Testing for Non-Reducing Sugars (CIE A Level Biology)

• Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons (a reducing sugar that is able to donate electrons is itself oxidised)
  • OILRIG in Chemistry
• If Benedict's test has been carried out on a solution and it shows that no reducing sugars are present then a modified version of the test can be carried out to test for the presence of non-reducing sugars

To test for non-reducing sugars:

• Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil
• Neutralise the solution with sodium hydrogencarbonate
  • Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for the Benedict’s test to work
• Then carry out Benedict’s test as normal; add Benedict’s reagent to the sample and heat in a water bath that has been boiled – if a colour change occurs (orange-red precipitate), a non-reducing sugar is present

Explanation

• The addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules
• The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

Reducing & Non-reducing Sugars Table