Testing for Non-Reducing Sugars (Cambridge (CIE) AS Biology): Revision Note

Testing for non-reducing sugars

• 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)

  • Remember "OILRIG" in Chemistry

• If Benedict's test has been carried out on a solution and it shows that no reducing sugars are present, 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

  • 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)

  • This allows a precipitate to form