Heat-Induced Changes in the Composition of Varietal Hop Essential Oils via Wort Boiling on a Laboratory Scale

We previously demonstrated that boiling of hop essential oil in water transforms sesquiterpene hydrocarbons into oxidation products (oxygenated sesquiterpenoids), which impart typical spicy/woody scents associated with "kettle hop" aroma when added to beer. In this study, boiling experiments in water as well as in wort with increasing hop oil (cv. Saaz)concentrations and subsequent headspace solid-phase microextraction GC-MS analysis revealed a positive correlation between the initial added hop oil concentration and formation of oxygenated sesquiterpenoids. Intervarietal differences were investigated by comprehensive GC-MS fingerprinting of hop-derived volatiles in unboiled and boiled hop essential oils (cv. Hallertau Magnum, Hallertau Tradition, Hallertau Perle, and Saaz) in wort. Unboiled essential oils of European aroma hops were shown to contain significantly higher levels of α-humulene, β-caryophyllene, and sesquiterpene oxidation products compared with cv. Hallertau Magnum. However, changes in the volatile profile upon boiling in wort appeared largely variety independent (i.e., chemically identical α-humulene and β-caryophyllene derivatives were formed). GC-olfactometry further showed that some of these derivatives formed upon boiling (i.e., 6(5→4)-abeocaryophyll-8(13)-en-5-al, caryophyllene oxide, humulene epoxide III,humulenol II, caryophylla-4(12),8(13)-diene-5α/β-ol, 3Z-caryophylla-3,8(13)-diene-5α/β-ol, and 14-hydroxy-β-caryophyllene) eluted in flavor-active zones.