Nutraceutical Valorization of Exhausted Olive Pomace from Olea europaea L. Using Advanced Extraction Techniques

Exhausted olive pomace (EOP) represents the principal residue of olive pomace. Several studies have optimized the extraction of specialized metabolites from the EOP of Olea europaea L., but a comparison between different extractive methods has not been made. For this reason, the present investigation aims to compare four different extractive methods by using water and 15% ethanol/water as extractive solvents. Specifically, based on extract antioxidant activity, the methods compared were maceration (MAC), microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), and Accelerated Solvent Extraction (ASE). Between these, the UAE and ASE hydroalcoholic EOP extracts were demonstrated to have the highest antioxidant activity. Subsequently, these extracts were investigated for their hypoglycemic and antiradical activity using in vitro cell-free and cell-based assays, respectively. ASE hydroalcoholic EOP extract demonstrated the greatest ability to inhibit the alpha-amylase enzyme and an in vitro antioxidant activity comparable to N-acetyl cysteine in HepG2 cells. UAE and ASE extracts' phytochemical characterization was also performed, identifying seven phenolic compounds, including 3-hydroxytyrosol, tyrosol, and, for the first time, salidroside. The ASE hydroalcoholic EOP extract was the richest from a phytochemical point of view, thus confirming its major biological activity. Therefore, ASE and 15% ethanol/water may represent the best extractive method for EOP nutraceutical valorization.