Food tests

• The Iodine test for starch


  1. Put some of the food sample into a test tube

  2. Add a few drops of iodine solution to the food sample using a pipette

  3. If starch is present, the solution turns from brown to blue-black
• The Benedict's test for reducing sugars


  1. Add an equal volume or excess of Benedict's solution to the food sample in a test tube

  2. Place in a hot water bath for a few minutes

  3. If reducing sugar is present, a brick red precipitate is formed. If reducing sugar is absent, the solution remains blue
• Test for protein


  1. Add a few drops of Biuret's reagent (sodium hydroxide and copper (II) sulphate) to the food sample in a test tube

  2. Shake the solution to mix and wait for a few minutes

  3. If protein is present, the solution turns from blue to purple
• Test for lipids


  1. Add a few cm3 of ethanol to the food sample

  2. Pour this mixture into a test tube of equal volumes of distilled water

  3. If lipids are present, a white emulsion is formed on the surface of the mixture
• Sources of error


  • Colour change of Benedict's test and Biuret test may be subtle and difficult to judge if the concentration of the tested molecule is low
• Safety precautions


  • Tie hair back and wear safety goggles when performing the Benedict's test using a Bunsen burner and hot water bath
  • Handle Biuret solution with care. It contains copper sulphate (poisonous) and sodium hydroxide (corrosive). Wash immediately if comes into contact with skin and wipe away any spills to surfaces
  • Keep ethanol solution away from flames as ethanol is highly flammable