Lactic acid is an organic acid with the molecular formula C3H6O3. In its solid state, it is white and in its liquid state is miscible with water. When dissolved, it forms a colorless solution. Production includes both artificial synthesis and natural sources. Lactic acid is an alpha-hydroxy acid (AHA) due to the presence of a hydroxyl group adjacent to the carboxyl group. It is a synthetic intermediate in many organic synthesis industries and in various biochemical industries. The conjugate base of lactic acid is called lactate (or the lactate anion). The name of the derived acyl group is lactoyl. Lactic acid is chiral, consisting of two enantiomers. One is known as L-lactic acid, (S)-lactic acid, or (+)-lactic acid, and the other, its mirror image, is D-lactic acid, (R)-lactic acid, or (−)-lactic acid. A mixture of the two in equal amounts is called DL-lactic acid, or racemic lactic acid. In animals, L-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise. It does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal, which is governed by a number of factors, including monocarboxylate transporters, concentration and isoform of LDH, and oxidative capacity of tissues. This reaction is reversible and redox-linked: LDH reduces pyruvate to lactate using NADH as an electron donor. Lactic acid is produced in human tissues when the demand for oxygen is limited by the supply. The process of lactic acidosis produces lactic acid, which results in an oxygen debt, which can be resolved or repaid when tissue oxygenation improves. The concentration of blood lactate is usually 1–2 mMTooltip millimolar at rest, but can rise to over 20 mM during intense exertion and as high as 25 mM afterward. In industry, lactic acid fermentation is performed by lactic acid bacteria, which convert simple carbohydrates such as glucose, sucrose, or galactose to lactic acid. These bacteria can also grow in the mouth; the acid they produce is responsible for the tooth decay known as cavities. In medicine, lactate is one of the main components of lactated Ringer's solution and Hartmann's solution. These intravenous fluids consist of sodium and potassium cations along with lactate and chloride anions in solution with distilled water, generally in concentrations isotonic with human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or burns.
History Swedish chemist Carl Wilhelm Scheele was the first person to isolate lactic acid in 1780 from sour milk. The name reflects the lact- combining form derived from the Latin word lac, meaning "milk". In 1808, Jöns Jacob Berzelius discovered that lactic acid (actually L-lactate) is also produced in muscles during exertion. Its structure was established by Johannes Wislicenus in 1873. In 1856, the role of Lactobacillus in the synthesis of lactic acid was discovered by Louis Pasteur. This pathway was used commercially by the German pharmacy Boehringer Ingelheim in 1895. Due to a combination of geographic and infrastructural factors, the Soviet Union, as well as several other members of the Warsaw Pact, experienced chronic shortages of citric and malic acid, among others. In order to combat this issue, the Narkomzem (Soviet Ministry of Agriculture) invested heavily in the development of suitable lactobacillus strains, which were able to produce lactic acid with relatively high efficiency from crude molasses feedstock. Despite synthetic citric acid being produced in some quantities across the Warsaw Pact, it proved far more difficult to purify, leading to lactic acid being, on average, a quarter of the cost of citric acid. The continued use of lactic acid in some Eastern European and Central Asian food production in the modern day, in favor of the more common citric or malic acids, lends it a distinctive flavor. Global demand for lactic acid continues to expand, with an estimated annual growth rate of 5–8% driven by the increasing use of biodegradable plastics, green solvents, and pharmaceutical intermediates. Worldwide production exceeded 1.5 million tonnes by the early 2020s, up from roughly 275,000 tonnes in 2006, and is projected to keep rising as biobased materials replace petroleum-derived products. Major producers include NatureWorks LLC, Purac, Galactic, and several Chinese manufacturers. NatureWorks operates one of the world’s largest polylactic acid (PLA) facilities in Blair, Nebraska, with a production capacity of about 140,000 tonnes per year, supplying feedstock for a wide range of biodegradable packaging and fiber applications.
Production Lactic acid is produced industrially by bacterial fermentation of carbohydrates, or by chemical synthesis from acetaldehyde. As of 2009, lactic acid was produced predominantly (70–90%) by fermentation. Production of racemic lactic acid consisting of a 1:1 mixture of D and L stereoisomers, or of mixtures with up to 99.9% L-lactic acid, is possible by microbial fermentation. Industrial production of the D-lactic acid enantiomer is technically more challenging because most naturally occurring lactic acid bacteria preferentially produce the L-form; obtaining high optical purity of D-lactic acid therefore requires genetically engineered microorganisms or specific D-lactate dehydrogenases.
Fermentative production Fermented milk products are obtained industrially by fermentation of milk or whey by Lactobacillus bacteria: Lactobacillus acidophilus, Lacticaseibacillus casei (Lactobacillus casei), Lactobacillus delbrueckii subsp. bulgaricus (Lactobacillus bulgaricus), Lactobacillus helveticus, Lactococcus lactis, Bacillus amyloliquefaciens, and Streptococcus salivarius subsp. thermophilus (Streptococcus thermophilus). As a starting material for industrial production of lactic acid, almost any carbohydrate source containing C5 (pentose sugar) and C6 (hexose sugar) can be used. Pure sucrose, glucose from starch, raw sugar, and beet juice are frequently used. Lactic acid producing bacteria can be divided in two classes: homofermentative bacteria like Lactobacillus casei and Lactococcus lactis, producing two moles of lactate from one mole of glucose, and heterofermentative species, producing one mole of lactate from one mole of glucose, as well as carbon dioxide and acetic acid/ethanol.
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