The process of cells digesting carbohydrates without using any oxygen causes fermentation. It converts the carbohydrate into energy and either an alcohol or an acid. Fermentation allows organisms to survive without oxygen, but it is far less efficient than oxygen-based cellular respiration.
Fermentation is most frequently performed by bacteria, yeast and animal muscles. Yeasts perform ethanol fermentation, which generates both ethanol and carbon dioxide. This is used by humans to create both alcoholic beverages and bread. Animal muscles perform lactic acid fermentation when they require more energy than they have oxygen to provide. Lactic acid fermentation requires water as well as carbohydrates. It is the only type of fermentation that does not produce a gas as a byproduct. Depending on the species, bacteria can perform either ethanol or lactic acid fermentation. Bacterial lactic acid fermentation is used to produce yogurt. Certain fungi also perform lactic acid fermentation.
Some types of organisms perform both lactic acid and ethanol fermentation. This occurs when they further process the lactic acid, which is very toxic in high concentrations. Ethanol and lactic acid are not the only waste products of fermentation. Some organisms create other useful products, such as hydrogen gas or methane, from fermentation.