M-19: The hunt for “hoppy” compounds: Formation and flavor-activity of hop-derived sesquiterpene oxidation products

T. PRAET (1), F. Van Opstaele (1), G. Aerts (1), L. De Cooman (1); (1) KU Leuven, KAHO Sint-Lieven, Ghent, Belgium

Raw Materials I
Friday, June 6 - 4:00 p.m.-5:45 p.m.
Level 4, Grand Ballroom

The “kettle hop” or “hoppy” aroma, one of the most complicated flavor characteristics of lager beer, has been a topic of interest for decades among both brewers and researchers. However, the knowledge on the hop-derived flavor-active constituents that play a role in hoppy aroma is still far from complete. This can be attributed to changes in the hop oil-derived volatile profile induced by several process steps such as wort boiling and fermentation. The oxygenated sesquiterpenoids in particular have been suggested to be correlated with the spicy aspect of hoppy aroma. These compounds arise when sesquiterpene hydrocarbons are oxidized during aging of hops, as well as in the case of kettle-hopping. To gain insight into the development of hoppy aroma, we here present a methodology in which hop essential oil is fractionated via solid-phase extraction (SPE) to obtain pure sesquiterpene hydrocarbons, which are subsequently boiled in an aqueous solution to mimic wort boiling. The resulting oxygenated sesquiterpenoids are then isolated by SPE and added to pitching wort (brewed on a 5 hL pilot scale), which is allowed to ferment on lab scale, in order to elucidate their behavior during fermentation. Samples were taken before and after fermentation, after maturation, and after centrifugation. About 30 oxygenated sesquiterpenoids were formed upon boiling of the pure sesquiterpene hydrocarbons. After isolation of these sesquiterpenoids, we obtained a fraction with a composition similar to the hop oil-derived volatile profile of lager beer, proving the relevance for brewing practice. Addition of this fraction to non-aromatized iso-alpha-bittered beer caused a shift in the aroma profile in favor of “woody,” “hay,” “hoppy,” and “spicy” notes. The fraction was also added to pitching wort. After fermentation, the change in the sesquiterpenoid-derived volatile profile could mainly be attributed to huge losses, probably due to adsorption to the yeast. However, some of the sesquiterpenoids were still detected via GC-O, even after this strong decline in their level, proving the flavor-activity of these compounds. The decrease in sesquiterpenoid level after maturation and centrifugation was less pronounced. The compounds that survived centrifugation are also found in commercial lager beers. In conclusion, our innovative approach delivers a new hop oil-derived oxygenated sesquiterpenoid fraction with clear sensorial impressions and evidence for the flavor-activity of individual oxygenated sesquiterpenoids. Further research will focus on application of this boiled hop oil fraction in brewing practice.

Tatiana Praet (born 1987) is a Ph.D. student in the Department of Biochemistry at the University of Leuven (KU Leuven, Belgium). She received a degree in industrial engineering in 2011. Currently, she is performing research in the area of brewing technology at the Laboratory for Enzyme, Fermentation and Brewing Technology at KAHO Sint-Lieven. Her work is focused on flavor-active hop-derived compounds that contribute to beer flavor. She is interested in application of analytical methods to hop research, specializing in gas chromatographic techniques (HS-SPME-GC-MS, GC-FID, GC-PFPD) and sensorial evaluation (GC-O, taste panel). Tatiana acquired an IWT grant from the Flemish government aimed specifically at research with significant practical applicability and the Barth-Haas Grant, awarded to students for relevant and innovative hop research.

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