Polyphenols from Slovenian hops (Humulus lupulus L.) of the Aurora variety were extracted by different methods and using classical solvents and several deep eutectic solvents (DES) based on choline chloride as the hydrogen bond acceptor component. The obtained extract solutions were analyzed by HPLC for the content of extracted α- and β-acids and extracted xanthohumol. It was found that choline chloride:phenol DES concentrated aqueous solution had an extraction efficiency close to that of diethyl ether, which is considered one of the best classical extraction solvents for polyphenols from hops. The comparison of the extraction efficiency with other choline chloride-based DESs showed that the chemical similarity of the phenol ring in the solvent DES with the polyphenols in hops may be crucial for a highly efficient extraction with choline chloride:phenol DES. On the other hand, the choice of extraction method and the viscosity of the solvents tested seem to play only a minor role in this respect. As far as we know, this is the first study to attempt to relate extraction efficiency in the extraction of hydrophobic solutes to the compressibility of the DES extractants, the latter of which may be correlated with the extent of hydrophobic hydration around the DES components. In addition, using the heating and stirring method for the preparation of choline chloride-based DES concentrated aqueous solutions we found no support for the occurrence of water in two different roles (in the structural and in the dilution role) in these solvents.

The efficiency of six deep eutectic solvents (DESs) based on choline chloride (ChCl) and various hydrogen bond donors (HBDs) was evaluated against a traditional organic solvent for extracting polyphenolic bioactive compounds from three different white mulberry samples (Morus alba), including branches, leaves, and fruits. Ultrasound-assisted extraction was performed under selected conditions identified for ChCl/glycerol DES: a 1:2 molar ratio of hydrogen bond acceptor to HBD, 20% water added to the DES, a temperature of 80 °C, and an extraction time of 30 min, providing a set of standard parameters for comparing the efficiency of different DESs. Extraction efficiencies were assessed using a developed and validated HPLC method, as well as total phenolic content and total flavonoid content assays. Among the tested DESs, those composed of ChCl and polyalcohols as HBDs showed the best performance. For branch and leaf samples, the ChCl/glycerol DES was the most effective, while for fruit samples, the ChCl/ethylene glycol DES showed the highest efficiency. In most polyphenol extractions tested, at least one DES achieved extraction efficiencies comparable to or higher than those obtained with methanol, except for flavonoids, for which DES yields were often lower. Overall, the results indicate that using DESs represents a greener and more sustainable approach to extracting bioactive compounds from white mulberry.