Acetic acid, often referred to as Volatile Acidity, results from the oxidation of ethanol by bacteria and yeasts, and can be formed either as a by-product of fermentation or due to spoilage of finished wine. It is the product of the further oxidation of acetaldehyde. Acetic acid bacteria, lactic acid bacteria, Brettanomyces and several yeasts found in the vineyard can all produce acetic acid. Of the acetic acid bacteria, Acetobacter and Gluconobacter produce the highest levels of acetic acid.
The sensory threshold for acetic acid is between 600 and 900 mg/L. At very low levels, acetic acid can add perceived complexity to a wine, but it is generally considered a fault, with concentrations above 1.2 g/L becoming unpleasant. At this level, the wine begins to acquire a vinegar-like character. Acetic acid is associated with a thin, sharp mouthfeel and a pungent aroma reminiscent of vinegar.