4-Ethylphenol

4-Ethylphenol (4-EP) is a phenolic compound.
Natural occurrences.
In wine and beer, 4-EP is produced by the yeast "Brettanomyces". When it reaches concentrations greater than the sensory threshold (140 µg/L) it can give the wine aromas described as "barnyard", "medicinal", "band-aids", and "mousy". In certain Belgian beer styles, a high 4-EP level may be desirable; however, very high levels of the compound in wine can render it undrinkable. The level of 4-EP is roughly proportional to "Brettanomyces" concentration and activity, and can therefore be used as an indicator of the yeast's presence. There are significant differences between strains of "Brettanomyces" in their ability to produce 4-EP.
4-EP is also a component of castoreum, the exudate from the castor sacs of the mature North American beaver ("Castor canadensis") and the Eurasian beaver ("Castor fiber"), used in perfumery.
Biochemistry.
4-EP is produced from the precursor "p"-coumaric acid. "Brettanomyces" converts this to 4-vinylphenol via the enzyme cinnamate decarboxylase. 4-Vinylphenol is further reduced to 4-ethylphenol by the enzyme vinyl phenol reductase. Coumaric acid is sometimes added to microbiological media, enabling the positive identification of "Brettanomyces" by smell.