Fermentation is a chemical process whereby organisms--either human or otherwise--process sugar in the absence of oxygen. Fermentation reactions produce waste products that vary with the species doing the fermenting, but lactic acid fermentation is a specific set of reactions for which the end product is lactic acid. Humans use lactic acid fermentation by bacteria to produce desirable foods.
Fermentation
All organisms rely upon fermentation to some extent. Larger organisms, like humans, use it only as a temporary and secondary energy-generating strategy, because it's much less efficient than respiration, another energy-generating strategy. Smaller organisms, like bacteria and yeast, can rely upon fermentation to a much greater extent--even exclusively. Because the end products of fermentation have some chemical properties that can be useful in food preparation, humans sometimes encourage fermentation of foods by bacteria.
Lactic Acid Fermentation
Specifically, lactic acid fermentation involves converting sugar into lactic acid, a process that releases a small amount of energy that's used by the fermenting cell. One example of a lactic-acid-fermenting species is Lactobacillus acidophilus, the bacteria partially responsible for the production of yogurt and other fermented dairy products. The bacteria convert lactose, or milk sugar, into lactic acid, which in turn ferments the milk.
Why Dairy Curdles
The reason that humans find lactic acid desirable in some foods is that it imparts a sour taste and changes the texture of the food. Milk, for instance, turns creamy or curdy when it ferments. This is a result of the lactic acid interacting with proteins in the milk, causing them to precipitate, or solidify, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." The creamy or curdy sour substance can then be further manipulated into yogurt or cheese.
Other Foods
Yogurt and cheese aren't the only foods produced by lactic acid fermentation. Sauerkraut is a cabbage mixture that's fermented by lactic-acid-producing bacteria, resulting in its characteristic sour taste. Sourdough bread, too, is the result of lactic-acid-producing bacteria, which convert the starches in the bread into lactic acid, after first digesting the starches into sugar. There are also fermented soy products, like tempeh, that rely upon formation of lactic acid in food.
Humans and Lactic Acid Fermentation
Interestingly enough, like Lactobacillus acidophilus, humans also employ lactic acid fermentation. In humans, fermentation is a very minor energy-production strategy, because it doesn't produce enough energy to sustain cells for very long. Still, explains Dr. Lauralee Sherwood in her book "Human Physiology," when you engage in a hard athletic effort and feel your muscles start to burn, the burn is the result of lactic acid formation. Hard efforts require cells to burn glucose without sufficient oxygen, requiring lactic acid fermentation.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
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