What is vinegar composed of?

Acetic acid (from fermented ethanol).

From http://www.versatilevinegar.org/faqs.html

Vinegar can be made from any fruit, or from any material containing sugar.

The following varieties of vinegar are classified by a U.S. Food and Drug Administration (FDA) Compliance Policy Guide for labeling purposes according to their starting material and method of manufacturing:

* Cider vinegar or Apple vinegar is made from the two-fold fermentation of the juices of apples. Vinegar can be made from other fruits such as peaches and berries with the labels describing starting materials.

* Wine vinegar or Grape vinegar is made from the two-fold fermentation of the juice of grapes.

* Malt vinegar, made by the two-fold fermentation of barley malt or other cereals where starch has been converted to maltose.

* Sugar vinegar, made by the two-fold fermentation of solutions of sugar syrup or molasses.

* Spirit or distilled vinegar, made by the acetic fermentation of dilute distilled alcohol.

* Blended Vinegar made from a mixture of Spirit vinegar and Cider vinegar is considered a combination of the products that should be labeled with the product names in the order of predominance. It is also the product made by the two-fold fermentation of a mixture of alcohol and cider stock.

* Rice or Rice Wine vinegar (although not part of FDA’s Compliance Policy Guide) has increased in popularity over the past several years and is made by the two-fold fermentation of sugars from rice or a concentrate of rice without distillation. Seasoned rice or rice wine vinegars are made from rice with the “seasoning” ingredients noted on the label.

* Balsamic vinegar (also not a part of FDA’s Compliance Policy Guide) continues to grow in market share and “traditional” and “commercial” forms are available. The products are made from the juice of grapes, and some juice is subjected to an alcoholic and subsequent acetic fermentation and some to concentration or heating. See the “Today’s Vinegar” section of the Web site for more information regarding Traditional and Commercial Balsamic Vinegar.

Acetic acid mostly. It’s what certain bacteria make out of alcohol. There will also be mostly water, plus traces of whatever fruit or whatnot formed the juice the bacteria were nomming on – apple cider vinegar will have apple traces in it, whereas red wine vinegar will be more grapey.

As others have said, acetic acid plus water to bring it to the appropriate concentration of acid. Among other ways, it’s what happens when good wine goes bad.

Thanks you guys. Oh, and wow. Thanks for the effort @jaytkay :) much appreciated!