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

• Leaves of the hop plant (Humulus lupulus L.) are an agricultural by-product.
• Two hop cultivars from four different hop-growing regions in Europe were studied.
• The leaves had 3-fold to 30-fold less total phenolics than the cones.
• The leaves had much lower DPPH radical scavenging activity than the cones’ extracts.
• All hop cones extracts had extraordinary antimicrobial activity against gram positive Staphylococcus aureus.

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 0.542 mg CAE/mL for the leaf extract and from 0.738 to 1.734 mg CAE/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 0.020 mg/mL was much higher than the cones’ extracts (0.005 to 0.010 mg/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 0.117 mL/μg of ferric ions in 25 min. Antimicrobial activity against gram positive Staphylococcus aureus was extraordinary for all hop cones extracts (minimal inhibitory concentrations, MICs < 0.003 mg/mL), while moderate antimicrobial activity (MICs > 0.16 mg/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 = 35.1 min for the leaves, while the cones had no such peak at that retention time.

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