Vinegar is one of the most common and most traditional of fermented foods. It has been made purposely and by accident for thousands of years.
The primary acid in vinegar is acetic acid which is generally produced as a result of a fermentation following an alcoholic fermentation. These alcohol fermentations generally start with fruits or grains.
Some examples of this cycle are grapes to wine to wine vinegar, apples to hard cider to apple cider vinegar, or grain to malt (sprouted / fermented grain), to vinegar.
Vinegar can be made from wild yeasts and organisms present in the environment it is fermenting in or through the inoculation of a mother culture such as you would find in raw apple cider vinegar.
Step One: Make Alcohol
To make vinegar you must first make alcohol. To do so you will need about a gallon of unpasteurized juice from apple cider pressing or other fruit juicing.
Place the juice in a sealable jug with an airlock system attached. The airlock is important because it allows the juice to ferment and release the carbon dioxide gas through the airlock while not allowing any foreign bacteria to taint the fermentation process.
Allow to ferment for 4 to 6 weeks at room temperature. During this time the natural yeasts present will work on the sugars in the juice to ferment and form alcohol. It should bubble and may form a gray foam at the top which is harmless yeast.
Once the bubbling has stopped all of the sugars have been converted to alcohol. Now you need to allow a second fermentation to occur which will produce acetic acid, the essential component for vinegar!
Step Two: Make Vinegar
If you are using a mother or starter culture now is the time to introduce it. You don’t need one, but it will ensure the inoculation of a specific strain of yeasts.
Now you must allow the alcohol to come in contact with oxygen so that the yeast spores can be fed. To do this, tie a clean tea towel or other clean fabric over the top, being sure it is tightly fastened so as to prevent the entry of fruit flies and other bugs.
This second fermentation can take quite a bit of time, as long as 6 to 9 months. This time frame is dependent on surface area and temperature. A warm room temperature of 70° to 80°F is ideal for this fermentation.
Once fermented it will be much stronger than store-bought vinegar so you can dilute it to taste.