Olive oil dregs as a novel source of natural antioxidants: Extraction optimization towards a sustainable process

Olive oil dregs (OOD), which are an underutilized by-product from oil mills, were used for the extraction of antioxidant compounds. The residues from three oil mills located in Campania (Southern Italy) were extracted with acidified methanol, and hydroxytyrosol (HT) was the main phenolic compound detected. Total phenolic content (TPC) and HT amount were measured. EVO Campania oil mill provided the residue with the highest TPC and HT quantities: 6.801 ± 0.159 mg Gallic Acid Equivalents (GAE)/g OOD and 519.865 ± 9.082 µg/g OOD, respectively. Eco-friendly extractions at different temperatures and times were performed on EVO Campania OOD, obtaining 9.122 ± 0.104 mg GAE/g OOD and 541.330 ± 64.087 µg/g OOD for TPC and HT, respectively, at 121^◦C for 60 min. Radical Scavenging Activity (RSA), Superoxide Scavenging Activity (SSA), and Ferric Reducing Antioxidant Power (FRAP) were measured in OOD aqueous extracts. Extract prepared at 37^◦C for 60 min showed the greatest RSA and SSA values (44.12 ± 1.82 and 75.72 ± 1.78, respectively), whereas extract prepared at 121^◦C for 60 min exhibited the highest FRAP value (129.10 ± 10.49 µg Ascorbic Acid Equivalents (AAE)/mg). OOD extracts were able to protect sunflower oil from oxidation for 4 weeks at 65^◦C. The overall results suggest that this novel residue can be usefully valorized by providing HT-rich extracts to use as antioxidant agents.