A reducing sugar is a type of sugar with an aldehyde group. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction. Reducing sugars include fructose, glucose, 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 is not a reducing sugar.

Benedict's reagent is used to determine if a reducing sugar is present. If it is a reducing sugar, the mixture will turn green/orange/red. Fehling's solution can also be used for the same purpose, as both contain copper (II) ions which are reduced to a brick red precipitate of copper (I) oxide when the solution is heated.

A reducing sugar occurs when its anomeric carbon is free, which is the chiral carbon. 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 ketal hydroxylgroup is free, it is not locked, not linked to another (sugar)molecule, the aldehyde (or keto-) form (i.e. the chain-form) is available for reducing copper (II) ions. When a sugar is oxidized it is likely to form a carbic acid.

Food science