Lactic acid

Ball-and-stick Stick Space-filling

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

12 atoms, 11 bonds.

IUPAC name: 2-hydroxypropanoic acid
Molar mass: 90.078(10) g/mol
Atoms: 12
SMILES: CC(C(=O)O)O
InChIKey: JVTAAEKCZFNVCJ-UHFFFAOYSA-N
PubChem CID: 612

Made of

Structural formula of lactic acid. Each unlabelled corner is a carbon atom, and the one hydrogen on carbon is not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Fermenting bacteria and a sprinter’s leg muscles turn sugar into the same small acid. Its IUPAC name, 2-hydroxypropanoic acid, spells out the structure. ChEBI, a database of small biological molecules, reads it as propanoic acid with a hydroxy group, –OH, on the carbon beside the acid group. Three atoms each of carbon and oxygen and six of hydrogen give C₃H₆O₃, and a mole weighs 90.08 g.

CAMEO Chemicals describes lactic acid as a syrupy liquid, colourless to yellow and without odour. It mixes with water in any proportion but hardly dissolves in benzene or chloroform, according to the Hazardous Substances Data Bank (HSDB). It is also hygroscopic, drawing moisture from the air. Its pK_a, a measure of how readily an acid gives up its hydrogen ion, is 3.86 at 20 °C, and it is a weak acid, as most organic acids are.

Structure and bonding

Lactic acid is built on a short chain of three carbons. One end carries three hydrogens; the middle carbon holds one hydrogen and an –OH group. The far carbon carries a double-bonded oxygen, drawn as two sticks, and a second –OH. With Labels on, the twelve atoms carry C, H and O symbols; the readout counts eleven bonds. The Stick view shows the chain bending at the middle carbon. In the Space-filling view the carboxyl oxygens cap one end and the methyl hydrogens the other.

The middle carbon’s four partners, H, CH₃, OH and COOH, all differ. Chem1 calls such a carbon chiral: it exists as two mirror-image forms, called enantiomers. The model has to fix one of them, though its structure carries no L or D label. The commercial acid is a racemic mixture of both, according to HSDB.

The drawing is the neutral acid. ChEBI calls lactic acid the conjugate acid of lactate, the ion left once the acid’s O–H hydrogen goes, and lactate is what blood tests measure. The concentrated acid also partly reacts with itself, HSDB notes, making a mixture of lactic acid and esters that split back into lactic acid when diluted.

Where it turns up

Human blood and muscle fluid hold small amounts of L-lactic acid, and HSDB records the level rising after vigorous activity. The same form turns up in the liver, kidneys, thymus gland and amniotic fluid. Outside animals, the LOTUS natural-products database reports it in the plants Sambucus ebulus and Festuca rubra. ChEBI lists it as a metabolite of algae and of the water flea Daphnia magna.

In the kitchen, CAMEO Chemicals ties lactic acid to cultured dairy products and food preservation. HSDB adds cheese and confectionery making, infant milk formulas, a sour note in drinks and the acidifying of worts, the sugary liquid of brewing. In the US Food and Drug Administration (FDA) list, it works in food as an antimicrobial, a curing and pickling agent, a flavour enhancer and a pH control agent.

Why it matters

Lactic acid sits at a junction of energy metabolism. The enzyme lactate dehydrogenase makes it from pyruvate (pyruvic acid) during fermentation. The US National Cancer Institute’s thesaurus notes that the same step also produces NAD (nicotinamide adenine dinucleotide). Glycolysis, the breakdown of glucose, then uses that NAD to make ATP (adenosine triphosphate), the cell’s energy source. When oxygen cannot reach muscle cells fast enough, Chem1 explains, they switch to this anaerobic route instead of making CO₂. HSDB adds that most mammalian cells make L-lactic acid routinely, as a normal intermediate.

Industry makes lactic acid either with bacteria, by fermentation, or by a synthetic method, according to HSDB. Fermentation strains such as Lactobacillus delbrueckii work on glucose, corn syrup, molasses, beet juice, whey and starches from rice, wheat, maize and potato. HSDB names polylactic acid (PLA) as lactic acid’s fastest-growing use. HSDB lists PLA’s uses as food and drink containers, packaging film, disposable cups and cutlery, clothing fibres, pillow filling and carpet.

Properties

Properties of lactic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 612.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₆O₃
Molar mass	90.078(10) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	6
Oxygen atoms	3
Bonds	11
Single bonds	10
Double bonds	1
PubChem molecular weight	90.08 g/mol	g/mol	PubChem

Safety

Concentrated lactic acid is corrosive, though the substance is also a food ingredient. Company notifications to the European Chemicals Agency carry the signal word Danger, with serious eye damage in 91.9% of them and skin irritation in 87.5%. The International Labour Organization adds irritation of the airways and corrosive harm if swallowed. The FDA affirms it as generally recognised as safe when added to food, and the PubChem Laboratory Chemical Safety Summary is the authoritative sheet.

Questions

What is the difference between L- and D-lactic acid?

The middle carbon is asymmetric, and lactic acid has two mirror-image forms, HSDB notes. Most mammalian cells make the L form, and people break it down in preference to the D form, by the same account. HSDB lists the D form melting at 52.8 °C and the L form at 53 °C, while the commercial acid, a racemic mix of the two, melts at 16.8 °C. The 3D model has to show one of them.

Why do muscles make lactic acid during exercise?

Hard physical work uses more oxygen, and MedlinePlus explains that this pushes cells to make energy without it, so lactate rises for a short while. The blood then carries it to the liver, where, according to HSDB, it is turned back into glucose by gluconeogenesis, a loop known as the Cori cycle. MedlinePlus adds the kidneys as a second site of that conversion.

What is lactic acidosis?

MedlinePlus describes it as a life-threatening condition in which lactate builds up until the blood becomes too acidic. HSDB traces it to disorders that raise pyruvate, speed up lactic acid formation or slow its breakdown. Its causes include conditions that cut the oxygen reaching tissues and conditions that disturb how cells use oxygen, make energy or clear lactate, according to MedlinePlus.

What is lactic acid used for besides food?

Haz-Map, drawing on HSDB, lists it as a solvent and acidulant, an ingredient that makes a product more acidic, in drugs and dyes. It also serves as a mordant, or dye-fixing agent, for printing woollen goods, a soldering flux, a dehairing agent, and in leather tanning, oil-well acidising and plant growth regulation. In medicine, the WHO drug classification files it among gynaecological anti-infectives and antiseptics, and the FDA classes its action as acidifying.

Made of the same elements

C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.