disacharid

A disaccharide is a carbohydrate composed of two monosaccharide units linked by a glycosidic bond formed through a condensation (dehydration) reaction, releasing a molecule of water. The two units may be the same or different, yielding examples such as maltose (glucose–glucose) and sucrose (glucose–fructose). Most disaccharides have the empirical formula C12H22O11 and are derived from monosaccharides such as glucose, galactose, or fructose. The glycosidic bond can be alpha or beta, and its position (for example, 1→4 or 1→2) determines the properties and the digestibility of the sugar. Depending on whether one of the anomeric carbons remains free, a disaccharide is classified as reducing or non-reducing.

Common disaccharides include maltose (two glucose units linked α1→4), lactose (galactose–glucose β1→4), and sucrose (glucose–fructose α1→β2). Maltose and lactose are reducing sugars, whereas sucrose is non-reducing because both anomeric carbons participate in the glycosidic bond. Trehalose (glucose–glucose α1→1) is another non-reducing example. In digestion, specific enzymes hydrolyze disaccharides to their monosaccharide components: sucrase breaks down sucrose to glucose and fructose; lactase yields glucose and galactose; maltase produces two glucose molecules. Lactase deficiency can cause lactose intolerance.

Disaccharides occur widely in foods and the diet; sucrose is common in table sugar from sugar cane or beets, lactose in milk products, and maltose in germinating grains and malt-flavored foods. Beyond nutrition, disaccharides are used as sweeteners and feedstocks in fermentation and food processing. Their properties, including sweetness and solubility, depend on the glycosidic linkage and anomeric configuration.