a disaccharide is a sugar (a carbohydrate) composed of two monosaccharides, such as glucose and fructose that make up the disaccharide sucrose. it is formed when two sugars are joined together and a molecule of water is removed. due to this, disaccharides cannot hydrolyse, meaning their molecules are unable to be broken down through a reaction with water. for example, milk sugar (lactose) is made from glucose and galactose whereas cane sugar (sucrose) is made from glucose and fructose.