The sugars are classified as reducing and non-reducing sugars. The reducing sugars are the carbohydrates in which the free aldehyde or free ketone group is present whereas the non-reducing sugars are those which do not contains the free aldehyde or ketone group.
It is important to note that the reducing sugars can be identified by different tests like benedict’s test and fehling solution test. All monosaccharides and disaccharides are reducing sugars except sucrose. In contrast, most polysaccharides are non-reducing sugars.
Reducing vs non reducing sugars
Here is a comparison table summarizing the main differences between reducing and non-reducing sugars:
| Reducing sugars | Non-reducing sugars |
| Sugars that have a free aldehyde or ketone group and can reduce other compounds | Sugars that do not have a free aldehyde or ketone group and cannot reduce other compounds |
| Common Examples: glucose, fructose, maltose | Common Examples: sucrose, lactose, cellulose |
| Positive result with Tollen's reagent (forms a silver mirror) | Negative result with Tollen's reagent (no reaction) |
| Color changes are observed in Benedict test | no color change in Benedict test |
| They can be used as precursors in millard reaction | Not used as precursors |
What are reducing and non-reducing sugars?
Reducing sugars
Reducing sugars contain free aldehyde and ketone groups. They can donate electrons to other compounds and cause the reduction of other compounds. However, reducing sugars contain free anomeric carbon. The anomeric carbon is that which is derived from the carbonyl group and contains two oxygen substituents.
Examples
All monosaccharides are reducing sugars. Such as:
- Glucose
- Fructose
- Galactose
- Maltose
Some disaccharides are also reducing sugars such as lactose.
Non-reducing sugars
Carbohydrate which does not contains free aldehyde and ketone group is known as non-reducing sugars. They do not contain anomeric carbon attached to the hydroxyl (-OH) group.
Examples
For instance, sucrose is a disaccharide but it is a non-reducing sugar. This is because the combination of glucose and fructose forms it. Hence, the carbonyl groups of both monosaccharides participate in the glycosidic bond. So, sucrose does not contain a free carbonyl group.
Note that, polysaccharides such as starch are non-reducing sugars.
Test to distinguish reducing vs non-reducing sugars
We can differentiate between reducing and non-reducing sugars by the following test:
Benedict’s Test
In this test, first, we take the food sample. It is dissolved in water and later benedict’s solution is added. After this, it is cold down. However, after ten to fifteen minutes note the color of the solution. If it changes to blue, it means that no reducing sugar is present. If the color of the solution changes to green orange or red confirms the presence of reducing sugars in the food sample.
Fehling solution test
In the Fehling test, a sample is first heated, and after that Fehling solution is added to it. If the red color precipitates are formed, it confirms the presence of reducing sugars. While non reducing sugars does not give this test.
Fehling solution:
CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4
Cu(OH)2 → CuO + H2O
Fehling test:
CH2OH(CHOH)4CHO + 2CuO → CH2OHCHOH4COOH + Cu2O (Red ppt)
Osazone formation
The reducing sugars can form osazone by reacting with phenylhydrazine. Whereas non-reducing sugars do not show this property.
Uses of reducing and non-reducing sugars
Reducing sugars
Reducing sugars are the most abundant organic molecules found in nature. They have the following uses:
- If we know the exact amount of glucose in our body, then we get an idea about the amount of insulin that a patient must be taken.
- In the food industry, the level of reduced sugars decides their quality.
- glucose is used in fermentation.
- Reducing sugars lowers the risk of developing obesity and diabetes.
- They are used as reducing agents in the chemical reactions
Non reducing sugars
The uses of non-reducing sugars are the following:
- Sucrose is the most common sweetener.
- Cellulose is used as a structural material to provide rigidity to plants.
- The cellulose act as the raw material for the formation of the cellulose acetate.
- Starch is used in the preservation of baked food.
- Glycogen in the liver helps to maintain the level of glucose.
Key Takeaways
Related Resources
Concepts Berg
How do you know if sugar is reducing or non-reducing?
The reducing sugars contain free aldehyde and ketone groups. They can reduce other compounds. At the same time, the non-reducing sugars do not contain free aldehyde and ketone groups. Therefore, they do not reduce others.
Why is fructose a reducing sugar?
Fructose is a reducing sugar. This is because it contains a free ketone group.
What are 5 examples of reducing sugars?
There are the following examples of reducing sugars:
- Glucose
- Fructose
- Galactose
- Ribose
- Xylose
What are five examples of non-reducing sugars?
There are examples of non-reducing sugars:
- Sucrose
- Insulin
- Raffinose
- Starch
- Glycogen
What test can be used to confirm a reduced sugar?
The benedict’s and feeling solution test is used to confirm the reducing sugars.
Why are reducing and non-reducing ends of sugars named so?
Reducing sugars can reduce others. Therefore, they are named reducing sugars. However, non-reducing cannot reduce others.
Is galactose a reducing sugar?
Galactose is a reducing sugar. This is because it contains a free aldehyde group.
References
- Biochemistry by U. Satyanarayana and U. Chakrapani
- https://www.biologyonline.com/dictionary/reducing-sugar
