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Lactose | Wikipedia audio article

October 10, 2019


Lactose is a disaccharide. It is a sugar composed
of galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes
up around 2–8% of milk (by weight). The name comes from lac (gen. lactis), the Latin
word for milk, plus the suffix -ose used to name sugars. The compound is a white, water-soluble,
non-hygroscopic solid with a mildly sweet taste. It is used in the food industry.==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 is hydrolysed to glucose and galactose, isomerised in alkaline solution to lactulose,
and catalytically hydrogenated to the corresponding polyhydric alcohol, lactitol. Lactulose is
a commercial product, used for treatment of constipation.==Occurrence and
isolation==Lactose comprises about 2–8% of milk by
weight. Several million tons are produced annually as a by-product of the dairy industry.
Whey or milk plasma is the liquid remaining after milk is curdled and strained, for example
in the production of cheese. Whey is made up of 6.5% solids, of which 4.8% is lactose,
which is purified by crystallisation. Industrially, lactose is produced from whey
permeate – that is whey filtrated for all major proteins. The protein fraction is used
in infant nutrition and sports nutrition while the permeate can be evaporated to 60–65%
solids and crystallized while cooling. Lactose can also be isolated by dilution of whey with
ethanol.Dairy products such as yogurt, cream and fresh cheeses have lactose contents similar
to that of milk. Ripened cheeses contain little to no lactose, as bacteria convert most of
it into lactic acid during the ripening process.==Metabolism==Infant mammals nurse on their mothers to drink
milk, which is rich in lactose. The intestinal villi secrete the enzyme 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.==Applications==
Its mild flavor and easy handling properties have led 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 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, i.e., compressibility
and cost effective use. For similar reasons it can be used to cut (dilute) illicit drugs.==History==
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. In 1856, Louis Pasteur named galactose “lactose”.
In 1860, Marcellin Berthelot renamed it “galactose”, and transferred the name “lactose” to what
is now called lactose. It has a formula of C12H22O11 and the hydrate formula C12H22O11·H2O,
making it an isomer of sucrose.==See
also==Lactose intolerance
Nectar Sugars in wine
lac operon

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