A reducing sugar is a carbohydrate that can be oxidized by a weak oxidizing agent in an aqueous solution. The primary characteristic of reducing sugars is that, when in an aqueous medium, they create compounds containing an aldehyde group.
A sugar can be classified as a reducing sugar if it contains an open-chain form with a free hemiacetal group or an aldehyde group. Monosaccharides that have a ketone group are called ketoses, and those with an aldehyde group are called aldoses. Aldehydes can be oxidized through a redox reaction, which involves the reduction of another compound. Therefore, a reducing sugar is a sugar that is capable of reducing some chemicals.
A sugar that contains ketone groups in its open chain form isomerizes through tautomeric shifts, producing an aldehyde group in a solution. Therefore, ketone-bearing sugars, such as fructose, are reducing sugars; however, in the actual sense, it is the isomer that contains an aldehyde group reducing because ketones cannot be reduced without the actual decomposition of a sugar. The base present in the solutions that test for the presence of aldehydes catalyzes this type of isomerization. Aldehyde-bearing sugars or aldoses are also reducing because when the oxidation process takes place, certain oxidizing agents are reduced. Benedict's reagent can be used to ascertain whether a sugar is a reducing sugar.