Enhanced-performance extraction of olive (Olea europaea) leaf polyphenols using L-lactic acid/ammonium acetate deep eutectic solvent combined with β-cyclodextrin: screening, optimisation, temperature effects and stability

This investigation had as objective to study the effective recovery of olive leaf (OLL) polyphenols using a blend of an L-lactic acid/ammonium acetate deep eutectic solvent (DES) with β-cyclodextrin (β-CD). Following an initial DES screening, the extraction process was optimised with response surface methodology by implementing Box-Behnken design. The optimised extraction conditions thus determined were stirring speed 300 rpm, DES concentration 56.4% (w/w), liquid-to-solid ratio 100 mL g and β-CD concentration 0.7% (w/v). Maximum extraction yield (113.66 mg caffeic acid equivalents g dry mass) was achieved at 80 °C, without compromising antioxidant activity. Comparative assessment of the extraction medium DES/β-CD with other green solvents showed that it was a high-performing system providing polyphenol-enriched extract with improved antioxidant characteristics. Stability test of the extract obtained under optimal conditions performed over a period of 30 days at ambient conditions revealed that, while ferric-reducing power fluctuated within narrow limits, the antiradical activity was enormously enhanced by 100%. These changes in the antioxidant activity were not associated with the decline of major polyphenols in the OLL extract. However, the overall polyphenolic profile remained largely unchanged. It is suggested that the DES/β-CD is an efficient green solvent that may confer the OLL polyphenol extract satisfactory stability.