| IUPAC name
| Other names
|3D model (Jmol)||Interactive image|
|Molar mass||342.30 g/mol|
|Melting point||202.8 °C (397.0 °F; 475.9 K)|
|Boiling point||668.9 °C (1,236.0 °F; 942.0 K)|
|21.6 g/100 mL|
Chiral rotation ([α]D)
Std enthalpy of
|5652 kJ/mol, 1351 kcal/mol, 16.5 kJ/g, 3.94 kcal/g|
|Flash point||357.8 °C (676.0 °F; 631.0 K)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|(what is ?)|
Lactose is a disaccharide sugar composed of galactose and glucose that is found in milk. Lactose makes up around 2–8% of milk (by weight), 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 (gen. lactis), the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C12H22O11 and the hydrate formula C12H22O11·H2O, making it an isomer of sucrose.
The first crude isolation of lactose, by Italian physician Fabrizio Bartoletti (1576–1630), was published in 1633. In 1700, the Venetian pharmacist Lodovico Testi (1640–1707) published a booklet of testimonials to the power of milk sugar (saccharum lactis) to relieve, among other ailments, the symptoms of arthritis. In 1715, Testi's procedure for making milk sugar was published by Antonio Vallisneri. Lactose was identified as a sugar in 1780 by Carl Wilhelm Scheele.
In 1812, Heinrich Vogel (1778-1867) recognized that glucose was a product of hydrolyzing lactose. In 1856, Louis Pasteur crystallized the other component of lactose, galactose. By 1894, Emil Fischer had established the configurations of the component sugars.
Lactose was named by the French chemist Jean Baptiste André Dumas (1800-1884) in 1843.
Structure and reactions
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 the anomeric form of the glucopyranose ring alone.
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. 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. 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 . Lactose can also be precipitated from whey using ethanol. Since it is insoluble in ethanol, lactose precipitates, in about 65% yield.
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 continuing consumption.
Many people with ancestry in Europe, West Asia, South Asia, the Sahel belt in West Africa, East Africa and a few other parts of Central 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. 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. 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 that of 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 most yeast during brewing, which may be used to advantage. For example, lactose may be used to sweeten stout beer; the resulting beer is usually called a milk stout or a cream stout.
Yeast belonging to the genus Kluyveromyces have a unique industrial application as they are capable of fermenting lactose for ethanol production. Surplus lactose from the whey by-product of dairy operations is a potential source of alternative energy.
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.
- Sigma Aldrich
- 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" (PDF), Fluid Phase Equilibria, 173 (1): 121–34, doi:10.1016/S0378-3812(00)00388-5. ds
- Carper, Steve. "The Really BIG List of Lactose Percentages". Lactose Intolerance Clearinghouse. Retrieved 30 January 2014.
- In 1856, Louis Pasteur named galactose "lactose". See:
- Pasteur (1856) "Note sur le sucre de lait" (Note on milk sugar), Comptes rendus, 42 : 347-351. From page 348: "Je propose de le nommer lactose." (I propose to name it lactose.)
- Fabrizio Bartoletti, Methodus in dyspnoeam … [Procedure for asthma … ], (Bologna ("Bononia"), (Italy): Nicolò Tebaldini for the heirs of Evangelista Dozza, 1633), p. 400. From page 400: "Manna seri hæc. Destilla leni balnei calore serum lactis, donec in fundo vasis butyracea fœx subsideat, cui hærebit salina quædam substantia subalbida. Hanc curiose segrega, est enim sal seri essentiale; seu nitrum, cujus causa nitrosum dicitut serum, huicque tota abstergedi vis inest. Solve in aqua propria, & coagula. Opus repete, donec seri cremorem habeas sapore omnino mannam referentem." (This is the manna of whey. [Note: "Manna" was the dried, sweet sap of the tree Fraxinus ornus.] Gently distill whey via a heat bath until the buttery scum settles to the bottom of the vessel, to which substance some whitish salt [i.e., precipitate] attaches. This curious [substance once] separated, is truly the essential salt of whey; or, on account of which nitre, is called "nitre of whey", and all [life] force is in this that will be expelled. [Note: "Nitre" was an alchemical concept. It was the power of life, which gave life to otherwise inanimate matter. See the philosophy of Sendivogius.] Dissolve it in [its] own water and coagulate. Repeat the operation until you have cream of whey, recalling, by [its] taste, only manna.)
