Exploring the potentail of microwaves and ultrasounds in the green extraction of bioactive compounds from Humulus lupulus for the food and pharmaceutical industry

Currently, there is a continuous growth in demand for medicinal and aromatic plant derived molecules obtained in a sustainable and eco-friendly way. Among the medicinal plants that have a plethora of health-related properties, there is definitely hops. In the present study, microwaves (MW) and ultrasounds (US) were used to increase the extraction efficiency of bioactive compounds from hop cones using water, ethanol and their mixture as food-grade solvents. A multidisciplinary approach was used for the in-depth characterization of green extracts. The bioactive components were determined by chemical methods, using both spectrophotometric assays as well as high-performance liquid chromatography with diode-array detection (HPLC-DAD) analysis of individual phenols, bitter acids and xanthohumol, and results were compared with those from a control extract, obtained using aqueous methanol (80 %) acidified with HCl (0.1 %). Moreover, we applied proton nuclear magnetic resonance (1H-NMR) spectroscopy to reveal insights on the different composition in terms of primary and secondary metabolites of the green extracts analyzed. Generally, microwave-assisted extraction (MAE) of hop cones with ethanol showed the strongest extraction power for almost all the bioactive compounds investigated, as well as the shortest extraction time (1 min) and the highest antiradical capacity. 1H-NMR analyses showed an overall greater influence of solvent polarity, rather than extraction technique, on the average composition of the final extract. 1H-NMR fingerprinting of hop aqueous extracts highlighted as main components sugars and amino acids, as well as other metabolites including trigonelline, an alkaloid with interesting biological properties. However, the two extraction techniques significantly affected the contribution of signals in the terpene or aromatic regions when ethanol and ethanol:water were used as solvents, proving a better extraction efficiency for these compounds of MW than US. Regardless the extraction technique and solvent used, the most abundant polyphenol compound in the green extracts analyzed was catechin, ranging from 2989 ± 10 to 14009 ± 248 μg g−1 (for UAEETOH and MAEETOH, respectively), followed by rutin and chlorogenic acid. 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2′-Azinobis [3-ethylbenzothiazoline-6-sulfonic acid]-diammonium salt (ABTS) radical scavenging methods were used to evaluate the biological properties of green hop extracts, showing a superior free-radical scavenging capacity in MW irradiated ethanol samples compared to other samples tested. The inter-relationships between the parameters analyzed and the different extraction conditions, as well as the relationships among variables, were investigated by principal component analysis.