A comparison of antioxidant and antimicrobial activity between hop leaves and hop cones

Abstract

The leaves of the hop plant (Humulus lupulus L.) are an agricultural by-product that is not currently being exploited. This study compared two hop cultivars cv. ‘Aurora’ and cv. ‘Hallertauer Magnum’ from four different hop-growing regions (Žalec, Slovenia; Leutschach (Kranach), Austria; Hüll, Germany; Žatec, Czech Republic). The leaves and cones were collected and their total phenolics and the antioxidative and antimicrobial activities of their ethanol extracts were determined. Samples were collected three years in succession (2008–2010). Total phenolics ranged from 0.099 to 0.542mgCAE/mL for the leaf extract and from 0.738 to 1.734mgCAE/mL for the cones, which had both higher levels and greater variability of phenolics. The leaves had much lower DPPH radical scavenging activity. Their IC50 of approximately 0.020mg/mL was much higher than the cones’ extracts (0.005 to 0.010mg/mL) regardless of the year and of the growing location. The best reducer was the extract from the Aurora leaves collected in the Czech Republic in 2010, which reduced 0.117mL/μg of ferric ions in 25min. Antimicrobial activity against gram positive Staphylococcus aureus was extraordinary for all hop cones extracts (minimal inhibitory concentrations, MICs<0.003mg/mL), while moderate antimicrobial activity (MICs>0.16mg/mL) against gram negative Escherichia coli O157:H7 was observed for hop cones and leaves extracts. The results of the profile analysis showed the caffeic acid peak at tr=35.1min for the leaves, while the cones had no such peak at that retention time.