Disaccharides are formed by condensation reaction between 2 monosachharides accompanied by the elimination of one water molecule.If this is the case,the 2 monosachharides get linked by glycosidic linkages i.e glycosidic linkages involve carbonyl groups of the 2 involved monosachharides.Then,the resulting sugar is a non-reducing sugar . But LACTOSE and MALTOSE are reducing sugars inspite of the glycosidic linkages between carbony groups of both of their monosachharide units. Why so ?
Sucrose is formed from a aldose and a ketose monosaccharide (glucose + fructose), where ketose forms a 5 membered ring and has a free carbon at the end of the chain.
Whereas, Lactose (glucose + galactose) and maltose (glucose + glucose) are formed from condensation of two aldoses, hence do not have a free carbon hence are called as non reducing sugars.