Disaccharides are a simple-sugar form of carbohydrate made up of two linked monosaccharides, of which there are three: glucose, fructose, and galactose. Examples include table sugar or sucrose, which is glucose linked to fructose, and milk sugar or lactose, which is glucose linked to galactose. They may also be formed by two of the same monosaccharides bonding to each other, and maltose, for instance, is a pair of glucose molecules. Sources of these carbohydrates include sugar beets and sugar cane, which produce sucrose, a sugar found in everything from baked goods to pasta sauce. They can also be found in milk, yogurt, and other dairy products, which contain lactose, and grains and beer, which contain maltose.
Also known as a biose, a disaccharide is a quick-digesting form of carbohydrate that the body uses for immediate energy. This sugar is created when two monosaccharides unite and produce a reaction in which a molecule of water is expelled, and it possesses many similar properties to a monosaccharide. Both are classified as simple sugars, in contrast to the polysaccharides, which contain three or more sugars and are alternately referred to as starches or complex carbohydrates. Also, both forms of simple sugar are generally water-soluble, take a crystalline shape, and are noticeably sweet-tasting.
The three best known disaccharides are sucrose, lactose, and maltose. In wide use commercially, sucrose is obtained from the sugar cane or sugar beet plants and sold as table sugar, refined to produce brown sugar, powdered sugar, or molasses, or used to sweeten a large variety of foods from beverages to baked goods. Lactose is the sugar in milk-based products, found in yogurt, cheese, and ice cream. People who have difficulty digesting lactose are said to be lactose-intolerant. Maltose is less sweet than the previous two and is best known as the carbohydrate found in beer, though it is prevalent in breads and other grains.
Three other common but less well known types are lactulose, trehalose, and melibiose. Lactulose is the combination of fructose and galactose. It is not actually found in food, as it cannot be digested, but rather is a medication taken in syrup form to treat constipation. Trehalose is common in microorganisms, small animals, insects, some plants, and fungi; as such it is found in foods ranging from shrimp, to sunflower seeds, to shiitake mushrooms. Melibiose is formed by the bonding of a specific molecule of galactose to glucose and is the sugar prevalent in legumes like peanuts, lentils, and peas.