Vol. XIV · Fermentation Plate 8 of 11vinegar, acetic acid bacteria, Acetobacter, mother of vinegar, wine vinegar, rice vinegar, black vinegar, 醋, 镇江香醋, 山西老陈醋, acidityMF

Vinegar.

A wine of 7 % alcohol can become a vinegar of about 7 % acidity. The bacteria need air; the acid they make is the vinegar.

Vinegar is made in two steps. First yeasts ferment sugar to alcohol (XIV.2). Then acetic acid bacteria (Acetobacter, Komagataeibacter) oxidise the alcohol to acetic acid: C2H5OH + O2 → CH3COOH + H2O. Unlike yeast fermentation this needs oxygen, so traditional vinegar sits in wide, shallow vessels or barrels with air above it, where the bacteria grow as a gelatinous film, the "mother of vinegar"; modern factories bubble air through the liquid and finish in a day.

The arithmetic is exact: 46 g of ethanol makes at most 60 g of acetic acid. So the alcohol in the base sets the strength — a 7 % wine gives about 7 % vinegar — and the base sets the flavour: wine, cider and malt in Europe, rice wine in Japan and southern China, and the solid-state grain vinegars of northern China — 山西老陈醋 from sorghum, aged and concentrated for years, and 镇江香醋 from glutinous rice, both dark, malty and round. The workshop turns a drink into vinegar.

Below: the acetic acid in the vinegar against the alcohol of the starting drink (exact stoichiometry, with the conversion efficiency you choose), and its pH (exact weak-acid equilibrium), with the 4–6 % strength of table vinegars marked.

What you will learn
Workshop
Ethanol (g per 100 mL) = ABV × 0.789; acetic acid = ethanol × 60/46 × efficiency (typical 85–95 %). pH from acetic acid's pKa of 4.76 at that concentration. Table vinegars are usually 4–6 % acetic acid; 5 % is the strength assumed by tested pickling recipes (XIII.7).

Alcohol in, acid out

At 90 % conversion a cider of 6 % alcohol makes a vinegar of about 5.6 % acetic acid, a beer of 4.5 % about 4.2 % (malt vinegar), and a 12 % wine about 11.1 % — stronger than table vinegar, so wine is usually diluted to 7–8 % alcohol first, because much above that the bacteria struggle. Once the alcohol is used up, the bacteria start to oxidise the acetic acid itself, which is why vinegar-makers stop and pasteurise or seal it.

Home vinegar is easy and safe in itself: leave unpasteurised wine or cider, diluted if strong, in a wide jar covered with cloth at 20–30 °C with a piece of mother or a splash of live vinegar, and taste after a few weeks. But never use home-made vinegar of unknown strength for preserving (XIII.7): its acidity must be known. In cooking, choose by flavour: rice vinegar for sushi and dressings, black vinegar for dumplings and 糖醋, sherry and wine vinegars for sauces (XI.4).

At the stove

Taste a drop each of rice vinegar, malt vinegar, cider vinegar, wine vinegar and Chinese black vinegar on a cube of bread. Note how different the same acid tastes with different bases.

The word vinegar comes from the French vin aigre, "sour wine"; Shanxi province in northern China has a saying that its people "cannot live without vinegar", and 山西老陈醋 can be aged for years in open jars, freezing in winter to concentrate it.

Terms

Acetic acid bacteria
Bacteria that oxidise alcohol to acetic acid, using oxygen.
Mother of vinegar
A gelatinous film of acetic bacteria and cellulose.
Acidity
The acetic acid content of vinegar, usually 4–6 %.
老陈醋
Aged Shanxi sorghum vinegar.
Acetic bacteria, with air, turn alcohol into acetic acid (46 g → 60 g). The drink's alcohol sets the strength, its base the flavour.

Sources

  1. Exact stoichiometry: glucose → 2 ethanol + 2 CO2 (180 g → 92 g + 88 g); ethanol + O2 → acetic acid + water (46 g → 60 g); ideal-gas molar volume 24.5 L at 25 °C.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.