Balsamic vinegar is made from the must of grapes, which is the freshly squeezed juice that contains the skins, seeds, and stems. The must is then cooked in large pots over an open flame until it is reduced by about half. This concentrated liquid is then placed into wooden barrels, where it undergoes a slow fermentation process that can take years.
During the fermentation process, the vinegar is exposed to the air and the natural yeasts and bacteria present in the wooden barrels. This causes the vinegar to develop its characteristic flavor and aroma.
As the vinegar ages, it is transferred to smaller and smaller barrels made from different woods, such as oak, cherry, chestnut, and juniper. Each type of wood imparts a slightly different flavor and aroma to the vinegar.
The aging process can take anywhere from a few months to several decades, depending on the desired flavor and quality of the vinegar. The longer the vinegar is aged, the thicker and sweeter it becomes.