All aldoses are reducing sugar because they have an aldehyde group or are capable of
forming one in solution through isomerism. 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 polysaccharide linkages, cannot easily become a free
aldehyde. The aldehyde functional group allows the sugar to act as a reducing agent, for
example in the Tollens' test or Benedict's test.