26.2 Vinegar

Alcohol made sour

Vinegar is made by bacteria that turn alcohol into acetic acid, the sharp acid in vinegar. In 1862 Pasteur gave the first account of this, and saw that a living thing was doing the work.

The workers are the acetic acid bacteria, often shortened to AAB, of the genus Acetobacter and its relatives. They live where there is sugar, alcohol or acid: on fruit and flowers, and above all in fermented drinks. Given enough oxygen, they turn ethanol into acetic acid. Producers feed them apple cider, wine or fermented grain mash. The same bacteria help make cocoa powder and kombucha, and they can also spoil food.

Those feeds are already alcoholic, so vinegar begins where an alcohol fermentation ends. How yeast makes alcohol is explained under Yeast.

How the bacteria do it

The change happens in two quick steps. Ethanol becomes acetaldehyde, and acetaldehyde becomes acetic acid. Each step is run by its own enzyme, and both enzymes sit on the outer side of the cell's membrane rather than inside the cell.

Ethanol, acetaldehyde and acetic acid are all poisonous to living cells, yet AAB thrive among them, for several reasons.

• The work is done outside the cell, so the poisons need not enter it.
• In many species the two enzymes are joined, so acetaldehyde is never set loose.
• Pumps actively remove hydrogen ions from the cell.
• The cell's membrane and its proteins are built to stay stable in strong acid.

The electrons taken from the alcohol are passed to the oxygen-using chain inside the bacterium. That is why vinegar bacteria need air: they cannot work without oxygen.

The second growth

Some Acetobacter strains, once the alcohol runs low, start to feed on the acetic acid itself, breaking it down to carbon dioxide and water. In a vinegar batch the bacteria grow in two bursts: first while making acid from alcohol, then again while eating that acid. Vinegar makers try to stop the second burst, since it destroys the product.

Vinegar's acid is also why it keeps. As an acidic food, vinegar does not support bacteria and can be used to preserve other foods.