Relatively frequent and dangerous wine defect (also acetic acid stain, volatile acid) caused by excessive amounts of acetic acid in the wine. The starting point of the process is Acetobacter (acetic acid bacteria), which can already form on injured or cracked grapes in the vineyard and get into the grape must. The vinegar fly also plays a role in transmission and spread. These bacteria form acetic acid from alcohol via the intermediate product acetaldehyde. Lactic acid bacteria can also be the cause. Incompletely filled wine containers with an empty space favour the process of acetic acid formation through the influence of oxygen. The acetic souring occurs most frequently during the mash fermentation of red wines, because the relatively high amount of oxygen in the mash intensifies the process.
The wine is highly coloured (brown tinge), and sometimes a slimy, slightly cloudy film forms on the surface. It smells unpleasantly sour and pungent like vinegar, the taste is acid-sharp. Such a wine is described as volatile when tasted. The best precaution is the most scrupulous hygiene, avoidance of oxygen ingress and sulphurisation of the mash. In wine, it is difficult to combat the defect, which is attempted by reverse osmosis and then ion exchange. If the wine defect Uhuton occurs due to large amounts of acetic acid ethyl ester through the reaction of acetic acid with alcohol and the metabolic products formed in the process, the acetic souring is sensory intensified. See also under vinegar.