Lactose

C 12
H 22
O 11

Ball-and-stick Stick Space-filling

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45 atoms, 46 single bonds.

IUPAC name: (2R,3R,4S,5R,6S)-2-(hydroxymethyl)-6-[(2R,3S,4R,5R)-4,5,6-trihydroxy-2-(hydroxymethyl)oxan-3-yl]oxyoxane-3,4,5-triol
Molar mass: 342.297(39) g/mol
Atoms: 45
SMILES: C(C1C(C(C(C(O1)OC2C(OC(C(C2O)O)O)CO)O)O)O)O
InChIKey: GUBGYTABKSRVRQ-QKKXKWKRSA-N
PubChem CID: 440995

Made of

C H O

Structural formula of lactose. Each unlabelled corner is a carbon atom, and the 14 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.

US food regulations allow two names for this ingredient: “Lactose” or, as an alternative, “Milk sugar”. ChEBI, a database of biological molecules, classes it as a disaccharide of galactose and glucose. The two units are held together by a beta glycosidic linkage, an oxygen bridge from carbon 1 of galactose to carbon 4 of glucose. The galactose unit is always in its beta form, while the glucose unit can be alpha or beta. Lactose contains only carbon, hydrogen and oxygen, as C₁₂H₂₂O₁₁, and its molar mass is 342.3 g/mol.

Pure lactose is a hard, white, odourless crystalline powder. Water dissolves it, CAMEO Chemicals records, while HSDB, the Hazardous Substances Data Bank, finds it barely soluble in alcohol and not at all in ether or chloroform. The IUPAC name, (2R,3R,4S,5R,6S)-2-(hydroxymethyl)-6-[(2R,3S,4R,5R)-4,5,6-trihydroxy-2-(hydroxymethyl)oxan-3-yl]oxyoxane-3,4,5-triol, describes two oxane rings of five carbons and one oxygen each, joined through a single “oxy” oxygen.

Structure and bonding

Lactose’s 45 atoms are joined by 46 single bonds, in two six-membered rings of five carbons and one oxygen each. In Ball-and-stick, one more oxygen, outside both rings, forms the bridge between them. With Labels on, eight O–H groups show around the rims, two of them ending CH₂OH arms, one per ring. Every carbon holds four single bonds reaching into three dimensions, the tetrahedral pattern Chem1 gives carbon in most organic compounds. Stick drops the balls, so the ring outlines and the bridge read as plain lines, while Space-filling swells the atoms into one solid outline.

The bonds drawn are real, but one molecule misses two features of the substance. Its alpha and beta forms differ only in how the hydrogen and hydroxyl group sit on the glucose ring’s carbon 1. In water each turns into the other by mutarotation, and HSDB gives the balance as 61.5 per cent beta and 38.5 per cent alpha. The model comes from a record that leaves that carbon unassigned, so it cannot be read as either form. Crystals usually hold water too, as alpha-lactose monohydrate, while the model is a single anhydrous molecule.

Where it turns up

The milk of mammals carries lactose, human and cow’s milk included, and so do milk products such as cheese and ice cream. When cheese and yogurt are made, fermentation breaks much of it down, MedlinePlus Genetics notes. In milk products, Chem1 explains, acidophilus bacteria that cannot use oxygen feed on lactose and turn it into lactic acid. Whey from a process that converts much of its lactose this way is called acid whey.

Industry takes lactose from whey, the liquid left when cheesemaking separates the curd from milk. In HSDB’s account, the whey is clarified and concentrated, lactose crystallises from it as the monohydrate, and the crystals are separated, washed, dried and milled. Spray drying gives amorphous, non-crystalline lactose instead, a mix of alpha and beta forms that readily takes up moisture. Its uses, according to HSDB, include filling tablets and capsules, foods for infants and convalescents, culture media, penicillin manufacture and silage.

Why it matters

Lactase, an enzyme of the small intestine, splits lactose into glucose and galactose for absorption. In lactose malabsorption, the intestine makes only low levels of lactase and undigested lactose passes into the colon. Bacteria there break it down, producing fluid and gas, according to NIDDK, the US institute for diabetes, digestive and kidney diseases. The leftover sugar also adds to the dissolved particles in the bowel, Chem1 explains, and that surplus pulls water from the intestinal walls, giving diarrhoea.

NIDDK estimates that about 68 per cent of the world’s people have lactose malabsorption. In Africa and Asia, most people do. MedlinePlus Genetics puts the share of humans whose ability to digest lactose drops after infancy at about 65 per cent.

Galactosaemia, a disorder in how the body processes galactose, a sugar found mainly within lactose, results from mutations in the GALT, GALK1 or GALE gene, MedlinePlus Genetics explains. Their enzymes turn galactose into glucose and other molecules the body stores or uses for energy, and a shortage of any one lets galactose build up to toxic levels.

Properties

Properties of lactose Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 440995.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₁₂H₂₂O₁₁
Molar mass	342.297(39) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	12
Hydrogen atoms	22
Oxygen atoms	11
Bonds	46
PubChem molecular weight	342.3 g/mol	g/mol	PubChem

Safety

Lactose is a food sugar and essentially harmless in its everyday forms: 263 of 264 reports under the Globally Harmonized System (GHS) find it meets no hazard criteria. No signal word or hazard statement applies to it. The PubChem Laboratory Chemical Safety Summary for lactose has the full entry.

Questions

What is the difference between lactose and sucrose?

Both have the formula C₁₂H₂₂O₁₁, and both 3D models hold 45 atoms and 46 single bonds. The rings set them apart. In lactose both are six-membered, five carbons and one oxygen apiece, whereas one of the two rings in sucrose has only five members. The taste differs too: HSDB rates lactose at about one-sixth of the sweetening strength of sucrose.

Why are some people lactose intolerant?

The usual cause is lactase nonpersistence, also called primary lactase deficiency. In most humans the activity of the LCT gene, and with it the supply of lactase, falls gradually after infancy. A regulatory stretch of DNA inside a neighbouring gene, MCM6, controls LCT. Rarer forms include congenital lactase deficiency, with little or no enzyme from birth, and secondary intolerance, which follows injury to the small intestine.

Is lactose intolerance the same as a milk allergy?

No. NIDDK treats them as separate conditions with separate causes. Lactose intolerance means digestive symptoms such as bloating, diarrhoea and gas after eating or drinking something that contains lactose, so it concerns the sugar. A milk allergy is the immune system reacting to one or more of the proteins that milk contains. Someone who cannot digest all their lactose but has no symptoms has malabsorption, not intolerance.

How is lactose removed from milk?

An enzyme splits it. US rules allow lactase from the yeast Kluyveromyces lactis, affirmed as generally recognised as safe in 1984, to turn milk's lactose into glucose and galactose. Lactase-treated milk holds less lactose than regular milk. Powder was different: splitting the sugar before drying makes milk brown and burn on a dryer's hot surfaces. For military field rations, chemist Virginia Holsinger dry-blended a lactase from the fungus Aspergillus oryzae into nonfat milk powder, the USDA reported in 1995.

Made of the same elements

C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.