Reducing sugars are sugars that are capable of acting as a reducing agent. A reducing agent reduces the oxidation state of an element or compound, for example, copper can be reduced from Cu2+ to Cu+.

Reducing sugars are reducing agents because they have an aldehyde group in solution. This can be an aldehyde group such as in the linear form of glucose.

Figure 1 - Glucose in its linear form. The aldehyde group is highlighted.

Alternatively, sugars with a free ketone group can rearrange in solution to form an aldehyde group, such as in fructose.

Examples of reducing sugars:

  • Glucose

  • Fructose

  • Lactose

Regular household sugar is not a reducing sugar. It does not have an aldehyde group or a free ketone group that could rearrange to form an aldehyde group in solution.