Vinegar is made by a chemical change called fermentation. During fermentation, the sugar in wine or juice is changed into alcohol and gas. As the gas evaporates, it leaves only the alcohol and fruit flavors.
The next step in the fermentation process is called oxidation, when the oxygen in the air mixes with the vinegar bacteria in the alcohol to change the alcohol into vinegar.
Fermentation takes place when the liquid is in large wooden barrels. These barrels have holes to permit air to circulate. Since the alcohol in the juice or wine is lighter than the rest of the liquid, it rises to the top of the barrel.
At the top, the alcohol comes in contact with the air and forms an acid called acetic acid. This acid increases the alcohol’s weight, carrying it to the bottom of the barrel in the form we know as vinegar.
It can take from one to two years for a barrel of juice or wine to ferment naturally into vinegar!