Fermentation can occur in food and drinks or within muscle cells. In food and drinks, fermentation occurs as bacteria or yeast digest sugars and convert them into gases, acids and/or alcohol. Fermentation can also occur within the muscle cells of the body when they have no oxygen, producing lactic acid.
In both these cases, the process of fermentation must take place in an environment without oxygen, meaning that fermentation is an anaerobic process. The basic chemical reaction for fermentation in food and drink involves sugar (glucose) being broken down into ethanol and carbon dioxide gas. When fermentation occurs in the muscles, this reaction is different, with only lactic acid and energy being produced.