lactic acid

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Lactic acid
IUPAC name	2-hydroxypropanoic acid
Identifiers
CAS number	598-82-3, D/L: [50-21-5] L: [79-33-4] D: [10326-41-7]
ATC code	G01AD01,QP53AG02
SMILES	CC(O)C(=O)O
ChemSpider ID	96860
Properties
Molecular formula	C3H6O3
Molar mass	90.08 g/mol
Melting point	L: 53 °C D: 53 °C D/L: 16.8 °C
Boiling point	122 °C @ 12 mmHg
Acidity (pKa)	3.86 at 25 °C
Related compounds
Other anions	lactate
Related carboxylic acids	acetic acid glycolic acid propionic acid 3-hydroxypropanoic acid malonic acid butyric acid hydroxybutyric acid
Related compounds	1-propanol 2-propanol propionaldehyde acrolein sodium lactate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Lactic acid (IUPAC systematic name: 2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It was first isolated in 1780 by a Swedish chemist, Carl Wilhelm Scheele, and is a carboxylic acid with a chemical formula of C₃H₆O₃. It has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid (AHA). In solution, it can lose a proton from the acidic group, producing the lactate ion CH₃CH(OH)COO⁻. It is miscible with water or ethanol, and is hygroscopic.

Lactic acid is chiral and has two optical isomers. One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(−)-lactic acid or (R)-lactic acid. L-(+)-Lactic acid is the biologically important isomer.

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. The concentration of blood lactate is usually 1–2 mmol/L at rest, but can rise to over 20 mmol/L during intense exertion.

Industrially, lactic acid fermentation is performed by Lactobacillus bacteria, among others. These bacteria can operate in the mouth; the acid they produce is responsible for the tooth decay known as caries.

In medicine, lactate is one of the main components of Ringer's lactate or lactated Ringer's solution (Compound Sodium Lactate or Hartmann's Solution in the UK). This intravenous fluid consists of sodium and potassium cations, with lactate and chloride anions, in solution with distilled water in concentration so as to be isotonic compared to human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or a burn injury.

Exercise and lactate

During power exercises such as sprinting, when the rate of demand for energy is high, lactate is produced faster than the ability of the tissues to remove it and lactate concentration begins to rise. This is a beneficial process since the regeneration of NAD⁺ ensures that energy production is maintained and exercise can continue. The increased lactate produced can be removed in a number of ways including

• oxidation to pyruvate by well-oxygenated muscle cells which is then directly used to fuel the Krebs cycle,
• conversion to glucose via the Cori cycle in the liver through the process of gluconeogenesis.

Contrary to popular belief, this increased concentration of lactate does not directly cause acidosis, nor is it responsible for delayed onset muscle soreness.^[1] This is because lactate itself is not capable of releasing a proton, and secondly, the acidic form of lactate, lactic acid, cannot be formed under normal circumstances in human tissues.^{[citation needed]} Analysis of the glycolytic pathway in humans indicates that there are not enough hydrogen ions present in the glycolytic intermediates to produce lactic or any other acid.

The acidosis that is associated with increases in lactate concentration during heavy exercise arises from a separate reaction. When ATP is hydrolysed, a hydrogen ion is released. ATP-derived hydrogen ions are primarily responsible for the decrease in pH. During intense exercise, aerobic metabolism cannot produce ATP quickly enough to supply the demands of the muscle. As a result, anaerobic metabolism becomes the dominant energy producing pathway as it can form ATP at high rates. Due to the large amounts of ATP being produced and hydrolysed in a short period of time, the buffering systems of the tissues are overcome, causing pH to fall and creating a state of acidosis, a natural process which facilitates the easier dissociation of Oxyhaemoglobin and allows easier transfer of oxygen from the blood^[2]. This may be one factor, among many, that contributes to the acute muscular discomfort experienced shortly after intense exercise.^{[citation needed]}

The effect of lactate on acidosis has been the topic of many recent conferences in the field of exercise physiology. Robergs et al. have accurately chased the proton movement that occurs during glycolysis. However, in doing so, they have suggested that [H⁺] is an independent variable that determines its own concentration. A recent review by Lindinger et al.^[3] has been written to rebut the stoichiometric approach used by Robergs et al. (2004).^[1] In using this stoichiometric process, Robergs et al. have ignored the causative factors (independent variables) of the concentration of hydrogen ions (denoted [H⁺]). These factors are strong ion difference [SID], PCO₂, and weak acid buffers. Lactate is a strong anion, and causes a reduction in [SID] which causes an increase in [H⁺] to maintain electroneutrality. PCO₂ also causes an increase in [H⁺]. During exercise, the intramuscular lactate concentration and PCO₂ increase, causing an increase in [H⁺], and thus a decrease in pH. (See Le Chatelier's principle)

Reference ranges for blood tests, comparing blood content of lactate (shown in violet at center-right) with other constituents.

Lactic acid as a polymer precursor

Two molecules of lactic acid can be dehydrated to lactide, a cyclic lactone. A variety of catalysts can polymerise lactide to either heterotactic or syndiotactic polylactide, which as biodegradable polyesters with valuable (inter alia) medical properties are currently attracting much attention.

Nowadays, lactic acid is used as a monomer for producing polylactic acid (PLA) which later has application as biodegradable plastic. This kind of plastic is a good option for substituting conventional plastic produced from petroleum oil because of low emission of carbon dioxide that can contribute to global warming. The commonly used process in producing lactic acid is via fermentation, and later to obtain the polylactic acid, the polymerization process follows.

Lactic acid in foods

Lactic acid is primarily found in sour milk products, such as: koumiss, leban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid.

Sauerkraut contains lactic acid, formed when lactic acid bacteria ferment the sugars in finely shredded cabbage.

Lactic acid in detergents

Lactic acid has gained importance in the detergents industry the last decade. Being a good descaler, soap-scum remover and being a registered anti-bacterial agent - a trend toward safer and natural ingredients has also contributed.

See also

• Cori cycle
• Alanine cycle
• Dental caries
• Biodegradable plastic
• 2-Hydroxybutyric acid
• Acids in wine

References

External links

• Lactic acid and running: myths, legends and reality
• Corn Plastic to the Rescue
• Lactic acid: a safe and natural ingredient
• Lactic Acid: Information and Resources
• Lactic Acid Is Not Muscles' Foe, It's Fuel