Non-reducing Sugars (Benedict's test). Sucrose is called a non-reducing sugar because it does not reduce copper sulphate, so there is no direct test for sucrose. However, if it is first hydrolysed (broken down) to its constituent monosaccharides (glucose and fructose), it will then give a positive Benedict's test. So sucrose is the only sugar that will give a negative Benedict's test before hydrolysis and a positive test afterwards. First test a sample for reducing sugars, to see if there are any present before hydrolysis. Then, using a separate sample, boil the test solution with dilute hydrochloric acid for a few minutes to hydrolyse the glycosidic bond. Neutralise the solution by gently adding small amounts of solid sodium hydrogen carbonate until it stops fizzing, then test as before for reducing sugars.
Examples of Disaccharides
Sucrose: glucose + fructose,
Lactose: glucose + galactose,
Maltose: glucose + glucose.
Maltose: glucose + glucose.
Sucrose is used in many plants for transporting food reserves, often from the leaves to other parts of the plant. Lactose is the sugar found in the milk of mammals and maltose is the first product of starch digestion and is further broken down to glucose before absorption in the human gut.
Aim: To test for reducing sugars using glucose, sucrose and lactose to see which one can be reduced.
Method
- First mix the one of the sample with about 5cm of distilled water in a test tube.
- Then add 3cm of Benedict’s reagent using a syringe in to a test tube.
- Repeat for the other two samples then place the sample in to a test tube holder.
- Place the test tube holder in to the hot water bath.
- Then after 5 min return to the test tube and note what has changed?
List of apparatus
- Spatula
- Samples to test, E.g. glucose, sucrose and lactose
- Test tube rack
- Distilled water
- Benedict’s regent
- A syringe or graduated pipette
- Test tube holder
- Eye protection (goggles)
Results
Table
Conclusions
I have discovered that lactose and glucose are reducing sugar because they are part of the aldehyde group, which reduce copper (CuSO4) in to copper (Cu2O), which makes it turn in to a red brown precipitate. As a result the aldehyde group is oxidised the carboxyl group.
I used Benedict’s reagent to test for reducing sugar, benedict’s regent used to test for monosaccharides e.g. glucose and fructose. Benedict’s reagent contains blue copper (II) sulphate (CuSO4), which is reduced to red copper (I) oxide by aldehyde thereby oxidising the aldehyde to carboxylic acids. I have discovered that sugars what are in the aldehyde group can either be oxidised or reduced by the benedict’s reagent, that is why glucose and lactose where changed colour and sucrose didn’t.
