A reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This 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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A sugar is classified as a reducing sugar only if it has an open-chain form with an aldehyde group. 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. Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses.
Reducing monosaccharides include glucose, fructose, 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.
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 hydrolysed starch contains more reducing sugars. The percentage of reducing sugars present in these starch derivatives is called dextrose equivalent (DE).
Benedict's reagent and Fehling's solution are used to test for the presence of a reducing sugar. The reducing sugar reduces copper(II) ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide precipitate. 3,5-Dinitrosalicylic acid is another test reagent that allows quantitative spectrophotometric measurement of the amount of reducing sugar present.
Sugars having acetal or ketal linkages are not reducing sugars, as they do not have free aldehyde chains. They therefore do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be hydrolysed using dilute hydrochloric acid. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.
All carbohydrates respond positively to Molisch's reagent.
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