On Food and Cooking

use alcohol as an energy source. Their
metabolism of alcohol leaves behind two by-
products, acetic acid and water.
CH 3 CH 2 OH + O 2 CH 3 COOH + H 2 O
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; see p. 509.)
Acetobacteria thrive in warm conditions, so
vinegar fermentations are often carried out at
relatively high temperatures, from 82 to
104ºF/28–40ºC.
The concentration of alcohol in the starting
liquid affects the acetic fermentation and the
stability of the resulting vinegar. An alcohol
concentration around 5% will produce a
vinegar that is around 4% acetic acid, which is