Disaccharide

A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides.

Chemistry
The two monosaccharides are bonded via a condensation reaction. This bond can be between the 1-, 4-, or 6-carbon on each component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations result in disaccharides with different chemical and physical properties.

Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting.

Common disaccharides

 * sucrose (known as table sugar, cane sugar, saccharose, or beet sugar) is composed of glucose + fructose
 * lactose (milk sugar) is glucose + galactose
 * maltose (produced during the malting of barley) is glucose + glucose
 * Trehalose is present in fungi and insects, is also glucose + glucose. It has been successfully produced at an industial scale by enzymatic treatment of starch for use as a food ingredient.
 * Cellobiose is another of the glucose + glucose disaccharides

Maltose and cellobiose are hydrolysis products of the polysaccharides, starch and cellulose, respectively.