<P> A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde group or a free ketone group . All monosaccharides are reducing sugars, along with some disaccharides, oligosaccharides, and polysaccharides . The monosaccharides can be divided into two groups: the aldoses, which have an aldehyde group, and the ketoses, which have a ketone group . Ketoses must first tautomerize to aldoses before they can act as reducing sugars . The common dietary monosaccharides galactose, glucose and fructose are all reducing sugars . </P> <P> Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing . Nonreducing disaccharides like sucrose and trehalose have glycosidic bonds between their anomeric carbons and thus cannot convert to an open - chain form with an aldehyde group; they are stuck in the cyclic form . Reducing disaccharides like lactose and maltose have only one of their two anomeric carbons involved in the glycosidic bond, meaning that they can convert to an open - chain form with an aldehyde group . </P> <P> The aldehyde functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test . The cyclic hemiacetal forms of aldoses can open to reveal an aldehyde and certain ketoses can undergo tautomerization to become aldoses . However, acetals, including those found in polysaccharide linkages, cannot easily become free aldehydes . </P>

Why all monosaccharides are reducing sugar but not all disaccharides