When wine turns to vinegar from contamination. More than an inch or two of air under the cork left in a standing bottle can lead to acetification.

The formation of vinegar, usually caused contamination of the must, liquor or finished product with vinegar-producing bacteria and the presence of air. Fermentation bottles should be filled as high as the froth caused by fermentation will allow. Stored wine should have no more than one inch of air under the cork in the standing bottle (1/2 inch is preferred). Adding one Campden tablet per gallon may halt acetification in its early stages, when the wine emits a slight smell of vinegar and an acid taste.