Required practical 4: food tests

• Carrying out chemical tests for carbohydrates, proteins and lipids

  1. Grind food sample with distilled water using mortar and pestle to make a paste

  2. Transfer paste to beaker and add more distilled water

  3. Stir to dissolve chemicals

  4. Filter solution to remove suspended food particles
• Carbohydrates


  Include starch and sugars such as glucose
• Test for starch

  1. Place 2cm3 of food solution in test tube

  2. Add a few drops of iodine solution

  3. If starch present, iodine solution turns blue-black

  4. If no starch, iodine stays orange
• Test for sugars (e.g. glucose)

  1. Place 2cm3 of food solution in test tube

  2. Add 10 drops of Benedict's solution

  3. Heat test tube in hot water bath for 5 minutes

  4. Green = small amount of sugar, yellow = more sugar, brick red = a lot of sugar
• Reducing sugars

  • Sugars that the Benedict's test works for (e.g. glucose)
  • Non-reducing sugars (e.g. sucrose) do not work with Benedict's test
• Test for proteins

  1. Place 2cm3 of food solution in test tube

  2. Add 2cm3 of Biuret solution

  3. If protein present, solution turns purple or lilac
• Test for lipids/fats

  1. Grind food with distilled water using mortar and pestle (do not filter)

  2. Transfer 2cm3 of solution to test tube

  3. Add a few drops of distilled water and ethanol

  4. Shake gently

  5. If lipids present, a white cloudy emulsion forms
• Ethanol is highly flammable, so no naked flames should be present