What is "lactic acid"

Lactic \Lac"tic\, a. [L. lac, lactis, milk: cf. F. lactique. See Lacteal, and cf. Galactic.] (Physiol. Chem.) Of or pertaining to milk; procured from sour milk or whey; as, lactic acid; lactic fermentation, etc.

Lactic acid (Physiol. Chem.), a sirupy, colorless fluid, soluble in water, with an intensely sour taste and strong acid reaction. There is one center of optical activity, and this results in the observation of three isomeric modifications all having the formula C3H6O3; one is dextrorotatory (L-lactic acid), the other levorotatory (D-lactic acid), and the third an optically inactive mixture of the first two (DL-lactic acid); chemically it is 2-hydroxypropanoic acid. Sarcolactic acid or paralactic acid occurs chiefly in dead muscle tissue, while ordinary lactic acid (DL-lactic acid) results from fermentation, such as the fermentation of milk by lactic acid bacteria. The two acids are alike in having the same constitution (expressed by the name ethylidene lactic acid), but the latter is optically inactive, while sarcolactic acid rotates the plane of polarization to the right. The third acid, ethylene lactic acid, accompanies sarcolactic acid in the juice of flesh, and is optically inactive.

Lactic ferment, an organized ferment ( Bacterium lacticum or Bacterium lactis), which produces lactic fermentation, decomposing the sugar of milk into carbonic and lactic acids, the latter, of which renders the milk sour, and precipitates the casein, thus giving rise to the so-called spontaneous coagulation of milk.

Lactic fermentation. See under Fermentation.

n. (context organic compound English) 2-hydroxy-propanoic acid (CH₃.CHOH.CO₂H), a syrupy liquid, soluble in water; found in milk, wine and many fruits; used as a food additive and in many industrial applications.

n. a clear odorless hygroscopic syrupy carboxylic acid found in sour milk and in many fruits

Lactic acid is an organic compound with the formula CHCH(OH)COH. In its solid state, it is white and water-soluble. In its liquid state, it is clear. It is produced both naturally and synthetically. With a hydroxyl group adjacent to the carboxyl group, lactic acid is classified as an alpha-hydroxy acid (AHA). In the form of its conjugate base called lactate, it plays a role in several biochemical processes.

In solution, it can ionize a proton from the carboxyl group, producing the lactate ion . Compared to acetic acid, its pK is 1 unit less, meaning lactic acid deprotonates ten times more easily than acetic acid does. This higher acidity is the consequence of the intramolecular hydrogen bonding between the α-hydroxyl and the carboxylate group.

Lactic acid is chiral, consisting of 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. A mixture of the two in equal amounts is called DL-lactic acid, or racemic lactic acid.

Lactic acid is hygroscopic. DL-lactic acid is miscible with water and with ethanol above its melting point which is around 17 or 18 °C. D-lactic acid and L-lactic acid have a higher melting point.

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 at rest, but can rise to over 20 mmol/L during intense exertion and as high as 25 mmol/L 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 caries.

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.