Among processes that deteriorate wine quality, the production of volatile phenols by Brettanomyces species is widespread and increasingly problematic. These compounds are characterised in particular by aromas of ink or glue, and horse sweat, leather or stable taints.
Volatile phenols, mainly 4-ethylphenol and 4-ethylguaiacol, are produced from p-coumaric acid and ferulic acid respectively after enzymatic decarboxylation (cinnamate decarboxylase, PAD) and reduction (vinylphenol reductase, VPR). These precursors are naturally present in grape musts. The decarboxylation step caused by cinnamate decarboxylase activity was described in many bacteria, yeast and fungi species, while the reduction step caused by vinylphenol reductase activity, or VPR, is more specific to the Brettanomyces/Dekkera species.
As Brettanomyces are present on grapes and on winemaking equipment, grape musts can be contaminated at a very early stage. However, these yeasts generally proliferate after alcoholic and/or malolactic fermentations, during wine maturing or after packaging.