Acetic Acid, the acid contained in vinegar (at 3%–6%), is also the main volatile fatty acid in alcoholic beverages. As the main acid generated from fatty acid metabolism in yeast, it is a key component (along with ethanol) in the generation of ethyl acetate, which is the most common flavor ester in beer. See ethyl acetate. Acetic acid (CH3COOH) is a weak, monocarboxylic volatile fatty acid produced by yeast as a natural by-product of metabolism, but it may also be produced by Acetobacteria (spoilage organisms). Acetobacter (acetic acid or vinegar bacteria), a genus of aerobic bacteria, can turn ethanol to acetic acid during fermentation if excessively aerated. This acid can also be generated from the oxidation of acetaldehyde and is thereby involved in complex reactions of flavor-generation during wood maturation of beer.

If present in amounts above the perception threshold, acetic acid conveys a sour vinegar taste and aroma to beer. However, this is not usually an issue in brewing, as most beers are produced under stringent quality controls to minimize both contamination and oxidation. Acetic acid is also not an issue in properly packaged beer, where oxygen is usually absent. In lambic beers and some other sour beer styles, acetic acid can be a desirable component that adds to the complexity of the flavor and aroma profile.