By Harold McGee
It takes three ingredients to make vinegar: an alcoholic liquid, oxygen, and bacteria of the genus Acetobacter or Gluconobacter, mainly A. pasteurianus and A. aceti. These bacteria are among the few microbes that are able to use alcohol as an energy source. Their metabolism of alcohol leaves behind two by-products, acetic acid and water.
CH3CH2OH + O2 → CH3COOH + H2O
Alcohol + oxygen → acetic acid + water
Acetic acid bacteria require oxygen, and so live on the surface of the fermenting liquid, where with other microbes they form a film sometimes called the “mother.” Especially thick films are created by Acetobacter xylinum, which secretes a form of cellulose. (Such mats are sometimes cultivated and eaten for themselves;.) Acetobacteria thrive in warm conditions, so vinegar fermentations are often carried out at relatively high temperatures, from 82 to 104°F/28–40°C.