Lactic Acid
Overview
Lactic acid (LAK-tik AS-id) is a colorless, odorless, syrupy liquid that occurs in two isomeric forms, D-lactic acid and L-lactic acid. Isomers are two or more forms of a chemical compound with the same molecular formula, but different structural formulas and different chemical and physical properties. The D form is produced during metabolic reactions that take place in muscle tissue, while the L form is produced by yeast cells. The synthetic production of lactic acid results in a product consisting of equal amounts of the D and L forms, a mixture known as a racemic mixture.
Key Facts
Other Names:
2-hydroxypropanoic acid; ±-hydroxypropanoic acid; milk acid
Formula:
CH3CHOHCOOH
Elements:
Carbon, hydrogen, oxygen
Compound Type:
Carboxylic acid (organic)
State:
Liquid
Molecular Weight:
90.08 g/mol
Melting Point:
16.8°C (62.2°F)
Boiling Point:
Not applicable; decomposes upon heating
Solubility:
Very soluble in water and ethanol; slightly soluble in ether
Lactic acid was first discovered in 1780 by the Swedish chemist Karl Wilhelm Scheele (1742–1786), who called his discovery "acid of milk." The two isomeric forms of the acid were first identified in 1863 by the German chemist Johannes Wislicenus (1835–1902), and the compound was first produced commercially in 1881 by American chemist Charles E. Avery. Avery patented his invention in 1885 and constructed a factory for the production of lactic acid in Littleton, Massachusetts.
About 30 million kilograms (72 million pounds) of lactic acid are produced annually in the United States. The most common method of production is the fermentation of glucose by yeast.
How It Is Made
In muscle cells, lactic acid is the product of anaerobic respiration, the process by which glucose is oxidized in the absence of oxygen to produce energy required by cells. Although some lactic acid is always produced in muscle cells in very low concentrations, it tends to accumulate during exercise, when cells do not receive adequate amounts of oxygen to metabolize oxygen by normal pathways. Lactic acid produced during exercise remains in the body for only short periods of time, sometimes in less than thirty minutes. It is metabolized in the muscle cells where it was produced, resulting in the production of energy, carbon dioxide, water, and other products.
Lactic acid is also produced by yeast during the process of fermentation. Fermentation is the process by which yeast cells convert glucose to an alcohol and carbon dioxide. Yeast cells use almost precisely the same enzyme in fermentation that muscle cells use in anaerobic respiration. The muscle cell enzyme and the yeast enzyme differ only in the orientation of one group of atoms, resulting in the production of the D isomer in one case and the L isomer in the other.
A synthetic process for the production of lactic acid was first introduced in 1963. That process begins with the addition of hydrogen cyanide (HCN) to acetaldehyde (ethanal; CH3CHO), resulting in the formation of lactonitrile (CH3CH2OCN). The lactonitrile is then hydrolyzed, using a strong acid, such as sulfuric acid, as a catalyst, to make lactic acid.
Interesting Facts
- For the better part of a century, athletes and physiologists have considered lactic acid a primary cause of fatigue during high-intensity exercise. However, scientists have learned that lactic acid actually helps to prevent muscle fatigue. Muscle soreness once thought to be caused by lactic acid is, instead, more likely to be a result of damaged muscle cells caused by excess use.
- Lactic acid present on the skin attracts mosquitoes.
- Lactic acid in the body exists in its ionic form, known as lactate.
Common Uses and Potential Hazards
The primary use for lactic acid in the United States is as a food additive, where it acts as an acidulant and a flavor additive. An acidulant is a compound that provides an acidic environment for foods, as is the case with yogurt, buttermilk, sauerkraut, green olives, pickles, and other acidic foods. As a flavor additive, it adds a tart or tangy flavor to foods and beverages, as well as acting as a preservative to keep them from spoiling. Lactic acid also has a number of important industrial uses, the most important of which is the production of other organic chemicals, especially ethyl lactate, acrylic acid, propylene glycol, and the polymer known as polyactide. Polyactide is used in the manufacture of plastic film, fiber, packaging material, and filling materials. Other commercial and industrial applications of lactic acid include:
- As a mordant in dyeing;
- As a solvent for dyes that are not soluble in water;
- For the treatment of animal hides in the preparation of leather products;
- As a catalyst in the production of certain types of plastics; and
- As an additive in electroplating baths.
Words to Know
A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure. Adding a layer of nickel, silver, or gold, on another type of metal using an electric current. The process by which yeast convert glucose to an alcohol and carbon dioxide. The process by which a compound reacts with water to form two new compounds. One of two or more forms of a chemical compound with the same molecular formula, but different structural formulas and different chemical and physical properties. A process that includes all of the chemical reactions that occur in cells by which fats, carbohydrates, and other compounds are broken down to produce energy and the compounds needed to build new cells and tissues. A substance used in dyeing and printing that reacts chemically with both a dye and the material being dyed to help hold the dye permanently to the material. A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.Lactic acid in normal concentrations poses no safety or health hazards to humans or other animals. One health consequence related to lactic acid, however, is a condition known as gout, a type of arthritis that causes severe pain in the joints. Gout is caused by an accumulation of uric acid in the blood. Since lactic acid blocks the elimination of uric acid from the body, individuals with excess lactic acid buildup, usually caused by high alcohol consumption, may develop an excess of uric acid crystals in the blood and joints, leading to gout.
For Further Information
"Cell Respiration." SparkNotes. http://www.sparknotes.com/testprep/books/sat2/biology/chapter6section1.rhtml (accessed on October 14, 2005).
Drake, Geoff. "The Lactate Shuttle—Contrary to What You've Heard, Lactic Acid Is Your Friend." Bicycling (August 1992): 36.
Friel, Joel. "All Athletes: Lactic Acid's Bad Rap." Ultrafit's e-Tips For Endurance Athletes. October 2004, Vol. 7, No. 10. http://www.ultrafit.com/newsletter/october04.html#Joe (accessed on October 14, 2005).
"Lactic Acid." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/l0522.htm (accessed on October 14, 2005).
Rogers, Palmer, Jiann-Shin Chen, and Mary Jo Zidwick. Organic Acid and Solvent Production, Part I: Acetic, Lactic, Gluconic, Succinic, and Polyhydroxyalkanoic Acids. Section 2: Lactic Acid. Available online at http://141.150.157.117:8080/prokPUB/chaphtm/306/04_00.htm (accessed on October 14, 2005).
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