Traditional vinegar is made by fermenting some type of alcohol—think beer, wine, cider, sake – into acetic acid. To make vinegar you need five key ingredients: alcohol, vinegar bacteria, oxygen, heat, and time.
Depending on the ABV (alcohol by volume) of the specific alcohol you may need to add water to dilute the alcohol. This is because when you get to a certain height in ABV the vinegar bacteria simply cannot operate and eat away at the alcohol to transform it into acetic acid or vinegar. This, by the way, is why your bottle of scotch will never turn into vinegar.
Over time and at the right temperature the vinegar bacteria will eat away at the wine or cider and turn into vinegar. Depending on the level of ABV, the environmental conditions, and the amount of vinegar you are trying to make this can take anywhere from two weeks to about six months simply to complete the fermentation. During this time you will see that a film, often thick, will form on top of the liquid. This is vinegar mother and is a byproduct of vinegar fermentation.
Better traditional vinegars are then aged for months to years after fermentation. This allows the vinegars to develop more nuanced flavor profiles and not just be acid bombs.
Reach out if you have any other questions on how vinegar is made.
