4-Ethylguaiacol

4-Ethylguaiacol, often abbreviated to 4-EG, is a phenolic compound with the molecular formula C₉H₁₂O₂ that's a key aromatic component in various products, from food and beverages to cosmetics and pharmaceuticals. It can be produced in wine and beer by Brettanomyces. It is also frequently present in bio-oil produced by pyrolysis of lignocellulosic biomass.

Winemaking

It is produced along with 4-ethylphenol in wine and beer by the spoilage yeast Brettanomyces. When it is produced by the yeast to concentrations greater than the sensory threshold of >600 μg/L, it can contribute bacon, spice, clove, or smoky aromas to the wine. On their own these characters can be quite attractive in a wine, however as the compound usually occurs with 4-EP whose aromas can be more aggressive, the presence of the compound often signifies a wine fault. The ratio in which 4-EP and 4-EG are present can greatly affect the organoleptic properties of the wine.

4-Ethylguaiacol can also be produced by pyrolysis of lignocellulosic biomass. It is produced from the lignin, along with many of the other phenolic compounds present in bio-oil. In particular, 4-ethylguaiacol is derived from guaiacyl in the lignin.