Biological molecules

• Carbohydrates, lipids and proteins are all biological molecules (molecules found in living organisms)
• Testing for Sugars Using Benedict's Reagent


  1. Add Benedict's reagent (which is blue) to a sample

  2. Heat in a water bath at 70°C

  3. Observe colour change and precipitate formation
• Starch test result

  • If starch is present, the sample changes from browny-orange to a blue-black colour
  • If there's no starch, it stays browny-orange
• Biuret test result

  • If no protein is present, the solution will stay blue
  • If protein is present, the solution will turn purple
• Use the Benedict's Test to Test for Glucose
• Using the Benedict's test

  1. Prepare sample and transfer to a test tube

  2. Prepare water bath at 70C

  3. Add Benedict's solution to test tube

  4. Place test tube in water bath for 5 minutes

  5. Observe colour change
• If the food sample contains glucose, the solution in the test tube will change from its normal blue colour to green or yellow (low conc) to brick-red (high conc)
• Using the Iodine test for starch


  1. Prepare sample and transfer to a test tube

  2. Add a few drops of iodine solution

  3. Observe colour change
• If the sample contains starch, the colour of the solution will change from browny-orange to blue-black
• Use the Biuret Test to Test for Proteins
• Using the Biuret test for proteins


  1. Prepare sample and transfer to a test tube

  2. Add Biuret solution

  3. Observe colour change
• If the food sample contains protein, the solution will change from blue to pink or purple. If no protein is present, the solution will stay blue
• Use the Sudan III Test to Test for Lipids
• Using the Sudan III test for lipids


  1. Prepare sample and transfer to a test tube

  2. Add Sudan III stain solution

  3. Observe separation of layers
• If the sample contains lipids, the mixture will separate out into layers with the top layer being bright red. If no lipids are present, no separate red layer will form at the top of the liquid