Disaccharides

Disaccharides

• Disaccharides are sugars composed of two monosaccharide units joined by a carbon–oxygen-carbon linkage (glycosidic linkage)
• There are three common disaccharides: maltose, lactose, and sucrose
• All three are white crystalline solids at room temperature and are soluble in water

Maltose

• Formed by the partial hydrolysis of starch and glycogen
• Liberated by the action of malt (germinating barley) on starch, hence also known as malt sugar
• About 30% as sweet as sucrose
• Broken down into two glucose molecules by the enzyme maltase
• α-1,4-glycosidic linkage between glucose molecules
• Reducing sugar, with an anomeric carbon that can open to form an aldehyde group

Lactose

• Known as milk sugar, occurs in the milk of humans and other mammals
• Reducing sugar composed of one molecule of D-galactose and one molecule of D-glucose
• Joined by a β-1,4-glycosidic bond
• Hydrolyzed by acid or the enzyme lactase to release galactose and glucose
• About one-sixth as sweet as sucrose

Sucrose

• Known as beet sugar, cane sugar, table sugar, or simply sugar
• Obtained from sugar cane and sugar beets by evaporation and recrystallization
• Unique among common disaccharides in having an α-1,β-2-glycosidic (head-to-head) linkage
• Non-reducing sugar, with anomeric carbons of glucose and fructose tied up
• Does not undergo reactions typical of aldehydes and ketones
• Hydrolyzed by dilute acid or the enzyme sucrase to form an equimolar mixture of glucose and fructose, known as invert sugar