For four years the word lactose belonged to an entirely different sugar
Milk sugar got its name from Jean-Baptiste Dumas in 1843. Then in 1856 Louis Pasteur crystallised one of its building blocks and called that lactose too. Marcellin Berthelot untangled the muddle in 1860, renaming Pasteur's sugar galactose and handing the original label back to the compound we pour on cereal.
Lactose is a pairing of two simple sugars, galactose and glucose, with the formula C12H22O11, which makes it an isomer of ordinary table sugar. Its name joins the Latin for milk to the chemists' suffix for sugars. Unravelling it took centuries: Fabrizio Bartoletti's rough isolation appeared in print in 1633, the Venetian pharmacist Lodovico Testi published glowing testimonials for milk sugar in 1700, Carl Wilhelm Scheele confirmed it was a sugar in 1780, and Heinrich Vogel found glucose among its breakdown products in 1812. Emil Fischer had fixed the shapes of both halves by 1894.
Most mammal milk contains it, though not all. Cow's milk runs to about 4.8 percent and rhesus macaque milk to 8 percent, while bears produce milk with none at all. Young mammals digest it with an enzyme called lactase, which normally fades after weaning. In populations with long dairy traditions, including those of Europe, West and South Asia, the Sahel and East Africa, genes keeping lactase switched on for life spread and arose independently several times. More than 70 percent of western Europeans digest it as adults, compared with under 30 percent in parts of Africa, East and Southeast Asia and Oceania.
As a sweetener it is feeble, rating 0.2 to 0.4 against sucrose at 1.0, and it dissolves poorly enough to feel gritty in food. That makes it more useful behind the scenes. It is a common filler in tablets and capsules, it is added to infant formula to mirror human milk, and it causes relatively little tooth decay because mouth bacteria ferment it slowly.
Brewers exploit another quirk. Most yeasts cannot ferment lactose, so adding it leaves sweetness behind in a stout, producing what is sold as milk stout or cream stout.
Source: Lactose
