A reducing sugar is any sugar that either has an aldehyde group or is 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.
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