T. PRAET (1), F. Van Opstaele (1), G. Aerts (1), L. De Cooman (1); (1) KU Leuven, Ghent, Belgium
Technical Session 3 - Hop Aroma
Monday, June 15
8:30–10:15 p.m.
Fiesta 3,4,6,8
Although the “dry hopping” process is gaining popularity, lager beers with a refined “kettle hop aroma,” imparted by (early) addition of hops to the boiling kettle, are still widely consumed. This highly desired spicy/herbal top note has been correlated to sesquiterpene oxidation products (SOPs), formed during ageing of hops. SOPs are also presumed to arise during kettle boiling by oxidation of sesquiterpene hydrocarbons (SHCs). However, whether SOPs are actually formed upon boiling, and how exactly they contribute to “kettle hop aroma,” is, despite the efforts of many scientists, still relatively unclear. Therefore, we aimed at obtaining scientific solid insights into formation and flavor-activity of these SOPs upon lab-scale boiling, and at investigation of the validity of these findings in real brewing practice. Hop essential oils (cv. Saaz, Hallertau Tradition, Perle, Magnum) and SHC fractions (isolated via solid phase extraction [SPE], cv. Saaz) were boiled (lab scale) in both aqueous and wort media and samples were analyzed via HS-SPME-GC-MS for comprehensive characterization of volatile hop-derived profiles. For sensory evaluation, GC-olfactometry and descriptive analysis (trained panel) were combined. The SOP fingerprint of commercial kettle-hopped American beers was recorded and flavor-active constituents were determined (GC-O). Specific hopped (cv. Saaz) lager beers were prepared at our pilot plant (4 hL) and samples were screened for SOPs to map their formation and behavior along the brewing process. We statistically demonstrated a discrimination between unboiled and boiled hop oil and established a general increase in the level of SOPs upon boiling, which is attributed to both quantitative (i.e. increased levels, typical α-humulene and β-caryophyllene derivatives) and qualitative (i.e. newly formed compounds) changes in the hop-derived volatile profile. Interestingly, SOP formation is clearly positively correlated with the initial boiled hop oil concentration (e.g. 10 g/L hop essential oil cv. Saaz: SOPs recovery up to 258% ± 23 [n = 3]) and independent of the investigated hop variety. SPE isolation of SOPs after boiling of a SHC fraction unambiguously proved oxidation of SHCs. Via GC-O, 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-caryohyllene were detected in flavor-active zones and non-aromatized iso-α-bittered lager beers, spiked with the SOP fraction, were described as “hoppy” and “spicy.” Remarkably, the newly formed compounds and constituents, characterized by increased levels, were also detected in commercial kettle-hopped American beers and the same α-humulene and β-caryophyllene oxidation products were found to be flavor-active in these beers upon GC-O analyses. In addition, pilot brews indicate that “early” kettle hopping does not lead to losses of all hop oil compounds, as believed by many brewers, but instead introduces SOPs to wort. In conclusion, the flavor-active SOPs are candidate key character impact compounds for “kettle hop aroma.” Insights into development of “kettle hop aroma” is of importance for both hop scientists and brewers and should lead to more deliberate hop additions with respect to the desired beer flavor.
Tatiana Praet (born 1987) is a Ph.D. student at the Catholic University of Leuven (KU Leuven, Belgium). In 2011, she received a master’s degree in industrial engineering (biochemistry) at KAHO Sint-Lieven. Currently, she is performing research in the area of brewing technology at the Laboratory of Enzyme, Fermentation and Brewing Technology, focusing on aroma-active hop-derived volatiles that contribute to beer flavor. Tatiana acquired an IWT-grant from the Flemish Government (2011) and the Barth-Haas Grant (2012) and received the MBAA Presidential Award (2014).
