Biochemical Tests: Sugars & Starch (AQA A Level Biology)

• There are a number of tests that can be carried out quickly and easily in a lab to determine if a sample contains a certain type of sugar
• The following tests are qualitative - they do not give a quantitative value as to how much of each type of molecule may be present in a sample
• 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

Benedict’s test for reducing sugars

• Benedict’s reagent is a blue solution that contains copper (II) sulfate ions (CuSO₄ ); in the presence of a reducing sugar copper (I) oxide forms
  • Copper (I) oxide is not soluble in water, so it forms a precipitate

Method

• Add Benedict's reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube
• Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes
• If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
• It is important that an excess of Benedict’s solution is used so that there is more than enough copper (II) sulfate present to react with any sugar present
• A positive test result is a colour change somewhere along a colour scale from blue (no reducing sugar), through green, yellow and orange (low to medium concentration of reducing sugar) to brown/brick-red (a high concentration of reducing sugar)
  • This test is semi-quantitative as the degree of the colour change can give an indication of how much (the concentration of) reducing sugar present

The Benedict's test for reducing sugars produces a colour change from blue towards red if a reducing sugar is present

Reducing & non-reducing sugars table

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 the 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, a 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

Test for starch

• To test for the presence of starch in a sample, add a few drops of orange/brown iodine in potassium iodide solution to the sample
  • The iodine is in potassium iodide solution as iodine is insoluble in water

• If starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour
• This test is useful in experiments for showing that starch in a sample has been digested by enzymes