Hop-derived ingredients not only enhance the shelf-life and add a bitter taste to beer, but they also play a crucial role in affecting its aroma. An important hop odorant is linalool, characterized by its floral and citrusy fragrance.
Under the leadership of the Leibniz Institute for Food Systems Biology at the Technical University of Munich, a team of scientists has now disproved a roughly 20-year-old assumption about this odorant. The new study contributes to a better understanding of changes in beer bouquets during the brewing process and beer aging.
Two molecular variants of the odorant linalool are found in hops and beer: the enantiomers (R)- and (S)-linalool. Both molecules consist of the same number and type of atoms, and show the same connectivity. Nevertheless, they have different spatial structures and differ like an image from its mirror image. This “small” but nevertheless crucial difference is also reflected in the different odor intensities of the molecules.
In addition, it has long been known that beer aroma changes during the brewing process and storage because, among other things, part of the (R)-linalool predominant in hops is converted to (S)-linalool.
Previously, based on a 1999 paper, researchers assumed that the odor threshold concentration of (R)-linalool is about a factor of 80 lower than that of (S)-linalool. Put simply, they assumed that (R)-linalool has a much stronger influence on beer aroma than its mirror-image counterpart. However, reliable data on the odor thresholds of both substances were lacking.
Preparative method optimized
To close this knowledge gap and enable more precise predictions of changes in beer aroma, the team led by brewing and beverage technologist Klaas Reglitz and food chemist Martin Steinhaus from the Leibniz Institute first optimized a preparative method. In close cooperation with the Research Center Weihenstephan for Brewing and Food Quality, the researchers thus succeeded for the first time in isolating enantiomerically pure (S)-linalool.
Having the pure substance available in sufficient quantities was an indispensable prerequisite for determining the specific odor threshold concentrations of the two odorant variants in water and unhopped beer using a trained sensory panel. This was essential because only (R)-linalool is commercially available as a pure substance.
As the team showed, the thresholds of (R)- and (S)-linalool in water were 0.82 and 8.3 micrograms per kilogram, respectively. In unhopped beer, the team determined thresholds of 6.5 micrograms per kilogram for (R)-linalool and 53 micrograms per kilogram for (S)-linalool.
Influence of (R)-linalool overestimated
“Our results thus confirm the previously postulated higher odor potency of (R)-linalool. However, they also refute the previous assumption that the odor threshold concentrations of the two enantiomers differ extremely. Instead, the study shows that the difference is only about eight to tenfold,” says Martin Steinhaus, head of Section I and the Food Metabolome Chemistry Research Group at the Leibniz Institute.
First author Klaas Reglitz adds, “The conversion of (R)- to (S)-linalool thus does not have as great an influence on beer aroma as has long been assumed. Thanks to our study, we now better understand how and why the aroma changes during storage.”
Reference: “Enantiospecific determination of the odour threshold concentrations of (R)- and (S)-linalool in water and beer” by K. Reglitz, J. Stein, J. Ackermann, V. Heigl, L. Brass, F. Ampenberger, M. Zarnkow and M. Steinhaus, July 2023, Brewing Science.