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Lactic acid is an organic acid with the molecular formula C3H6O3. In its solid state, it is white and miscibility 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.
In solution, it can ionize by a loss of a proton to produce the lactate ion CH3CH(OH)CO2–, also known as 2-hydroxypropanoate. Compared to acetic acid, its p K is 1 unit less, meaning that lactic acid is ten times more acidic than acetic acid. This higher acidity is the consequence of intramolecular hydrogen bonding between the α-hydroxyl and the carboxylate group.
Lactic acid is chiral, consisting of two . One is known as -lactic acid, ( S)-lactic acid, or (+)-lactic acid, and the other, its mirror image, is -lactic acid, ( R)-lactic acid, or (−)-lactic acid. A mixture of the two in equal amounts is called -lactic acid, or racemic lactic acid. Lactic acid is hygroscopy. -Lactic acid is miscible with water and with ethanol above its melting point, which is . -Lactic acid and -lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely -lactic acid. On the other hand, lactic acid produced by fermentation in animal muscles has the () enantiomer and is sometimes called "sarcolactic" acid, from the Greek , meaning "flesh".
In animals, -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, simultaneously regenerating NAD⁺ required for glycolysis under anaerobic conditions. The concentration of blood lactate is usually at rest, but can rise to over 20mM during intense exertion and as high as 25mM afterward. In addition to other biological roles, -lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA), which is a G protein-coupled receptor (GPCR).
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 Tooth decay. 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 along with lactate and chloride in solution with distilled water, generally in concentrations isotonicity with human blood. It is most commonly used for fluid resuscitation after blood loss due to physical trauma, surgery, or burns.
Lactic acid is produced in human tissues when the demand for oxygen is limited by the supply. This occurs during tissue ischemia when the flow of blood is limited as in sepsis or hemorrhagic shock. It may also occur when demand for oxygen is high, such as with intense exercise. The process of lactic acidosis produces lactic acid, which results in an , which can be resolved or repaid when tissue oxygenation improves.
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 acid 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.
As a starting material for industrial production of lactic acid, almost any carbohydrate source containing (pentose sugar) and (hexose sugar) can be used. Pure sucrose, glucose from starch, raw sugar, and beet juice are frequently used. (2010). 9780470293669, Wiley. ISBN 9780470293669 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. (2026). 9783540854623, Springer-Verlag. ISBN 9783540854623
Raw, fermentatively produced lactic acid often contains impurities, such as residual sugars, proteins, peptides, and colored impurities. Since only purified lactic acid — and often ultrapure lactic acid that passes the heat stability test — has numerous applications in food, pharmaceuticals, and plastics (especially biodegradable plastics/PLA), the raw lactic acid must be purified. A multi-stage, low-pressure distillation process involving falling film, thin film, and short path distillation in a corrosion-proof plant can produce ultrapure, colorless, and stable lactic acid.
The resulting lactate can be used in two ways:
Lactate is continually formed at rest and during all exercise intensities. Lactate serves as a metabolic fuel being produced and oxidatively disposed in resting and exercising muscle and other tissues. Some sources of excess lactate production are metabolism in red blood cells, which lack mitochondria that perform aerobic respiration, and limitations in the rates of enzyme activity in muscle fibers during intense exertion. Lactic acidosis is a physiology characterized by accumulation of lactate (especially -lactate), with formation of an excessively high proton concentration H+ and correspondingly low pH in the tissues, a form of metabolic acidosis.
The first stage in metabolizing glucose is glycolysis, the conversion of glucose to pyruvate− and H+:
When sufficient oxygen is present for aerobic respiration, the pyruvate is oxidized to and water by the Krebs cycle, in which oxidative phosphorylation generates ATP for use in powering the cell.
When insufficient oxygen is present, or when there is insufficient capacity for pyruvate oxidation to keep up with rapid pyruvate production during intense exertion, the pyruvate is converted to lactate− by lactate dehydrogenase), a process that absorbs these protons:
The combined effect is:
The production of lactate from glucose (), when viewed in isolation, releases two H+. The H+ are absorbed in the production of ATP, but H+ is subsequently released during hydrolysis of ATP:
Once the production and use of ATP is included, the overall reaction is
The resulting increase in acidity persists until the excess lactate and protons are converted back to pyruvate, and then to glucose for later use, or to and water for the production of ATP.
