Lactic acid fermentation is a metabolic strategy that humans, other animals, and various bacterial species use to generate energy from glucose molecules under certain conditions. Specifically, when oxygen is scarce, cells can burn glucose anaerobically, meaning without oxygen, to generate small amounts of energy. There are several uses, both cellular and outside the body, for lactic acid fermentation.
Cellular Energy
Cells use lactic acid fermentation as a metabolic strategy, meaning they use it to generate energy. Cells generally have two metabolic strategies available to them. One of these, called respiration, generates large quantities of energy, but requires oxygen. The other strategy, called fermentation, generates only about 1/15 of the energy generated by respiration, but isn't oxygen dependent. Multicellular organisms can't support life through fermentation alone, notes Dr. Lauralee Sherwood in her book "Human Physiology," but cells can use fermentation for short periods of time if cells are working too hard and demand oxygen in quantities that outstrip supply.
Generation of NAD+
Lactic acid fermentation actually occurs in two parts. The first part, called glycolysis, is common to both lactic acid fermentation and respiration. This reaction splits glucose into two molecules of pyruvate, and uses a molecule called NAD+ to accomplish the split. Glycolysis is the only energy-yielding part of fermentation. The second part of fermentation converts pyruvate into lactic acid, which doesn't yield additional energy, note Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry." The purpose of the second conversion is that it also regenerates NAD+, allowing glycolysis to continue, so that cells can continue producing energy.
Fermentation of Food
Humans can not only run fermentation reactions in their own cells, they can also take advantage of other species' abilities to ferment. The bacterial genus Lactobacillus, for instance, metabolizes glucose by lactic acid fermentation. Humans take advantage of this, purposely seeding milk--the favorite food source of Lactobacillus--with strains of bacteria. As the bacteria ferment sugar and produce lactic acid, the acid reacts with proteins in the milk. This curdles the proteins, chemically denaturing them, explain Drs. Mary Campbell and Shawn Farrell in "Biochemistry." The products, including yogurt and cheese, have characteristically sour flavors that come from the acid secreted by Lactobacillus bacteria.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
