Lactose
IUPAC name	β-D-galactopyranosyl-(1→4)-D-glucose
Other names	Milk sugar 4-O-β-D-galactopyranosyl-D-glucose
Identifiers
CAS number	63-42-3 Y
PubChem	6134
EC-number	200-559-2
SMILES	O[C@H]2[C@H](O[C@@H]1O[C@H](CO)[C@H](O)[C@H](O)[C@H]1O)[C@@H](CO)O[C@@H](O)[C@@H]2O
Properties
Molecular formula	C12H22O11
Molar mass	342.30 g/mol
Appearance	white solid
Solubility in water	21.6 g/100 mL
Hazards
EU Index	not listed
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Lactose (also referred to as milk sugar) is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from lacte, the Latin word for milk, plus the -ose ending used to name sugars.

The molecular structure of α-lactose, as determined by X-ray crystallography.

[edit] Chemistry

Lactose is a disaccharide that consists of galactose and glucose fragments bonded through a β-1→4 glycosidic linkage. Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose. The glucose fragment can be in either the α-pyranose form or the β-pyranose form, whereas the galactose fragment can only have the β-pyranose form: hence α-lactose and β-lactose refer to anomeric form of the glucopyranose ring alone.

As it gives free radicals by mechanochemistry, it is possible to use lactose to follow by ESR (electron spin resonance) the energy used during a milling process.^[1]

[edit] Solubility

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.^[2]

[edit] Digestion of lactose (catabolism)

Infant mammals nurse on their mothers to drink milk, which is rich in the carbohydrate lactose. The intestinal villi secrete an 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 constant consumption.

Many people with ancestry in Europe, the Middle East, India, and parts of East 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 lactose tolerance have evolved independently in various ethnic groups.^[3]

People who are lactose intolerant may suffer uncomfortable or socially unacceptable symptoms of too much lactose consumption. In these people, lactose is not broken down and provides food for gas-producing gut flora. This can lead to bloating, flatulence, and other gastrointestinal symptoms.

[edit] References