Question

What is meant by ‘reducing sugars’?

Answer 1

The name itself should be enough to understand what a reducing sugar is, they themselves get oxidized in order to reduce others. Sugars are basically long chains of single or various carbohydrates, some of the sugars can get oxidized and some cannot get oxidized.

Complete answer:
The sugars which can act as a reducing agent in certain reactions are known as reducing sugars. By this, it means that the sugars themselves get oxidized and reduce other substances.
Sugars are nothing but carbohydrates, the reducing nature of any sugar can be identified by the groups it has. If any sugar contains an aldehyde or a keto group, they can work as reducing agents and thus are called reducing sugars.
Aldehyde or keto groups have reduced character towards weak oxidizing agents such as Tollen’s reagent or Fehling’s or Benedict’s solution in their respective tests. Thus carbohydrates containing free aldehyde or keto groups can act as reducing sugars.
Some examples of reducing sugars are glucose and lactose.
On the contrary, if the carbohydrates of any sugar does not contain any free aldehyde or keto group, they cannot act as a reducing agent and therefore cannot be oxidized. Therefore, they are called non-reducing sugars.
In other words, a non-reducing sugar is a carbohydrate that is not oxidized by a weak oxidizing agent in basic aqueous solution. This occurs as they do not provide free aldehyde or keto group in a basic aqueous solution.
Some examples of non-reducing sugars are sucrose and trehalose.

Note:
Another way that we can distinguish between reducing and non-reducing sugar is by checking for hemi-acetal groups. A carbohydrate in sugar containing a hemi-acetal group will act as a reducing agent as they can provide a free aldehyde group during equilibrium. Thus, reducing sugars contain a hemi-acetal group and non-reducing sugar does not contain a hemi-acetal group. In contrast, a non-reducing sugar contains an acetal group which does not give a free aldehyde group during equilibrium.