The test for reducing sugars is also known as the Benedict's test because it requires Benedict's reagent (which is an alkaline solution of copper(II) sulphate).
All monosaccharides and most disaccharides (with the exception of sucrose) are reducing sugars. This means they will reduce the copper present in Benedict's reagent from a blue coloured aqueous form to a red coloured precipitate.
The method for performing the test is fairly simple. A solution of the test substance is created. To this an equal volume of Benedict's reagent is added. The solution must then be heated above 60oC for a few minutes. If reducing sugars are present the solution will have colour. The original pale blue colour will disappear and be replaced by a red or brown precipitate.
|Figure 1 : Reducing Sugar Test
This test is quantitative so the amount of precipitate formed indicates how much reducing sugar was present in the original sample.
Non-reducing sugars are sugar molecules that are composed of only a few monomers i.e. disaccharides. They do not give a positive Benedict's test but can be easily hydrolysed into their monosaccharide components, which will give a positive test.
The method used to test for non-reducing sugars is as follows
- First perform the Benedict's test on a sample of the test, to see if there are any reducing sugars present
- If this test is negative take a separate sample of test solution and boil with dilute hydrochloric acid for a few minutes to break the glycosidic bonds
- Then neutralise by adding a small amount of sodium hydrogen carbonate
- Finally perform the Benedict's test for reducing sugars
- If a red/brown precipitate is formed it shows that the sample contained non-reducing sugars
There is no general test for polysaccharides but starch has it's own test.
The method is very simple. Add some drops of iodine solution to the test sample. A colour change from brown to blue/black indicates starch is present.