Olive tree farming practices and the olive oil industry generate huge amounts of wastes every year. One of the main components of these agroindustrial residues is the extractive fraction and interestingly it contains non-structural components, including bioactive compounds. Particularly, exhausted olive pomace (EOP) is the final residue resulting from the industrial extraction of olive-pomace oil with hexane and the extractives account for more than 40% of its chemical composition.

In this work, different extraction methods, including green technologies, have been compared to evaluate the extraction of antioxidants from EOP. Hydrothermal extraction using medium (85 ºC) and high thermal conditions (200 ºC) were carried out. Aqueous accelerated extraction was also assessed at different temperatures (55 ºC and 190 ºC). Likewise, the effect of organosolv extraction (50% ethanol at 200 ºC), alkaline hydrolysis at 55 ºC and different NaCl concentrations (3-9% w/v) were also tested. Moreover, the influence of the time extraction (up to 120 min) and the solid loading (up to 25%) were evaluated in the aforementioned experiments.

The extracts obtained were characterized regarding the content of total phenols by the Folin-Ciocalteu method. In general, the solid loading showed a negative influence on the phenolic compounds extraction with all techniques used. Using water as extractive agent, the values ranged between 26.8 and 50.8 mg gallic acid equivalents (GAE)/g EOP, depending on the conditions. In particular, when the accelerated solvent extraction was used, the temperature showed a clear positive effect on the phenolic compound extraction and the extration time was considerably reduced. It enables the obtention of 40.6 mg GAE/g EOP at 190 ºC in 10 min. Nevertheless, the best results were found using organosolv extraction at 200 ºC (82.1 mg/g GAE/g EOP). After characterization, hydroxytyrosol was found to be one of the potential active compounds in EOP.