acetic acid, a simple two-carbon fatty acid which is the main flavour constituent responsible for the aroma and sour taste of vinegar. In wine it is the main component of what is called volatile acidity (VA).

Acetic acid is produced by a range of microbial activity including primary fermentation, malolactic conversion, and other fermentations carried out by spoilage organisms (lactic acid bacteria, acetic acid bacteria, and spoilage yeasts including brettanomyces). If wine is exposed to oxygen after fermentation, acetobacter can produce high levels of acetic acid from ethanol.