The specific rotation of a freshly prepared α–D–glucose gradually decreases from +112° to + 52.5° while that of β–D–glucose increases from +19.2 to +52.5° with time. This phenomenon is known as mutarotation. All reducing sugars, monosaccharides and disaccharides undergo mutarotation in aqueous solution. To understand this phenomenon we need to know the cyclic structure of glucose.