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.

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