Stale beer is sometimes described as smelling like cardboard. Yet adding the compound responsible for that impression to fresh beer did not reproduce the full flavor after storage. A 1995 study explored how combining it with another compound brought the flavor closer to stale beer, and how that second compound formed during storage.
Connecting compounds that increase during storage with actual odor
The study focused on 2-furfuryl ethyl ether (2-FEE) and (E)-2-nonenal, known for its cardboard-like odor. The researchers quantified compounds previously linked to flavor changes during storage and tested their contribution by adding them to fresh beer. They also investigated the precursors of 2-FEE in model solutions and beer.
The storage experiment used Japanese bottled draft beer from one production lot, comparing a control kept at 0°C with samples stored in the dark at 37°C for up to four weeks. Analyses included gas chromatography, mass spectrometry, and liquid chromatography. Three experienced Asahi Breweries assessors evaluated the odor after taking the samples into their mouths.
Two compounds together came closer to stale flavor
Many of the compounds studied increased during storage. An increase, however, is not the same as reproducing stale odor. For example, adding furfural produced a sweet impression and linalool a powdery impression, but neither alone reproduced the target stale flavor.
Adding 2-FEE produced an astringent impression that represented part of the stale flavor. Here, “astringent” describes the odor after the sample was taken into the mouth. Adding (E)-2-nonenal produced a cardboard-like impression and a stronger stale character than 2-FEE alone. Still, neither single compound reproduced the full flavor after storage.
Adding the two compounds together strengthened the stale flavor further and brought it closer to the stored beer. The original addition test used 6 ppb of 2-FEE and 0.3 ppb of (E)-2-nonenal. A mixture of eight other compounds produced only a weak stale flavor. Even at trace concentrations, the combination of compounds mattered.
This was a descriptive sensory test involving few assessors. The finding that the two-compound combination was effective should be distinguished from statistical proof of synergy, or a rule explaining the stale odor of all beers with just these two compounds.
Precursors change during storage
Furfuryl acetate and furfuryl alcohol provided clues to the formation pathway. The researchers added furfuryl acetate to an acidic buffer and compared solutions with and without ethanol. After storage, the ethanol-containing solution contained furfuryl alcohol and 2-FEE. The solution without ethanol produced furfuryl alcohol, but no detectable 2-FEE. An ethanol-containing solution to which furfuryl alcohol was added directly also contained 2-FEE after storage.
Together, these findings suggest a pathway in which furfuryl acetate reacts with water to form furfuryl alcohol, which then reacts with ethanol to form 2-FEE. Experiments adding precursors thus provided support beyond simply observing two substances increase together.
In beer, adding furfuryl acetate and storing it at 25°C for 70 days produced a greater increase in 2-FEE than without the addition. Adding furfuryl alcohol and storing the beer at 37°C for six days also increased 2-FEE. The authors concluded that this pathway operates during beer storage. They also discussed the possibility that other reactions supply furfuryl alcohol, rather than establishing a single exclusive origin.
What the study shows about storage and aroma
Investigating flavor changes during storage benefits not only from identifying compounds that increase markedly, but also from adding them back to the original drink and tracing their precursors. This study connects all three approaches.
The main comparison used accelerated storage at 37°C, while a separate formation experiment included 25°C. These results cannot be used to calculate how many days the same changes take in a household refrigerator, or a shelf life applicable to all current products. The number of bottles and analytical replicates was not clearly reported either, so generalization must take the specific lot and experimental conditions into account.
Summary
Combining cardboard-like (E)-2-nonenal with the astringent impression of 2-FEE brought the flavor closer to stored beer than adding either alone. The findings also supported a pathway from furfuryl acetate through furfuryl alcohol, followed by reaction with ethanol to form 2-FEE. Understanding stale aroma requires looking beyond one prominent compound to combinations of compounds and reactions occurring during storage.
At the time of the study, the first author was affiliated with Asahi Breweries’ Nagoya Brewery, and experienced company assessors performed the sensory evaluation. The coauthors were affiliated with Meiji University and the University of Tokyo. No separate funding or conflict-of-interest declaration section was found in the original paper.
Reference
Harayama K, Hayase F, Kato H. Contribution to Stale Flavor of 2-Furfuryl Ethyl Ether and Its Formation Mechanism in Beer. Bioscience, Biotechnology, and Biochemistry. 1995;59(6):1144–1146. doi:10.1271/bbb.59.1144
