Ultrasound-Assisted Eutectic Solvent-Based Process Intensification for Sustainable Recovery of Oleuropein from Olive Leaves

Olive leaves, a significant source of agri-food waste, can be valorized as feedstock in biorefineries due to their high content of antioxidant compounds, mainly polyphenols. This study aims to valorize olive leaves through an efficient solid/liquid extraction of oleuropein, its main polyphenol, using green solvents and advanced technologies. Accordingly, three natural eutectic solvents formed with 1,2-butanediol and choline chloride, betaine, or proline, which incorporated water or ethanol as cosolvents, and ultrasound-assisted extraction to enhance the process were used in this work. Additionally, the effect of the cosolvent composition on the physical properties of the solvent mixtures (i.e., density and viscosity as a function of temperature) was evaluated. The extraction time was optimized for both conventional and intensified extractions, and the antioxidant activity of the extracts was assessed over time to determine their stability. Measurements through high-performance liquid chromatography and antioxidant activity assays concluded that ultrasound-assisted extraction using the solvent proline:1,2-butanediol prepared with ethanol at 25-75% composition yielded the best results at 37.00 mg of oleuropein per gram of dry sample (g/ds), obtained after just 1 min of intensified extraction, with a notable reduction in both time and energy consumption from conventional extraction, while providing significant antioxidant activity and stability.