Lactose

Lactose (also referred to as milk sugar) is a sugar which is found most notably in milk. Lactose makes up around 2–8% of the solids in milk. The name comes from the Latin word for milk, plus the -ose ending used to name sugars.

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Chemistry

Lactose is a disaccharide that consists of β-D-galactose and D-glucose molecules bonded through a β1-4 glycosidic linkage.

Solubility

Lactose has a solubility of 1 in 4.63 measured %w/v. This translates to 0.216g of lactose dissolving readily in 1mL of water.

The solubility of lactose in water is 18.9049g at 25°C, 25.1484g at 40°C and 37.2149g at 60°C per 100g solution. Its solubility in ethanol is 0.0111g at 40°C and 0.0270g at 60°C per 100g solution.^[1]

Digestion of lactose

Main article: Lactose intolerance

Infant mammals are fed on milk by their mothers. To digest it an enzyme called lactase (β-D-galactosidase) is secreted by the intestinal villi, and this enzyme cleaves the molecule into its two subunits glucose and galactose for absorption.

Since lactose occurs mostly in milk, in most mammals the production of lactase gradually decreases with maturity. However, production never ceases completely; it is controlled by the presence of lactose in the diet, which in turn deactivates the repressor of the lac operon, allowing lactase to be synthesized. In other words, lactose induces the synthesis of the enzyme which breaks it down.^[2] (See also: gene expression.)

Many people with ancestry in Europe, the Middle East, India, or parts of East Africa, maintain normal lactase production into adulthood. In many of these cultures mammals such as cattle, goats, and sheep are milked for food. Hence, it was in these regions that genes for lifelong lactase production first evolved.^[3]

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