What Are Reducing Sugars?
Reducing sugars are types of sugars/carbohydrates that have ability of being oxidized and cause the reduction of other substances without having to be hydrolyzed. The aldehyde functional group allows the sugar to act as a reducing agent. The presence of reducing sugars is determined chemically by test methods such as dinitrosallicillic (quantitative) and qualitative methods using Benedict’s reagent, Fehling’s solution and Tollen’s reagent.
The main characteristic of reducing sugars is that in aqueous medium, they generate one or more compounds containing an aldehyde group. All monosaccharides are reducing sugars along with some disaccharides, some oligosaccharides and some polysaccharides.
Examples of reducing sugars include:
- Glucose
- Galactose
- Fructose
- Lactose
- Arabinose
- Maltose
- Mannose
- Glyceraldehyde
- Ribose
- Xylose
- cellobiose
Characteristics Of Reducing Sugars
- A reducing sugar has a free aldehyde (-CHO) or ketone (-C=O) group that can act as a reducing agent.
- Reducing sugars give a dark-red color (brick color) when they react with Benedict’s solution.
- Reducing sugars have a sweet taste.
- Molecular weight of reducing sugars is relatively low.
- Reducing sugars give a positive reaction towards the Fehling’s test.
- Reducing sugars have the ability to reduce cupric ions of Benedict’s or Fehling solution to cuprous ions.
- Reducing sugars give positive result with Tollen’s test.
- Presence or absence of reducing sugars can be identified by carrying out different tests.
- Most of the reducing sugars are monosaccharides.
What Are Non-reducing Sugars?
Non-reducing sugars are carbohydrates which are unable to be oxidized and do not reduce other substances. Non-reducing sugars do not have an OH group attached to the anomeric carbon and therefore they cannot reduce other compounds.
The main characteristic of non-reducing sugars is that, in basic aqueous medium, they do not generate any compounds containing an aldehyde group.
Examples of non-reducing sugars include:
- Sucrose
- Trehalose
- Raffinose
- Gentiarose
- Melezitose
- Stachyose
- Verbascose
Sucrose is the most commonly known non-reducing sugar. The linkage between the glucose and fructose units in sucrose, which involves aldehyde and ketone groups, is responsible for the inability of sucrose to act as a reducing sugar.
Characteristics Of Non-reducing Sugars
- A non-reducing sugar does not have a free aldehyde or ketone and therefore it cannot act as a reducing agent.
- Non-reducing sugars do not give a red color but remains green in color when it reacts with Benedict’s solution.
- Non-reducing sugars have a less sweet taste.
- The molecular weight of reducing sugars is relatively high when compared to that of reducing sugars.
- Non-reducing sugars give a negative reaction towards the Fehling’s test.
- Non-reducing lack ability to reduce cupric ions of Benedict’s solution to coprous ions.
- Non-reducing sugars give negative result with Tollen’s test.
- The presence or absence of non-reducing sugars cannot be identified by different tests.
- Most of non-reducing sugars are polysaccharides whereas others are disaccharides.
Difference Between Reducing And Non-reducing Sugars In Tabular Form
| BASIS OF COMPARISON | REDUCING SUGARS | NON-REDUCING SUGARS |
| Aldehyde And Ketone Group | A reducing sugar has a free aldehyde (-CHO) or ketone (-C=O) group that can act as a reducing agent. | A non-reducing sugar does not have a free aldehyde or ketone and therefore it cannot act as a reducing agent. |
| Benedict’s Solution | They give a dark-red color (brick color) when they react with Benedict’s solution. | They do not give a red color but remains green in color when it reacts with Benedict’s solution. |
| Sweetness | They have a sweet taste. | They have a less sweet taste. |
| Molecular Weight | Molecular weight of reducing sugars is relatively low. | The molecular weight of reducing sugars is relatively high when compared to that of reducing sugars. |
| Fehling’s Test | They give a positive reaction towards the Fehling’s test. | They give a negative reaction towards the Fehling’s test. |
| Ability To Reduce Cupric Ions | They have the ability to reduce cupric ions of Benedict’s or Fehling solution to cuprous ions. | They lack ability to reduce cupric ions of Benedict’s solution to coprous ions. |
| Tollen’s Test | They give positive result with Tollen’s test. | They give negative result with Tollen’s test. |
| Identification | Presence or absence of reducing sugars can be identified by carrying out different tests. | The presence or absence of non-reducing sugars cannot be identified by different tests. |
| Classification | Most of the reducing sugars are monosaccharides. | Most of non-reducing sugars are polysaccharides whereas others are disaccharides. |