Acetic Acid is an organic acid produced by certain bacteria (such as Acetobacter) and yeast (such as Brettanomyces) in the presence of oxygen and ethanol. Acetic acid is the acid found in vinegar, and it has a rougher, more warming acidity than lactic acid. While individual preferences for acetic acid levels may vary, excessive amounts are generally considered a flaw in sour beer. Acetic acid is also an important precursor for acetate esters.
Production of Acetic Acid
It is important to note that while oxygen is necessary for the production of acetic acid, and some of the microorganisms which produce acetic acid require oxygen for metabolism, the presence or absence of oxygen in a fermenting beer will not 'produce' or 'create' these microorganisms. If a mixed culture has acetobacter or brettanomyces present, oxygen added after primary fermentation may allow acetic acid to be produced. However without the presence of an organism which produces acetic acid, exposing the beer to oxygen will not result in the creation of acetic acid or in the production of acetic acid producing microbes.
Acetic acid bacteria
Acetic acid bacteria can produce acetic acid under aeobic (oxygen-containing) conditions when alcohol is present. Acetobacter is a commonly known example of acetic acid bacteria
Some bacteria are able to produce acetic acid directly from a carbohydrate source rather than from ethanol.
Characteristics of Acetic Acid
Organic acids that make beer taste sour can have significant differences in terms of the additional flavors of the acids and how that acidity is perceived. While acetic acid tends to be a harsher acidity than other organic acids such as lactic acid.