In 1688, the German physician Michael Ettmüller (1644–1683) reprinted Bartoletti's preparation. See: Ettmüller, Michael, Opera Omnia … (Frankfurt am Main ("Francofurtum ad Moenum"), [Germany]: Johann David Zunner, 1688), book 2, page 163. From page 163: "Undd Bertholetus praeparat ex sero lactis remedium, quod vocat mannam S. [alchemical symbol for salt, salem] seri lactis vid. in Encyclopaed. p. 400. Praeparatio est haec: … " (Whence Bartoletti prepared a medicine from milk whey, which he called manna or salt of milk whey, see in [his] Encyclopedia [note: this is a mistake; the preparation appeared in Bartoletti's Methodus in dyspnoeam … ], p. 400. This is the preparation: … )
- Lodovico Testi, De novo Saccharo Lactis [On the new milk sugar] (Venice, (Italy): Hertz, 1700).
- Ludovico Testi (1715) "Saccharum lactis" (Milk sugar), Academiae Caesareo-Leopoldinae naturae curiosorum ephemerides, … , 3 : 69-79. The procedure was also published in Giornale de' letterati d'Italia in 1715.
- Carl Wilhelm Scheele (1780) "Om Mjölk och dess syra" (About milk and its acid), Kongliga Vetenskaps Academiens Nya Handlingar (New Proceedings of the Royal Academy of Science), 1 : 116-124. From page 116: "Det år bekant, at Ko-mjölk innehåller Smör, Ost, Mjölk-såcker, … " (It is known, that cow's milk contains butter, cheese, milk-sugar, … )
- Carl Wilhelm Scheele (1780) "Om Mjölk-Såcker-Syra" (On milk-sugar acid), Kongliga Vetenskaps Academiens Nya Handlingar (New Proceedings of the Royal Academy of Science), 1 : 269-275. From pages 269–270: "Mjölk-Såcker år et sal essentiale, som uti Mjölken finnes uplöst, och som, för dess sötaktiga smak skull, fått namn af såcker." (Milk sugar is an essential salt, which is found dissolved in milk, and which, on account of its sweet taste, has the name of "sugar".)
- 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
- Vogel (1812) "Sur le sucre liquide d'amidon, et sur la transmutation des matières douces en sucre fermentescible" (On the liquid sugar of starch, and on the transformation of sweet materials into fermentable sugars), Annales de chemie et de physique, series 1, 82 : 148-164; see especially pages 156-158.
- H. A. Vogel (1812) "Ueber die Verwandlung der Stärke und andrer Körper in Zucker" (On the conversion of starches and other substances into sugar), Annalen der Physik, new series, 42 : 123-134; see especially pages 129-131.
- Pasteur (1856) "Note sur le sucre de lait" (Note on milk sugar), Comptes rendus, 42 : 347-351.
- Fischer determined the configuration of glucose in:
- Emil Fischer (1891) "Ueber die Configuration des Traubenzuckers und seiner Isomeren" (On the configuration of grape sugar and its isomers), Berichte der Deutschen Chemischen Gesellschaft, 24 : 1836-1845.
- Emil Fischer (1891) "Ueber die Configuration des Traubenzuckers und seiner Isomeren. II" (On the configuration of grape sugar and its isomers), Berichte der Deutschen Chemischen Gesellschaft, 24 : 2683-2687.
- Emil Fischer and Robert S. Morrell (1894) "Ueber die Configuration der Rhamnose und Galactose" (On the configuration of rhamnose and galactose), Berichte der Deutschen chemischen Gesellschaft zu Berlin, 27 : 382-394. The configuration of galactose appears on page 385.
- Dumas, Traité de Chimie, Appliquée aux Arts, volume 6 (Paris, France: Bechet Jeune, 1843), p. 293.
- Ranken, M. D.; Kill, R. C. (1997), Food industries manual, Springer, p. 125, ISBN 0-7514-0404-7
- Wong SY, Hartel RW.: Crystallization in lactose refining-a review. J Food Sci 2014, 79(3):R257-72.
- Minard, R. Introduction to Organic Laboratory Techniques: A Microscale Approach. Pavia, Lampman, Kriz & Engel, Saunders. 1990.
- Wade, Nicholas (2006-12-10), "Study Detects Recent Instance of Human Evolution", New York Times.
- Ridley, Matt (1999), Genome, HarperCollins, p. 193, ISBN 978-0-06-089408-5.
- Ling, Charles (2008), Whey to Ethanol: A Biofuel Role for Dairy Cooperatives? (PDF).