Malic acid

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15 atoms, 14 bonds.

IUPAC name: (2S)-2-hydroxybutanedioic acid
Molar mass: 134.087(14) g/mol
Atoms: 15
SMILES: C(C(C(=O)O)O)C(=O)O
InChIKey: BJEPYKJPYRNKOW-REOHCLBHSA-N
PubChem CID: 222656

Made of

Structural formula of malic acid. Each unlabelled corner is a carbon atom, and the three hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Of the two mirror-image forms malic acid can take, only the L form occurs in nature, according to the Hazardous Substances Data Bank (HSDB). The model shows that form, (2S)-2-hydroxybutanedioic acid by its IUPAC name. ChEBI (Chemical Entities of Biological Interest) describes it as succinic acid with a hydroxy group, –OH, in place of one hydrogen on a carbon. It is a dicarboxylic acid, carrying two carboxyl groups, –COOH, the group Chem1 identifies as the mark of an organic acid. Four carbon, six hydrogen and five oxygen atoms give a molar mass of 134.09 g/mol.

HSDB describes malic acid as a white, crystalline material and calls its taste smoothly tart. Malic acid dissolves readily in water, and also in methanol, ethanol and acetone. Heated, the L form breaks down at about 140 °C.

Structure and bonding

Malic acid has no ring: its 15 atoms hang on an unbranched chain of four carbons. Each end carbon belongs to a carboxyl group, –COOH: it holds one oxygen by a double bond, drawn as two sticks, and an OH group by a single bond. Around a carboxyl carbon the model’s three bond angles add up to 360°, so its neighbours lie flat. Stick thins the frame to its bonds, and Space-filling draws only overlapping spheres, so double and single bonds look alike.

With Labels on, the second carbon shows four different partners: H, OH, COOH and CH₂COOH. This carbon is the stereocentre behind malic acid’s two mirror-image forms, and the structural formula marks it with a wedge. The model fixes the (S) arrangement, the L form.

The model is the neutral acid, a hydrogen still on each carboxyl group. In water malic acid can give up both as hydrogen ions. HSDB lists 3.51 and 5.03 at 20 °C for the two steps, values on the pK_a scale that the phosphoric acid page explains. ChEBI pairs the (S) acid with (S)-malate, the ion with a 2− charge left once both have gone.

Where it turns up

Haz-Map calls L-malic acid the natural form present in apples and other plants, and places malic acid in unripe apples and other fruit. HSDB finds relatively high levels in many fruits and vegetables and calls L-malic acid a common metabolite, a product of living chemistry, in plants and animals. The LOTUS natural-products database reports it in Rehmannia glutinosa and Punica granatum.

The US Food and Drug Administration (FDA) lists L-malic acid as an antimicrobial agent, a flavour enhancer, a flavouring agent and a pH control agent. It also lists it as a synergist, an additive that strengthens the action of others. Commercial acid, mostly the DL form, also goes into wine, stucco plaster, cosmetics, toothpastes and medicines. The Cosmetic Ingredient Review judges malic acid and sodium malate safe as pH adjusters in cosmetics.

Why it matters

In cells malic acid works as malate, which HSDB places in every living organism as a stage of the citric acid cycle. Within the cycle, malate forms from fumaric acid and is then oxidised to oxaloacetic acid. Malic enzyme, present in many organisms including bacteria and plants, also converts it to pyruvic acid. ChEBI gives malic acid two roles, fundamental metabolite and food acidity regulator.

Natural L-malic acid has a specific optical rotation of −5.7° in acetone at 18 °C, on HSDB’s figure. The synthetic acid, a mixture of both mirror forms, is optically inactive. Commercial makers in the United States, Canada and Europe produce only that synthetic R,S-malic acid. The tart acidulant made there is therefore not the single form of fruit that the model shows. Japan produces both the S and the R,S forms. Reports to the US Environmental Protection Agency put the 2023 national volume at 10 million to under 50 million pounds.

Properties

Properties of malic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 222656.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₄H₆O₅
Molar mass	134.087(14) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	4
Hydrogen atoms	6
Oxygen atoms	5
Bonds	14
Single bonds	12
Double bonds	2
PubChem molecular weight	134.09 g/mol	g/mol	PubChem

Safety

Pure malic acid seriously irritates the eyes and can irritate the skin. Notifications to the European Chemicals Agency give it the signal word Warning, with serious eye irritation in 97.7% of reports and skin irritation in 18%. When heated to decomposition it gives off acrid smoke and irritating fumes, HSDB notes. The PubChem Laboratory Chemical Safety Summary for malic acid is the sheet for anyone handling the powder.

Questions

Why does the malic acid model show only one mirror-image form?

It is built from PubChem's record for L-malic acid, (2S)-malic acid, the form found in fruit, so it draws that arrangement. ChEBI names (R)-malic acid as its enantiomer, its mirror-image partner. Malic acid with neither arrangement specified has a record of its own, CID 525, which describes the synthetic acid sold to food makers, a mixture holding both forms.

Is the malic acid in food the same as the malic acid in apples?

Not always in form. Apples and other plants carry L-malic acid, which industry can also obtain from maleic acid or by fermenting sugars. The DL acid is made from maleic and fumaric acids, and HSDB calls it a widely used food acidulant, an ingredient added to raise the acidity of a food.

Why is malic acid added to sweets and drinks?

For its sourness. HSDB reports that malic acid ionises in water to the same degree as citric acid yet has a much stronger apparent acid taste, and less of it usually gives the same effect. In 1983, on HSDB's figures, half of all malic acid went into dry drink powders and a quarter into carbonated and still drinks. Food and sweets took 15 per cent, metal cleaning and finishing 5 per cent and other uses the last 5 per cent.

Is malic acid safe to eat?

As a food flavouring, Haz-Map records it as safe, following an international expert committee on food additives that found no safety concern in that use. Malates are ordinary parts of human and animal diets and are metabolised rapidly and completely to carbon dioxide, HSDB notes. People and laboratory animals break down both mirror forms readily, and HSDB records no reason to tell L- and DL-malic acid apart when judging their safe use in food.

Made of the same elements

C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 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.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.