Q.What are reducing and non–reducing sugars?

A disaccharide yields exactly two monosaccharide units on hydrolysis, joined by an oxide bridge called a glycosidic linkage, formed between the anomeric carbon of one monosaccharide and a hydroxyl group of the other (general formula Cₙ(H₂O)ₙ₋₁, one water short of two free monosaccharides).

Sucrose links C1 of α-D-glucose to C2 of β-D-fructose (an α-1,2 bond); because this consumes both units' carbonyl carbons, sucrose is a non-reducing sugar. Hydrolysis (acid or the enzyme invertase) gives equal glucose and fructose; since dextrorotatory sucrose (+66.6°) and glucose (+52.5°) are outweighed by strongly levorotatory fructose (-92.4°), the mixture's rotation flips sign on hydrolysis -- hence sucrose's alternate name, invert sugar. …