Ultrasound-Assisted Extraction to Recover Antioxidant Compounds: A Comparative Analysis of Green Solvents for the Sustainable Valorization of Olive Leaves

The olive industry produces large quantities of olive leaves, an agro-industrial byproduct without commercial value and with significant environmental disposal challenges. However, olive leaves are a rich source of antioxidant compounds known for their diverse biological properties. This highlights the need for green processes that utilize environmentally friendly solvents. Thus, this research aimed to evaluate the effectiveness of two different solvents: ethanol (at concentrations of 0%, 15%, and 30%) and glycerol (also at 0%, 15%, and 30%), combined with moderate temperatures (50 °C and 70 °C) using ultrasound-assisted extraction (20 kHz and 60% amplitude for 30 min) to recover antioxidant compounds from olive leaves. Our results showed that 30% glycerol at 70 °C was more effective at recovering polyphenols (14.85 ± 0.09 mg GAE g−1 dw) compared to ethanol (12.26 ± 0.13 mg GAE g−1 dw) under the same conditions. Additionally, the antioxidant capacity, measured using DPPH and ORAC assays, yielded higher results with 30% glycerol, showing values of 2.65 ± 0.16 mg mL−1 and 684.65 ± 20.79 µmol TE g−1 dw, respectively. The polyphenol profile quantified different families of polyphenols like phenolic acids, flavanols, flavonols, stilbenes, and secoiridoids. However, oleuropein, a secoiridoid, was the most predominant polyphenol found in the extracts (1122.52 ± 33.72 μg g−1 dw). Interestingly, the use of 30% ethanol at 70 °C reduced the glucose and fructose content by 48% and 84%, respectively. Therefore, glycerol emerges as an efficient green solvent for the extraction of antioxidant compounds. These findings support the valorization of this agricultural residue by recovering its valuable components, contributing to food sustainability and a circular economy.