During oxygen debt, a muscle cell needs more energy than its oxygen supply normally allows, so it breaks down glucose only partially and produces lactic acid as a waste product instead of water and carbon dioxide. This lactic acid must be broken down by oxygen once levels rise again.
The use of glucose is far less efficient than usual in a muscle cell accumulating oxygen debt. Each glucose molecule broken down into lactic acid only gives about 5 percent of the energy a glucose molecule would give with sufficient oxygen. This lactic acid contributes to muscle fatigue and failure. Once the high energy use stops, the lactic acid is combined with oxygen to form carbon dioxide and water just as with normal aerobic respiration.
Lactic acid fermentation is a type used by both muscle cells and some bacteria. Other bacteria and yeasts use ethanol fermentation, creating alcohol instead of lactic acid as a waste product. Both types of fermentation are used in the production of food. Lactic acid fermentation is used in the production of yogurt and sauerkraut. Ethanol fermentation is used in the production of bread and alcoholic beverages. It is also used in the production of ethanol fuels.