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Lactose (milk sugar)
CAS number 63-42-3 YesY
PubChem 6134←←
ChemSpider 5904 YesY
EC-number 200-559-2
ChEBI CHEBI:36218 YesY
Jmol-3D images Image 1
Molecular formula C12H22O11
Molar mass 342.30 g/mol
Appearance white solid
Density 1.525 g/cm3
Melting point 202.8 °C (397.0 °F; 475.9 K)[2]
Boiling point 668.9 °C (1,236.0 °F; 942.0 K)[2]
Solubility in water 21.6 g/100 mL[1]
Std enthalpy of
5652 kJ/mol, 1351 kcal/mol, 16.5 kJ/g, 3.94 kcal/g
EU Index not listed
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point 357.8 °C (676.0 °F; 631.0 K)[2]
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Lactose is a disaccharide sugar derived from galactose and glucose that is found in milk. Lactose makes up around 2–8% of milk (by weight),[3] although the amount varies among species and individuals, and milk with a reduced amount of lactose also exists. It is extracted from sweet or sour whey. The name comes from lac or lactis, the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C12H22O11.


Lactose was discovered in milk in 1619 by Fabrizio Bartoletti, and identified as a sugar in 1780 by Carl Wilhelm Scheele.[4]

Structure and reactions[edit]

The molecular structure of α-lactose, as determined by X-ray crystallography.

Lactose is a disaccharide derived from the condensation of galactose and glucose, which form a β-1→4 glycosidic linkage. Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose. The glucose can be in either the α-pyranose form or the β-pyranose form, whereas the galactose can only have the β-pyranose form: hence α-lactose and β-lactose refer to anomeric form of the glucopyranose ring alone..

Lactose is hydrolysed to glucose and galactose, isomerised in alkaline solution to lactulose, and catalytically hydrogenated to the corresponding polyhydric alcohol, lactitol.[4]

Lactose monohydrate crystals have a characteristic tomahawk shape that can be observed with a light microscope.


Several million tons are produced annually as a by-product of the dairy industry. Whey is made of up 6.5% solids of which 4.8% is lactose that may be purified by crystallisation.[5] Whey or milk plasma is the liquid remaining after milk is curdled and strained, for example in the production of cheese. Lactose makes up about 2-8% of milk by weight[citation needed]. Industrially, lactose is produced from whey permeate – that is whey filtrated for all major proteins. The protein fraction is used in infant nutrition and sport nutrition while the permeate can be evaporated to 60-65 % solids and crystallized while cooling .[6] Lactose can also be precipitated from whey using ethanol. Since it is insoluble in ethanol, lactose precipitates, in about 65% yield .[7]


Infant mammals nurse on their mothers to drink milk, which is rich in lactose. The intestinal villi secrete the enzyme called lactase (β-D-galactosidase) to digest it. This enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can be absorbed. Since lactose occurs mostly in milk, in most mammals, the production of lactase gradually decreases with maturity due to a lack of constant consumption.

Many people with ancestry in Europe, West Asia, South Asia, and parts of East Africa maintain lactase production into adulthood. In many of these areas, milk from mammals such as cattle, goats, and sheep is used as a large source of food. Hence, it was in these regions that genes for lifelong lactase production first evolved. The genes of adult lactose tolerance have evolved independently in various ethnic groups.[8] By descent, more than 70% of western Europeans can drink milk as adults, compared with less than 30% of people from areas of Africa, eastern and south-eastern Asia and Oceania.[9] In people who are lactose intolerant, lactose is not broken down and provides food for gas-producing gut flora, which can lead to diarrhea, bloating, flatulence, and other gastrointestinal symptoms.


Food industry applications have markedly increased since the 1960s. For example, its bland flavor has lent to its use as a carrier and stabiliser of aromas and pharmaceutical products. Lactose is not added directly to many foods, because its solubility is less than other sugars commonly used in food. Infant formula is a notable exception, where the addition of lactose is necessary to match the composition of human milk.

Lactose is not fermented by yeast during brewing, which may be used to advantage.[4] For example, lactose may be used to sweeten stout beer; the resulting beer is usually called a milk stout or a cream stout.

Another major use of lactose is in the pharmaceutical industry. Lactose is added to pills as a filler because of its physical properties, i.e., compressibility, and low price. For similar reasons it can be used to dilute heroin.

See also[edit]


  1. ^ The solubility of lactose in water is 18.9049 g at 25 °C, 25.1484 g at 40 °C and 37.2149 g at 60 °C per 100 g solution. Its solubility in ethanol is 0.0111 g at 40 °C and 0.0270 g at 60 °C per 100 g solution.Machado, José J. B.; Coutinho, João A.; Macedo, Eugénia A. (2001), "Solid–liquid equilibrium of α-lactose in ethanol/water", Fluid Phase Equilibria 173 (1): 121–34, doi:10.1016/S0378-3812(00)00388-5 . ds
  2. ^ a b c Sigma Aldrich
  3. ^ Carper, Steve. "The Really BIG List of Lactose Percentages". Lactose Intolerance Clearinghouse. Retrieved 30 January 2014. 
  4. ^ a b c Linko, P (1982), "Lactose and Lactitol", in Birch, G.G. & Parker, K.J, Natural Sweeteners, London & New Jersey: Applied Science Publishers, pp. 109–132, ISBN 0-85334-997-5 
  5. ^ Ranken, M. D.; Kill, R. C. (1997), Food industries manual, Springer, p. 125, ISBN 0-7514-0404-7 
  6. ^ Wong SY, Hartel RW.: Crystallization in lactose refining-a review. J Food Sci 2014, 79(3):R257-72.[]
  7. ^ Minard, R. Introduction to Organic Laboratory Techniques: A Microscale Approach. Pavia, Lampman, Kriz & Engel, Saunders. 1990.
  8. ^ Wade, Nicholas (2006-12-10), "Study Detects Recent Instance of Human Evolution", New York Times .
  9. ^ Ridley, Matt (1999), Genome, HarperCollins, p. 193, ISBN 978-0-06-089408-5 .