The beer brewing waste spent hops (Humulus lupulus H.) as a valuable source of repellents useful to ward off insect attacks to stored food

Hop, Humulus lupulus L., is an aromatic plant largely utilized in the brewing industry to add flavour and bitterness to beer. As a consequence, a large amount of residual material, named “spent hops”, is produced by the brewing industry. Since only about 15% of the hop constituents end up in the beer, spent hops should be further valorised as a low-cost source of bioactive substances. In this study, hops and spent hops were evaluated as a source of essential oil (EO) with repellent activity against two of the main stored food insect pests. Hops and spent hops EO yields were about 2.4% and 0.1%, respectively. The main components of both the EOs were the terpenes myrcene, α-humulene, and β-caryophyllene. The main chemical difference between hops and spent hops was the myrcene percentage. Area preference method bioassays showed that spent hops EO exerted a strong repellency against the lesser grain borer, Rhyzopertha dominica (F.) (Bostrichidae) and the granary weevil, Sitophilus granarius (L.) (Curculionidae). RD50 values indicated that R. dominica was about 20 time more susceptible to spent hops EO than S. granarius. Among the EO components, myrcene was the strongest repellent against R. dominica while limonene was the most effective compound against S. granarius. However, overall, for its much higher content, myrcene can be considered the compound that mostly contributes to the repellent activity of spent hops EO. These results show that spent hops could be an excellent, virtually costless, source of EO to be utilized as eco-friendly repellent in the protection of stored food from insect pests.