The aroma of beer is influenced by many factors, but one of the most important is the presence of hop-derived compounds. These compounds not only make beer bitter and extend its shelf life, but also give it a distinctive smell. One of these compounds is linalool, which has a pleasant floral and citrus-like aroma.
A group of scientists from the Leibniz Institute for Food Systems Biology at the Technical University of Munich has recently challenged a long-held belief about this compound. Their study helps to understand how the aroma of beer changes during the brewing process and over time.
Linalool exists in two forms that are mirror images of each other: (R)- and (S)-linalool. They have the same atoms and bonds but different shapes. This makes them smell differently, with (R)-linalool being more potent than (S)-linalool.
It is known that during brewing and ageing, some of the (R)-linalool in hops is converted to (S)-linalool, which affects the aroma of beer. However, until now, there was no reliable data on how much each form of linalool contributes to the smell of beer.
To fill this gap, the researchers led by Klaas Reglitz and Martin Steinhaus from the Leibniz Institute developed a method to isolate pure (S)-linalool, which is not commercially available. They worked with the Research Centre Weihenstephan for Brewing and Food Quality to achieve this goal.
They then used a trained sensory panel to measure the odour threshold concentrations of both forms of linalool in water and unhopped beer. The odor threshold concentration is the lowest amount of a compound that can be detected by smell.
They found that the odour threshold concentrations of (R)- and (S)-linalool in water were 0.82 and 8.3 micrograms per kilogramme, respectively. In unhopped beer, they were 6.5 and 53 micrograms per kilogramme, respectively.
This means that (R)-linalool is indeed more potent than (S)-linalool, but not as much as previously thought. The difference between them is only about eight to ten times, not 80 times, as suggested by a 1999 paper.
“Our results confirm the higher odour potency of (R)-linalool, but also show that the previous assumption was greatly exaggerated. We now have a better understanding of how and why the aroma of beer changes during storage,” says Martin Steinhaus, head of Section I and the Food Metabolome Chemistry Research Group at the Leibniz Institute.
Klaas Reglitz, the first author of the study, adds, “The conversion of (R)- to (S)-linalool does not have such a big impact on beer aroma as previously assumed. Our study provides more accurate predictions of changes in the beer bouquet.”
Linalool is a common compound in many essential oils, often as a main component. It is found in various spices, such as coriander, star anise, hops, nutmeg, ginger, basil, and many others. Linalool is used both as a part of essential oils and as a pure compound for flavouring purposes.
Hops (Humulus lupulus) are mainly used for making beer. The female hop plants produce cones that are used for brewing. In 2022, Germany had about 20,600 hectares of hop fields, which is about one-third of the world’s hop area. Germany is one of the leading hop producers and exporters in the world. In 2023, Germany exported about 30% of its hop production to other countries.
Linalool is not only important for beer aroma but also for other aspects of beer quality. For example, a recent study found that linalool can enhance foam stability and reduce haze formation in beer. Another study showed that linalool can inhibit bacterial growth and biofilm formation in beer. These findings suggest that linalool has potential applications for improving beer quality and safety.
Reference:
Reglitz, K. et al, Enantiospecific determination of the odour threshold concentrations of (R)- and (S)-linalool in water and beer. BrewingScience (2023). DOI: 10.23763/BrSc23-07reglitz
