Lactose comprises about 2–8% of milk by weight. Several million tons are produced annually as a by-product of thedairy industry.[citation needed]
Whey or milk plasma is the liquid remaining after milk is curdled and strained, for example in the production ofcheese. Whey is made up of 6.5% solids, of which 4.8% is lactose, which is purified by crystallisation.[11] Industrially, lactose is produced from whey permeate – whey filtrated for all majorproteins. The protein fraction is used ininfant nutrition andsports nutrition while the permeate can be evaporated to 60–65% solids and crystallized while cooling.[12] Lactose can also be isolated by dilution of whey withethanol.[13]
Infantmammals nurse on their mothers to drink milk, which is rich in lactose. Theintestinal villi secrete theenzymelactase (β-D-galactosidase) to digest it. This enzyme cleaves the lactose molecule into its two subunits, the simplesugars 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 weaning; the removal of lactose from the diet removes the metabolic pressure to continue to produce lactase for its digestion.[14][15]
Many people with ancestry inEurope,West Asia,South Asia, theSahel belt inWest Africa,East Africa and a few other parts ofCentral Africa maintain lactase production into adulthood due to selection for genes that continue lactase production. In many of these areas, milk from mammals such ascattle,goats, andsheep is used as a large source of food. It was in these regions that genes for lifelong lactase production firstevolved.[16] The genes of adult lactose tolerance have evolved independently in various ethnic groups.[17] By descent, more than 70% of western Europeans can digest lactose as adults, compared with less than 30% of people from areas of Africa, eastern and south-eastern Asia and Oceania.[18] In people who are lactose intolerant, lactose is not broken down and provides food for gas-producinggut flora, which can lead to diarrhea, bloating, flatulence, and other gastrointestinal symptoms.
Thesweetness of lactose is 0.2 to 0.4, relative to 1.0 forsucrose.[19] For comparison, the sweetness of glucose is 0.6 to 0.7, offructose is 1.3, of galactose is 0.5 to 0.7, ofmaltose is 0.4 to 0.5, ofsorbose is 0.4, and ofxylose is 0.6 to 0.7.[19]
When lactose is completely digested in thesmall intestine, itscaloric value is 4 kcal/g, or the same as that of othercarbohydrates.[19] However, lactose is not always fully digested in the small intestine. Depending on ingested dose, combination with meals (either solid or liquid), and lactase activity in theintestines, the caloric value of lactose ranges from 2 to 4 kcal/g.[19] Undigested lactose acts asdietary fiber. It also has positive effects on absorption ofminerals, such ascalcium andmagnesium.[19]
Theglycemic index of lactose is 46 to 65.[20] For comparison, the glycemic index of glucose is 100 to 138, of sucrose is 68 to 92, of maltose is 105, and of fructose is 19 to 27.[19][20]
Lactose has relatively lowcariogenicity among sugars.[21] This is because it is not a substrate fordental plaque formation and it is not rapidlyfermented byoralbacteria.[21] The buffering capacity of milk also reduces the cariogenicity of lactose.[19]
Its mild flavor and easy handling properties have led to its use as a carrier and stabiliser of aromas and pharmaceutical products.[5] Lactose is not commonly added directly to food, because its low solubility can lead to a gritty mouthfeel.[22]Infant formula is a notable exception, where lactose is added to match the composition of human milk.[23] However, lactose-reduced formulas are increasing in popularity.[24]
One of the undesirable properties of lactose utilization is its low solubility, which can result in crystallization, giving a gritty and sandy mouthfeel in the final product. Usually, in supersaturated solution, sugars tend to crystallize, also forming big agglomerates, depending on the process condition.
Lactose is not fermented by mostyeast during brewing, which may be used to advantage.[9] For example, lactose may be used to sweeten stout beer; the resulting beer is usually called amilk stout or a cream stout.
Yeast belonging to the genusKluyveromyces 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.[25]
Another significant lactose use is in the pharmaceutical industry. Lactose is added to tablet and capsule drug products as an ingredient because of its physical and functional properties (examples areatorvastatin,levocetirizine orthiamazole among many others).[5][26] For similar reasons, it can be used to dilute illicit drugs such as cocaine or heroin.[27]
The first crude isolation of lactose, by Italian physician Fabrizio Bartoletti (1576–1630), was published in 1633.[28] 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.[29] In 1715, Testi's procedure for making milk sugar was published by Antonio Vallisneri.[30] Lactose was identified as a sugar in 1780 byCarl Wilhelm Scheele.[31][9]
In 1812, Heinrich Vogel (1778–1867) recognized that glucose was a product of hydrolyzing lactose.[32] In 1856,Louis Pasteur crystallized the other component of lactose, galactose.[33] By 1894,Emil Fischer had established the configurations of the component sugars.[34]
Lactose was named by the French chemistJean Baptiste André Dumas (1800–1884) in 1843.[35] In 1856, Pasteur named galactose "lactose".[36] In 1860,Marcellin Berthelot renamed it "galactose", and transferred the name "lactose" to what is now called lactose.[37] It has a formula of C12H22O11 and the hydrate formula C12H22O11·H2O, making it anisomer of sucrose.
^The solubility of lactose in water is 189.049 g at 25 °C, 251.484 g at 40 °C and 372.149 g at 60 °C per kg solution. Its solubility inethanol is 0.111 g at 40 °C and 0.270 g at 60 °C per kg 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
^abcGerrit M. Westhoff; Ben F.M. Kuster; Michiel C. Heslinga; Hendrik Pluim; Marinus Verhage (2014). "Lactose and Derivatives".Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. pp. 1–9.doi:10.1002/14356007.a15_107.pub2.ISBN978-3-527-30673-2.
^abcLinko, P (1982), "Lactose and Lactitol", in Birch, G.G.; Parker, K.J (eds.),Natural Sweeteners, London & New Jersey: Applied Science Publishers, pp. 109–132,ISBN978-0-85334-997-6
^Pavia, Donald L.; Lampman, Gary M.; Kriz, George S. (1990),Introduction to Organic Laboratory Techniques: A Microscale Approach, Saunders,ISBN0-03-014813-8
^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:"Mannaseri 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 themanna of whey. [Note: "Manna" was the dried, sweet sap of the treeFraxinus 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 ofSendivogius.] 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 physicianMichael 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.Archived 2018-11-09 at theWayback Machine From page 163:"UnddBertholetus praeparat ex sero lactis remedium, quod vocatmannam S. [alchemical symbol for salt, salem]seri lactis vid. inEncyclopaed. p. 400. Praeparatio est haec: ... " (Whence Bartoletti prepared from milk whey a medicine, which he calledmanna orsalt of milk whey; see in [his]Encyclopedia [note: this is a mistake; the preparation appeared in Bartoletti'sMethodus in dyspnoeam ... ], p. 400. This is the preparation: ... )
^Ludovico Testi (1715)"Saccharum lactis" (Milk sugar),Academiae Caesareo-Leopoldinae naturae curiosorum ephemerides, ... ,3 : 69–79. The procedure was also published inGiornale 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".)
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.
Fischer established the configuration of galactose in:
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.
^Pasteur (1856)"Note sur le sucre de lait" (Note on milk sugar),Comptes rendus,42 : 347–351. From page 348:"Je propose de le nommerlactose." (I propose to name itlactose.)
^Marcellin Berthelot,Chimie organique fondée sur la synthèse [Organic chemistry based on synthesis] (Paris, France: Mallet-Bachelier, 1860), vol. 2,pp. 248–249 andpp. 268–270.
