Reducing Sugar – Definition and Comparison with Non-reducing Sugar

Introduction to Reducing Sugar

Sugar is basically a chain of carbohydrates, soluble in nature. It is a compound of carbon, hydrogen, and oxygen. There are many types of sugar. Sucrose, obtained from sugarcane is the most common type. In this topic, we will tell about reducing sugar and non-reducting sugar and their comparison.

However, its other types include:- fructose – found in fruits and honey, galactose – found in milk, glucose – in vegetables and fruits, maltose – it is found in barley, xylose – in wood. The simplest form of sugar is monosaccharides.

Then comes disaccharides and polysaccharides. There is another way to categorize sugar. Based on their reducing nature they are of two types – reducing sugar and non-reducing sugar. Let us see them in details.


 reducing sugar


Reducing Sugar

Sugars behave differently at low pH. It can be oxidized by a weak oxidizing agent e.g tollen’s reagent, in basic aqueous solution. In aqueous solution, they generate one or more compounds containing aldehyde or ketone group.

After oxidization, they cause a reduction of other substances. For eg. Maltose, which contains a hemiacetal group, reacts with water to form an open-chain containing an aldehyde group.

Furthermore, these are mostly monosaccharides like glucose, fructose, and galactose, or the disaccharides like lactose and maltose. Let us perform a lab test to determine it.

Benedict’s Test for Reducing Sugar

This test determines the presence of it in a solution. The principal reagent is Benedict’s solution which contains – copper (II) sulphate (blue colour), sodium carbonate, sodium citrate.

  • Take a liquified food sample in a test tube.
  • Take an equal amount of benedict’s solution and mix it with the sample.
  • Then we place the test tube in a boiling water bath for about 4-6 minutes.
  • Observe the colour changes in the mixture from starting to end.
  • Blue colour – This shows that no reducing sugar is present in the sample.
  • Yellow colour – Low amount of reducing sugar present.
  • The orange colour – Shows the presence of a moderate amount of it.
  • Brick red colour – It shows that the sample has a large amount of it.
  • Similarly, it will show the negative result with Fehling’s solution.

Non-Reducing Sugar

Sugar which does not have free aldehyde or ketone functional group is the non-reducing sugar. Also, they do not get oxidized. Sucrose is their most common source. They give a negative reaction for Fehling’s as well as Benedict’s test. All polysaccharides are non-reducing sugars.

Comparison Between Reducing and Non-Reducing Sugar

  1. They are carbohydrates with free aldehyde or ketone group while non-reducing one does not have free aldehyde or ketone instead they are present in bond formation.
  2. It is in hemiacetal or hemiketal form whereas non-reducing form is in acetal or ketal form.
  3. It exhibits mutarotation, on the other hand, its non-reducing form does not exhibit it.
  4. Reducing sugar form osazones while non-reducing does not form osazones.
  5. It is mainly monosaccharides while non-reducing is mainly polysaccharides.

Examples of Reducing Sugar

  1. Glucose- It serves as the main source of energy as it can be directly absorbed in the bloodstream. Moreover, the presence of free aldehyde group makes it a reducing sugar. Plants store it as starch and human bodies as glycogen.
  2. Fructose – Fructose is the sweetest among all the natural sugars. Most fruits contain fructose in them. Furthermore, the presence of free ketone group makes it a reducing sugar. They make a bond with glucose to form sucrose, a non-reducing one.
  3. Lactose – They are the combination of glucose and galactose. Moreover, the presence of sugar makes it a reducing agent. It is mostly present in cow milk.
  4. Maltose – They are made of two molecules of glucose. They are reducing in nature. Also, they are mostly found in grains and cereals. Maltose ferment to form alcohols.

Solved Question for you

Q- What is the use of reducing sugar in our life processes?

Ans – Oxidation and reduction process produce energy in our body. Oxidation is the loss of electrons while reduction is the gain of electrons. In addition, this redox reaction is important for human life, which provides the necessary energy to the body. Also, both these sugars act as the catalyst for the reaction.

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