How vinegar is made
Vinegar begins as a weak alcoholic liquid. A second fermentation turns the alcohol into acetic acid, which is the sharp taste.
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Vinegar is a sour liquid used in the kitchen. The sourness is acetic acid, made by a fermentation that comes after an earlier one.
The first fermentation is the one that makes alcohol. Yeast turns sugars from apple juice, wine, malted barley, or another sweet liquid into alcohol. If that liquid is left where air can reach it, a second group of microbes can move in. These are acetic acid bacteria. They take the alcohol and oxygen from the air and leave acetic acid. That second change is what makes vinegar vinegar, rather than a weak wine or a cider.
The mother
In a jar of vinegar you sometimes see a slippery disc or a cloudy strand. That is a mother, a mat of the bacteria and the film they build. It is the colony that did the work, gathered where the air meets the liquid. A vinegar maker keeps a mother and adds it to the next batch so the second fermentation starts promptly.
The vessel matters because the bacteria need air. A sealed bottle of finished wine does not turn into vinegar in a hurry. An open cask, a wide jar, or a barrel that is not filled to the bung does. Traditional vinegar lofts set rows of wooden barrels in a warm roof space and move the liquid from a younger barrel to an older one. Wood lets a little air through, and the long wait rounds the flavour. A quicker works can run the same chemistry by spreading the liquid over a packing that gives the bacteria plenty of air.
What the starting liquid decides
The acid is the same idea each time. The flavour is not. Wine vinegar tastes of the wine it came from. Cider vinegar tastes of apples. Malt vinegar, the kind served with fish and chips, starts from barley that was brewed. A plainer spirit vinegar tastes sharper and thinner because the alcohol it starts from was plainer. Rice vinegar is milder because the brew it starts from is milder. The second fermentation does not invent those flavours. It keeps what the first liquid already had, and adds the sourness.
A very high alcohol level, or a liquid that is kept boiling, slows the bacteria. A cook who boils a vinegar sauce drives off some of the aroma, which is why a dressing is often mixed off the heat. Salt and herbs are additions. They are not what makes the liquid vinegar.
Keeping it
Once the alcohol has been turned, the vinegar is stable in a way the sweet juice was not. The acid makes life hard for many of the microbes that would spoil juice. A fresh mother can still grow if air and a little alcohol remain, so a bottle is kept closed. Cloudiness in a living vinegar is usually the mother returning, not a sign that the bottle has failed.
The kitchen uses follow from the acid. It pickles, it cuts the richness of a sauce, and it reacts with bicarbonate in a batter. All of that is the same sour liquid, made by letting bacteria finish a job the yeast began.
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