Acetic acid is a compound that causes off flavors and aromas in beer, often described as a sour or acidic taste or a vinegar character. In fact, acetic acid is the active ingredient in vinegar, and beer with this character has likely had some of its alcohol metabolized into vinegar by acetic acid bacteria.
Acetic acid in beer
Acetic acid is generally considered an off flavor in beer. However, a very few sour beer styles feature it as a desirable flavor characteristic, notably Flanders Red Ale and Oud Bruin.
However, most sour mash beers, such as Berliner Weisse, depend on the different souring action of lactic acid bacteria; a vinegar character is generally considered undesirable in these beers.
Causes of acetic acid
Acetic acid is a bacterial byproduct, and is always caused by an acetic acid infection during the brewing process.
Preventing acetic acid
Because it is a bacterial product, sanitation is the only way to keep acetic acid out of your beer.
Creating acetic acid
Traditional Belgian brewers add acetic acid character to their beers through open fermentation; however, commercial cultures are also available, either from distributors of brewers' yeast or from vinegar making suppliers. If you already make vinegar at home, you can also sour beer with your existing vinegar mother.
For palate testing or training purposes, an acetic character can also be added to a finished beer by simply adding a small amount of neutral vinegar. However, this does not give as complex an acetic character as a true acetic acid fermentation, and is not the recommended method of adding a desired acetic character to beer.