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Lactic acid is a carboxylic acid and its chemical formula is C₃ H₆ O₃ and its structure is reflected in its systematic name, 2- hydroxy propanoic acid.

In solution, it can lose a protonFor alternative meanings see proton (disambiguation). Proton Classification Subatomic particle Fermion Hadron Baryon Nucleon Proton Properties Mass: 938 MeV/ c2 Electric Charge: 1. 6 × 10−19 C Spin: 1/2 In physics, the proton is a subatomic particle from the COOH carboxy group, turning into the lactate ion CH₃CHOHCOO^-.

There are two optical isomers of lactic acid (and of lactate) since the central carbon atom is bound to four different groups. The first isomer is known as L(+)-lactic acid or (S)-lactic acid and the second is D(-)-lactic acid or (R)-lactic acid.

During one form of anerobic glycolysisAnerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O) are available. This is only an effective means of energy during short, intense exercise, providing energy for 30 seconds to 2 minutes. It replenishes very q or fermentationIn its strictest sense fermentation is the energy-yielding anaerobic metabolic breakdown of a nutrient molecule, such as glucose, without net oxidation. Fermentation yields lactate, acetic acid, ethanol, or some other simple product. Fermentation is also, L-lactate is produced from pyruvatePyruvate CHCOCOO&minus is the ionized form of pyruvic acid. It is an important chemical compound in biochemistry. It is the output of the breakdown of glucose known as glycolysis, and (in aerobic respiration) the main input for the citric acid cycle. via the enzymeAn enzyme is a protein, or protein complex, that catalyzes a chemical reaction. Like any catalyst, enzymes work by lowering the activation energy of a reaction, thus allowing the reaction to proceed to its steady state or completion much faster than it ot lactate dehydrogenase. This conversion also oxidizes one molecule of NADH to NAD⁺, and this is the reason for the conversion: NAD⁺ has to be regenerated so that glycolysis can continue.

This lactic acid fermentation occurs in red blood cells since they don't have mitochondria, and in skeletal muscle during intense exertion when sufficient amounts of oxygen cannot be supplied fast enough. This lactate is released into the bloodstream. The typical lactate concentration in the blood is 1-2 m mole/ L. The liver takes up about 60% of the lactate and reoxidizes it to pyruvate, which is then reconverted to glucose in a process known as gluconeogenesis. The glucose enters the bloodstream and can be used by the tissues. This glucose → lactate → glucose cycle, originally described by Carl and Gerti Cori, is known as the Cori cycle. About 40% of the lactate is taken up by well oxygenated muscle cells and oxidized to pyruvate, which is then directly used to fuel the citric acid cycle.

This lactic acid fermentation is also performed by lactic acid bacteria which are responsible for the sour taste of old milk and which are used in the production of dairy products such as cheese, yoghurt and kefir. Lactic acid fermentation also gives the sour taste to fermented vegetables such as traditionally cultured sauerkraut and pickles and many fermented starches such as poi. Lactic acid bacteria also operates in the mouth, and the resulting lactic acid is responsible for the tooth decay known as caries.

Lactic acid is also the result of malolactic fermentation, a process used in winemaking to convert sharp-tasting malic acid into the gentler lactic acid.

The CAS number of lactic acid is 50-21-5. To distinguish between the optical isomers, one can use CAS 79-33-4 for L-lactic acid and CAS 10326-41-7 for D-lactic acid and CAS 598-82-3 for an optically inactive mixture of the two.