Fehling′s reagent I for sugars

The principle behind Fehling′ test is basically based on the reducing property of monosaccharide and disaccharides, which in turn depends on the presence of free keto or an aldehyde group. This test is performed by adding a few drops of sugar solution to a mixture of Fehling′s A solution (i.e., CuSO₄) and Fehling′s B solution (i.e., KOH and Na-K tartarate). Fehling′s solution gets reduced by aldose sugars to insoluble yellow or red colored cuprous oxide. Monosaccharides like glucose, galactose and fructose and disaccharides like lactose and maltose resond freely to Fehling′s test as they possess a free keto or an aldehyde group, whereas sucrose fails to answer the test as it does not possess a free aldehyde or an keto group. Hence, Fehling′s test is also used to distinguish reducing discaccharides like maltose from non-reducing sugars like sucrose.