The three most common disaccharides are sucrose, lactose, and maltose.
Sucrose is table sugar, the product most people think of when they use the term “sugar.” Chemically, sucrose is one glucose molecule and one fructose molecule linked together.
A glucose molecule bound to a galactose molecule forms lactose, one of the few saccharides derived from an animal source. About 5 percent of fluid milk is lactose, or milk sugar. Some people are unable to digest lactose to its monosaccharide because they lack sufficient lactase, the enzyme responsible for breaking down milk sugar into glucose and galactose. The symptoms of lactase deficiency, or lactose intolerance, include gas, bloating, and abdominal pain caused by the disaccharides not being properly absorbed. In some cheeses, yogurt, and other fermented dairy products, the lactose is broken down by bacteria to lactic acid, which can usually be digested satisfactorily by lactase-deficient individuals.
Two glucose molecules linked together create maltose, or malt sugar. Maltose is primarily used in the production of beer and breakfast cereals, and in some infant formulas. This saccharide is produced whenever starch breaks down; for example, in germinating seeds and in human beings during starch digestion.