  a reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerism. the aldehyde functional group allows the sugar to act as a reducing agent, for example in the tollens' test or benedict's reagent, or the maillard reaction, important in the browning of many foods. 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. ==chemistry== a sugar is classified as a reducing sugar only if it has an open-chain form with an aldehyde group or a free hemiacetal group.campbell & farrell 2012, p. 459. monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses. the aldehyde can be oxidized via a redox reaction in which another compound is reduced. thus, a reducing sugar is one that reduces certain chemicals. sugars with ketone groups in their open chain form are capable of isomerizing via a series of tautomeric shifts to produce an aldehyde group in solution. therefore, ketone-bearing sugars like fructose are considered reducing sugars but it is the isomer containing an aldehyde group which is reducing since ketones cannot be oxidized without decomposition of the sugar. this type of isomerization is catalyzed by the base present in solutions which test for the presence of aldehydes. aldoses or aldehyde-bearing sugars are reducing also because during oxidation of aldoses, there are certain oxidizing agents that are reduced.campbell & farrell 2012, p. 459. ==examples== reducing monosaccharides include glucose, glyceraldehyde and galactose. many disaccharides, like lactose and maltose, also have a reducing form, as one of the two units may have an open-chain form with an aldehyde group. however, sucrose and trehalose, in which the anomeric carbons of the two units are linked together, are non-reducing disaccharides since neither of the rings is capable of opening. thumb|center|upright=3|equilibrium between cyclic and open-chain form in one ring of maltose in glucose polymers such as starch and starch-derivatives like glucose syrup, maltodextrin and dextrin the macromolecule begins with a reducing sugar, a free aldehyde. more hydrolyzed starch contains more reducing sugars. the percentage of reducing sugars present in these starch derivatives is called dextrose equivalent (de). ==detection== benedict's reagent and fehling's solution are used to test for the presence o
Biomolecules Carbohydrate_chemistry Carbohydrates