Efficient recovery of antioxidants from olive leaves through green solvent extraction and enzymatic hydrolysis: Experimental evaluation and cosmo-rs analysis

The present study focuses on the valorization of olive leaves (Olea europaea L.) through the recovery of high-value antioxidants using green solvents (i.e., bio-based solvents (BioS) and natural eutectic solvents (NAES)) while evaluating the implementation of an enzymatic hydrolysis step for biomass pretreatment. Overall, 12 BioS including water, alcohols, ethers, esters, and terpenes, and 7 NAES composed of choline chloride, proline, and betaine combined with polyols, were studied as green solvents for the solid–liquid extraction of antioxidants from olive leaves. NAES generally performed better than BioS, with Proline:1,2-butanediol [1:4] being the most efficient, yielding 75 mg of total polyphenols/g of dried sample in the non-pretreated extract. As for the composition, 95 % of the non-pretreated extract was constituted by oleuropein, while, on average, 65 % of the pretreated extract was found to be a mixture of hydroxytyrosol and oleuropein. In all cases, the polyphenols extracted were determined by high-performance liquid chromatography. Finally, the experimental results were analyzed using COSMO-RS methodology, obtaining reliable solubility trends for target antioxidants in the proposed green solvents, while explaining the thermodynamic interactions of the systems in terms of molecular behavior. This work can serve as a guideline to develop flexible biorefinery schemes for the production of oleuropein and hydroxytyrosol based on different extraction approaches, with in silico tools providing further insights into the role of novel solvents in olive-derived polyphenols recovery processes.