different compounds, such as biophenols and pigments. Little information is available about the influence of the components of the unsaponifiable fraction on oil stability, except for tocopherols and carotenoids. The aim of this work was to study the composition and antioxidant capacity of total sterols (4-desmethyl-sterols) of extravirgin olive oils obtained with different extraction technologies. The olives were harvested at two different ripening stages in the Aprutino Pescarese area (Pescara-Abruzzo, Italy) and were processed with three extraction systems: by pressure (traditional system) and by centrifugation (continuous system) with and without mill waste water recycling. The unsaponifiable fraction of the oils was obtained by liquid extraction after cold saponification, and the sterol fraction was purified by silica TLC. The qualitative and quantitative determination and identification of the total 4-desmethyl-sterols was performed by CGC and GC-MS, using apolar capillary columns (95% dimethyl-5% diphenyl-polysiloxane). Particular attention was focused on the determination of compound looses and the identification of interferences that can occur at the different steps of the analytical method. The antioxidant capacity of the total sterol fraction was evaluated by measuring its forced oxidative stability with an Oxidative Stability Instrument. A commercial refined vegetable oil, subjected to further elimination of its minor components, was used as model system. The latter was enriched with the 4-desmethyl sterols fractions, extracted from the unsaponifiable matter of the extravirgin olive oils, and was then subjected to forced oxidation.