Fermentation, sugar is changed to alcohol and gas. The gas evaporates, leaves the alcohol and fruit flavors. Oxygen in the air mixes with bacteria in the alcohol to change the alcohol into vinegar (oxidation)Fermentation takes place in large wooden barrels with holes to for air to circulate. The alcohol is lighter so it rises to the top. At the top, the alcohol comes in contact with air and forms acetic acid. This increases the alcohols weight, carrying it to the bottom of the barrel as vinegar.