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A sugar capable of reducing an oxidizing agent. To do this it must have a potentially active aldehyde or ketone group. All monosaccharides have such a group and consequently they are all reducing sugars. For a disaccharide to be reducing one of the reducing groups of either of the two component monosaccharides must be left intact. Maltose, which consists of two glucose units linked by an α(1-4) glycosidic linkage, is reducing because the second glucose residue can undergo oxidation, having an aldehyde at carbon five. Sucrose however is nonreducing because the component glucose and fructose units are linked by their aldehyde (carbon one of glucose) and ketone (carbon two of fructose) groups. Various solutions, e.g. Benedict's and Fehling's, are used to detect reducing sugars.
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