Ultrasound‐Assisted Natural Deep Eutectic Solvents as Separation‐Free Extraction Media for Hydroxytyrosol from Olives

Abstract A range of natural deep eutectic solvents (NADESs) as separation‐free media were applied as extraction solvents in combination with ultrasound‐assisted extraction (UAE) for the extraction of hydroxytyrosol from olive fruits. Following the investigation of 7 different NADESs, a tailor‐made DES composed of malic acid and betaine ( BeMa ) in a 1 : 1 molar ratio was screened for its efficiency. High performance liquid chromatography (HPLC) was used to test the content of hydroxytyrosol in the extraction. Response surface methodology (RSM) combined with central composite design (CCD) was used for the experimental design. The optimum extraction efficiency was obtained using a 1 : 20 sample: solvent mass/volume ratio upon sonication for 33.65 min in an aqueous solution containing 42.13 % (w/v) DES at 43.86 °C. In addition, the zebrafish embryo toxicity test showed that the selected DES exhibited essentially no biotoxicity, while BeMa was found to increase the antibacterial ability of the target subject. Our results indicated that NADESs are promising as green and safe solvents for the extraction and analysis of active substances from natural plants.