The production of olive oil is paramount for the Mediterranean agro-industrial sector but this activity generates a great amount of residues. Their adequate valorization could make this sector more sustainable and economically stronger. Among them, the exhausted olive pomace (EOP) is the main residue generated after the extraction of the residual oil contained in the olive pomace. Due to its carbohydrate and extractive content, EOP is an interesting bioresource for the production of bioenergy and high added-value products, such as phenolic compounds, in a biorefinery context.

The aim of this work was to evaluate the EOP phenolic content. In this way, ultrasound assisted extraction (UAE) was performed using sequential extraction with water and 70% acetone as solvents. Moreover, it was compared with an optimized extraction using water at 85 ºC under agitation. All these experiments were carried out with a 10% solid load. The extracts thus obtained were lyophilized and characterized in terms of the total phenolic content (TPC) by the Folin-Ciocalteu method, and their antioxidant activity was determined by two different in-vitro tests (DPPH and ABTS radical scavenging). The phenolic extracts were tentatively identified by capillary zone electrophoresis (CZE).

Results showed that EOP presents a significant amount of phenolic compounds, in all tested conditions. However, UAE extraction with 70% acetone revealed to be more effective than with water, with total phenolic content of 95.7 versus 74.4 mg GAE/g extract, respectively. The water extract obtained at 85 ºC showed a TPC of 66.1 mg GAE/g extract. Antioxidant activities of the extracts revealed to be in accordance with the TPC.

The analysis by CZE showed that hydroxytyrosol was the major compound found in all the extracts, which is considered a powerful antioxidant compound. Therefore, EOP is a promising source of antioxidants, in particular of hydroxytyrosol.