Hop (Humulus lupulus L.) volatiles variation during storage

Storage conditions play an important role in maintaining the integrity of hop products during processing, transport and storage, and their ultimate quality when used to brew beer. Several groups have investigated variation in alpha acids, beta acids and polyphenols under different storage conditions, but there are few studies directly linking changes in hop acid composition under pro-oxidative conditions, to either: (i) industry standard measures of the oxidative quality of hop alpha acids (hop storage index [HSI]); or (ii) changes in the volatile composition of hops. The aim of this work was to present useful information regarding the variation in hop acids and volatiles in different storage conditions, such as temperature and exposure to ambient air. A comparison between pellets and pressed hop flowers was investigated. Long term storage in oxidizing conditions can lead not only to a decrease in the essential oil content, in particular at the expense of the more volatile fraction, but also to the degradation and/or transformation of some of the main compounds to their oxidized forms. With this work, we hope to give some insight into the inter-relationship of measurable hop quality parameters (such as HSI, hop acid chemistry and volatile chemistry), as well as demonstrate how easily hop chemistry can be affected by improper storage conditions.