Tailloring deep eutectic solvents for the extraction of valuable compounds from natural sources using choline chloride and carboxylic acids mixtures: optimization of the extraction of phenolic compounds from Juglans regia L. leaves

Phenolic compounds are a wide group of plant secondary metabolites with increasing interest due to their antioxidant ability and marked effects on oxidative processes related to several chronic diseases. Nowadays, they find applications in pharmaceutical products, functional foods and natural-based cosmetics. Therefore, the extraction and identification of these valuable compounds from different plants have become a major research area. Nevertheless, conventional solid-liquid extraction of phenolics usually involves the use of flammable, toxic and volatile organic solvents. Deep eutectic solvents (DES) are gaining much interest as alternative solvents to extract valuable compounds from natural matrices. DES can be considered “designer solvents” due to the possibility of combining different HBA (hydrogen bond acceptor) and HBD (hydrogen bond donor) to obtain solvents with specific properties to the target application. In this study, DES composed of choline chloride (CC) and different groups of organic acids were prepared, in order to extract phenolic compounds from leaves of Juglans regia L. (walnut leaves). The initial screening involved monocarboxylic (acetic, propionic, butyric, valeric, lactic and glycolic acids), dicarboxylic (malonic, glutaric and malic acids), tricarboxylic (citric) and aromatic acids (phenylacetic acid, 3-phenylpropanoic acid). The initial extraction conditions were: 50 ºC, 60 min extraction time and 20% water content. The main phenolic compounds (neochlorogenic acid, quercetin 3-O-glucoside and quercetin O-pentoside) were quantified by high performance liquid chromatography coupled to a diode de array detector (HPLC-DAD at 280 and 370 nm). Higher extraction yields were obtained using CC:butyric acid and CC:3-phenylpropanoic acid. For these systems, the stoichiometric ratio of HBA and HBD as well as the water content in the DES solutions were further evaluated. Better results were obtained using DES compared to the conventional water + ethanol mixed solvent. The present work contributes to the valorization of walnut leaves extracts using alternative solvents that could be tailored for potential applications in the food and pharmaceutical areas.The authors thank the Foundation for Science and Technology (FCT, Portugal) and FEDER under Programme PT2020 for financial support to CIMO (UID/AGR/00690/2013), L. Barros (SFRH/BPD/107855/2015) and V. Vieira (SFRH/BD/108487/2015) grants. To POCI-01-0145-FEDER-006984 (LA LSRE-LCM), funded by ERDF, through POCI-COMPETE2020 and FCT. To Xunta de Galicia for financial support for the post-doctoral researcher of M.A. Prieto.