food tests

• Describe the test for lipids 


  1.    Take a completely dry and grease free test tube

  2.    Take 2cm3 of the sample being tested and add 5cm3 of ethanol 

  3.    Shake the tube thoroughly to dissolve any lipid in the sample

  4.    Add 5cm3 of water and shake gently 

  5.    A milky white emulsion indicates the presence of a lipid

  6.    As a control, use water instead of the sample but using the same process. The solution should remain colourless/clear
• describe the result for the test for lipids

  The cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing through this emulsion is refracted as it passes from oil droplets to water droplets, making appear cloudy.
• Describe the test for proteins

  Biuret test

  1.    Place a sample of the solution to be tested in a test tube and add an equal volume of sodium hydroxide solution at room temperature

  2.    Add a few drops of very dilute (0.05%) copper (II) sulphate solution and mix gently 

  3.    A purple colouration indicates the presence of peptide bonds. If no peptide bond is present, the solution remains blue.
• Describe the test for starch

  • iodine


  1.    Place a 2cm3 sample being tested into a test tube 

  2.    Add two drops of iodine solution and shake or stir

  3.    The presence of starch is indicated by a blue-black colouration