Reducing sugar

A reducing sugar is any sugar that, in an alkaline solution, forms some aldehyde or ketone. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction.

Examples

Reducing sugars include glucose, fructose, glyceraldehyde, lactose, arabinose and maltose. All monosaccharides which contain ketone groups are known as ketoses, and those which contain aldehyde groups are known as aldoses.

Significantly, sucrose and trehalose are not reducing sugars.

Chemistry

A reducing sugar occurs when its anomeric carbon (the carbon which is linked to two oxygen atoms) is in the free form. Since sugars occur in a chain as well as a ring structure, it is possible to have an equilibrium between these two forms. When the hemi-acetal or hemi-ketal hydroxyl group is free, i.e. it is not locked in an acetal or ketal linkage, the aldehyde or ketone (i.e. the chain-form) is present. The aldehyde can be oxidized to a carboxyl group via a redox reaction. The chemical that causes this oxidation becomes reduced. Thus, a reducing sugar is one that reduces certain chemicals. Even though a ketone cannot be oxidized directly, a keto sugar can sometimes be converted to an aldehyde via a series of tautomeric shifts to migrate the carbonyl to the end of the chain. Therefore, keto sugars are also sometimes reducing.

Detection

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 form 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 to convert the acetal or ketal into a hemiacetal or hemiketal. 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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