Lactose

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.

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.

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

Isolation

Several million tons are produced annually as a by-product of the dairy industry. Whey is made up of 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.

Metabolism

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.

Applications

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.