Open field and vitro-derived plants of ‘Cascade’ hop: a comparison through total polyphenol content and antioxidant activity of vegetative biomass

Hop (Humulus lupulus L.) stores (both in cones and in the whole vegetative body) valuable bioactive metabolites such as terpenoids, phenolic compounds (i.e. xanthohumol), alkaloids and bitter acids (humulone and lupulone) with antioxidant, antimicrobial and antiviral properties. Characterizing hop vegetative biomass, considered a waste, is the first step for its exploitation as source of bioactive compounds, every day more requested by various industrial sectors (pharmaceutical, cosmetic, agri-food, etc.), thus turning a burden into an additional income for growers. Moreover, to guarantee a continuous supply of hop plant materials for stakeholders, in vitro-derived plantlets could be an important solution. With these premises, this study aims at comparing the chemical composition of open field- and vitro-derived hop vegetative biomass, since they synthetize the same bioactive compounds. Specifically, total polyphenol content (TPC) and antioxidant activity (AO) of leaves and stems (LS) and only leaves (OL) of hop, ‘Cascade', were measured. Moreover, water alone was tested as only solvent to test the sustainability of the process, in comparison to conventional extractions made with ethanol/water (80/20 v/v). Results showed that the open-field plants have significantly higher TPC and AO (measured by DPPH and FRAP tests) when compared to those from in vitro-derived plants. However, when the LS are taken in consideration, in vitro-derived plants showed statistically higher TPC than those from open field (6.20±0.91 vs 5.74±1.52 mg GAE g‑1). Regarding the extraction method, ethanol remains the most efficient solvent, but promising results were obtained also using water, both for the TPC (8.32±1.57 vs 5.73±1.10 mg GAE g‑1 respectively) and antioxidant activity. Results reported in this study, although preliminary, are very interesting as they highlight how vitro-derived hop plantlets are a wealth source of bioactive compounds; moreover, to make the extraction process more sustainable, further research are worth to be done to increase the efficiency of water as green solvent.