4-Ethylphenol
4-Ethylphenol (4-EP) is an organic compound with the formula CHCHOH. It is one of three isomeric ethylphenols. A white solid, it occurs as an impurity in xylenols and as such is used in the production of some commercial phenolic resins. It is also a precursor to 4-vinylphenol. Natural occurrences. In wine and beer, 4-EP is produced by the yeast "Brettanomyces". At concentrations greater than 140 µg/L (typical sensory threshold) it gives 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 biosynthesized in two steps from "p"-coumaric acid. Decarboxylation gives  4-vinylphenol as catalyzed by 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.