Write short notes on reducing and non-reducing sugars.

Carbohydrates are generally sweet in taste and hence referred to as sugars. First let us know the exact definition of carbohydrates.


Carbohydrates are defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis.

The substance most people refer to as “sugar” is the sucrose disaccharide, which is extracted either from sugar cane or beets. Sucrose is the disaccharides most sweet. It’s approximately three times sweet as maltose, and six times sweet as lactose.


Carbohydrates are also called sugars in general some partially methylated sugars and amino sugars and amino sugars naturally and one natural nitro sugar is known. All carbohydrates are polyhydroxy aldehydes or ketones or substances that yield these on hydrolysis. They are mainly classified into two types which are listed below

Reducing sugars:  Any carbohydrate whose structure comprises of an aldehyde, or a hemiacetal in equilibrium with an aldehyde. This aldehyde group can be oxidized, with resultant reduction of the oxidizing agent. Aldehydes and keto groups have reducing character and reduce Tollens reagent and Fehling’s (Benedict’s) solution. Carbohydrates containing free aldehyde and keto functional group are thus reducing sugars.

Example: Glucose, lactose.

Non-reducing sugars: A nonreducing sugar is a carbohydrate that is not oxidized by a weak oxidizing agent (an oxidizing agent that oxidizes aldehydes but not alcohols, such as the Tollen’s reagent) in basic aqueous solution. The characteristic property of nonreducing sugars is that, in a basic aqueous medium, they do not generate any compounds containing an aldehyde group.  If the groups are not free, then they do not reduce Tollens reagent and Fehling’s solution and are, therefore, classified as Non-reducing sugars.

Example: Sucrose, trehalose

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