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Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol（醇）, lactic acid（乳酸 ）, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid（丁酸 ） and acetone（丙酮 ）. Yeast carries out fermentation in the production of ethanol in beers, wines and other alcoholic drinks, along with the production of large quantities of carbon dioxide. Fermentation occurs in mammalian muscle during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid . It may seem that fermentation would yield only meager（贫乏的） amounts of energy(2ATPs maximum per glucose).What actually happens,however ,is that may bacteria can grow as fast as they would in the presence of oxygen.This rapid growth is made possible by an increase in the rate of glycolysis. From another stand-point ,fermentation permits independence from molcuular oxygen and allows colonization of anaerobic environments.It also enables microorganisms with a versatile meatbolismto adapt to variations in the availability of oxygen.For them,fernentation provides a means to grow enven when oxygen levels are too low for aerobic respiration. Bacteria that digest cellulose（纤维素） in the rumens(瘤胃 ) of cattle are largely fermentative.After initially hydrolyzing(水解 ) cellulose to glucose,they ferment the glucose to organic acids,which are then absorbed as the bovine,s(牛) principal energy source. Even human muscle cells can undergo a form of fementation that permits short periods of activity after the oxygen supply in the muscle has been exhuasted.Muscle cells convert pyruvic acid（丙酮酸） into lactic acid（乳酸）,which allows anaerobic production of ATP to proceed for a time .But this can not go on indefinitely,and after a few minutes,the accumulated lactic acid causes muscle fatigue（疲劳）. Products of Fermentation in Microorganisms Alcoholic beverage(饮料 ) Acetone(丙酮 ) Organic acid Foods Fermentation products can