Studies of brain slices of mice show that β-hydroxybutyrate, lactate, and pyruvate act as oxidative energy substrates, causing an increase in the NAD(P)H oxidation phase, that glucose was insufficient as an energy carrier during intense synaptic activity and, finally, that lactate can be an efficient energy substrate capable of sustaining and enhancing brain aerobic energy metabolism in vitro. The study "provides novel data on biphasic NAD(P)H fluorescence transients, an important physiological response to neural activation that has been reproduced in many studies and that is believed to originate predominantly from activity-induced concentration changes to the cellular NADH pools."
Lactate can also serve as an important source of energy for other organs, including the heart and liver. During physical activity, up to 60% of the heart muscle's energy turnover rate derives from lactate oxidation.
During childbirth, lactate levels in the fetus can be quantified by fetal scalp blood testing.
Lactic acid containing bacteria have shown promise in reducing oxaluria with its descaling properties on calcium compounds.
In lists of nutritional information lactic acid might be included under the term "carbohydrate" (or "carbohydrate by difference") because this often includes everything other than water, protein, fat, ash, and ethanol. If this is the case then the calculated food energy may use the standard that is often used for all carbohydrates. But in some cases lactic acid is ignored in the calculation.For example, in
While not normally found in significant quantities in fruit, lactic acid is the primary organic acid in akebia fruit, making up 2.12% of the juice.
pH Regulation
Lactate production and export contribute significantly to intracellular pH regulation in metabolically active tissues. In skeletal muscle, accumulation of lactic acid lowers intracellular pH, and monocarboxylate transporters facilitate the efflux of both lactate and H⁺, thereby helping maintain acid-base homeostasis and delaying fatigue.
Neural tissue energy source
Although glucose is usually assumed to be the main energy source for living tissues, there is evidence that lactate, in preference to glucose, is preferentially metabolized by in the of several mammalian species that include mouse, , and . According to the lactate shuttle, cells are responsible for transforming glucose into lactate, and for providing lactate to the neurons. Because of this local metabolic activity of glial cells, the extracellular fluid immediately surrounding neurons strongly differs in composition from the blood or cerebrospinal fluid, being much richer with lactate, as was found in microdialysis studies.
Brain development metabolism
Some evidence suggests that lactate is important at early stages of development for brain metabolism in prenatal and early postnatal subjects, with lactate at these stages having higher concentrations in body liquids, and being utilized by the brain preferentially over glucose. It was also hypothesized that lactate may exert a strong action over networks in the developing brain, making them more inhibitory than it was previously assumed, acting either through better support of metabolites, or alterations in base intracellular pH levels, or both.
Blood testing
for lactate are performed to determine the status of the acid base homeostasis in the body. Blood sampling for this purpose is often arterial (even if it is more difficult than venipuncture), because lactate levels differ substantially between arterial and venous, and the arterial level is more representative for this purpose.
+ Reference ranges | 0.5Derived from mass values using molar mass of 90.08 g/mol || 2.2 | mmol/L
! rowspan =2 Arterial
mg/dL
| 0.5 || 1.6 | mmol/L
Uses
In 2023, lactate was the 289th most commonly prescribed medication in the United States, with more than 500,000 prescriptions.
Polymer precursor
Two molecules of lactic acid can be dehydrated to the lactone lactide. In the presence of catalysts lactide polymerize to either atactic or syndiotactic polylactic acid (PLA), which are biodegradable . PLA is an example of a plastic that is not derived from .
Pharmaceutical and cosmetic applications
Lactic acid is also employed in pharmaceutical technology to produce water-soluble lactates from otherwise-insoluble active ingredients. It finds further use in topical preparations and cosmetics to adjust acidity and for its disinfectant and keratolytic properties.
Foods
Fermented food
Lactic acid is found in many fermented foods.
target="_blank" rel="nofollow"> this USDA database entry for yoghurt the food energy is calculated using given coefficients for carbohydrate, fat, and protein. (One must click on "Full report" to see the coefficients.) The calculated value is based on 4.66 grams of carbohydrate, which is exactly equal to the sugars. The actual energy density of lactic acid is .
Separately added
As a food additive it is approved for use in the EU, United States and Australia and New Zealand; it is listed by its INS number 270 or as E number E270. Lactic acid is used as a food preservative, curing agent, and flavoring agent. It is an ingredient in processed foods and is used as a decontaminant during meat processing. Lactic acid is produced commercially by fermentation of carbohydrates such as glucose, sucrose, or lactose, or by chemical synthesis. Carbohydrate sources include corn, beets, and cane sugar.
Forgery
Lactic acid has historically been used to assist with the erasure of inks from official papers to be modified during forgery.
Cleaning products
Lactic acid is used in some liquid cleaners as a descaling agent for removing hard water deposits such as calcium carbonate. It is also used as an antibacterial agent in some hard surface liquid cleaners.
See also
External links
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