Acetic acid fermentation

Weak fermented liquors will become sour on exposure to the air. This is owing to the conversion of the alcohol in acetic acid. Acetic acid is produced by fermenting various substrates (starchy solution, sugar solutions or alcoholic foodstuffs such as wine or cider) with Acetobacter bacteria. Examples of acetic fermentation:

What is vinegar

Acetobacter aceti, which is normally used to produce vinegar can make vinegar up to 14 per cent acetic acid. When cider, wine or malt is used as starting material about 5 per cent acetic acid will be produced. The colour and taste of the vinegar will depend on the source (cider, wine, beer, barley malt) and production method. Traditionally, vinegar was produced in casks filled with wood shavings on which wine was sprayed. When vinegar is distilled it will have no colour.

Legislation in many countries require that the acetic acid in vinegar must be produced by fermentation rather than by chemical processes. There are also three chemical processes to produce acetic acid: methanol carbonylation, butane oxidation, or acetaldehyde oxidation. Vinegar contains 4-8% acetic acid by volume.