Lactic acid-based deep eutectic solvents for the extraction of bioactive metabolites of Humulus lupulus L.: supramolecular organization, phytochemical profiling and biological activity

In the present study, natural deep eutectic solvents were used to increase the extraction efficiency (EE) of bioactive compounds from hop (Humulus lupulus L) cones, under ultrasounds irradiation. Three lactic acid-based deep eutectic solvents (LaDES) were developed, physicochemical characterized and their EE values were compared with those of a control extract, obtained using aqueous methanol (80%) acidified with HCl (0.1%). The supramolecular organization of LaDES and their extracts were analysed using FTIR spectroscopy and Raman spectroscopy to clarify the interactions among LaDES components and between LaDES and the bioactive components of the plant matrix. The phytochemical profiles of different green extracts were evaluated by chemical methods, using both spectrophotometric assays and HPLC-DAD analysis of individual phenols and bitter acids, and the results were compared with those from the control samples. LaDES showed remarkable effects on the EE of all phytochemicals considered in the study. Among the different combinations of LaDES tested, lactic acid: sucrose showed the highest EE against hop bitter acids, while lactic acid: urea extracted more non-phenolic pigments than the other two LaDES combinations. Polyphenols were extracted better by lactic acid: glycine than the other solvents tested, also showing the highest antiradical potential in vitro tests. Finally, all LaDES inhibited both Gram- positive and negative bacteria, while non activity was recorded against tested yeasts.