Lactic acid is a waste product from the cellular breakdown of glucose in some organisms. The respiration process, where every cell in an organism takes in food and water to utilize glucose for energy, produces this lactic acid as a by-product. Lactic acid is a by-product because the cells that perform this form of respiration do not take in oxygen. This form of respiration, where lactic acid is produced from no oxygen, is also called fermentation.
Not all bacteria produce lactic acid. Specifically, only "non-respiring, non-spore forming" bacteria, according to the Food and Agriculture Organization of the United Nations, form lactic acid. These bacteria are utilized by humans because their lactic acid helps in the fermentation process of breads, alcohol, and dairy products for human consumption. Some examples of such bacteria include Streptococcus thermophilus and Lactobacillus lactis, which are both used in yogurts and cheeses.
Homofermenters and Heterofermenters
Two types of bacterial organisms produce lactic acid. One type is the homofermenters, which only produce lactic acid as a waste product. Heterofermenters produce lactic acid and two extra waste products, ethanol and carbon dioxide. Heterofermenters are usually used for beer products and the ethanol and carbon dioxide is removed in the distillation process.
The bacteria that produce lactic acid are found within certain environments where glucose is present, such as in organisms. For example, some lactic acid bacteria may be found in only green vegetables or in the intestines of animals. This is because the bacteria needs particular carbohydrates in a reduced oxygen environment to thrive. In return, the lactic acid that is produced often protects the cells of plants or animals from breaking down. This is why the bacteria are used in foods to help them from spoiling.
The muscles of animals perform lactic acid fermentation under certain circumstances. This happens when oxygen is not adequately given to the muscles when they are stressed, such as during exercise. The muscle cells, as a reaction to little oxygen being given to the tissues begins a fermentation process. Lactic acid from the fermentation helps the muscles gain energy during intense workouts.
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