Required Practical: Food Tests (AQA GCSE Biology): Revision Note

Food tests

• Aim: To use qualitative reagents to test for a range of carbohydrates, lipids and proteins.

• Process:

  • To include: Benedict’s test for sugars, iodine test for starch, the emulsion test for lipids and the Biuret reagent for protein

  • Safely use appropriate heating devices and techniques including the use of a Bunsen burner and a water bath

• A qualitative food test indicates if a substance is present or absent in a sample (it doesn’t indicate how much is present)

• Observations are essential in this practical: colour changes indicate if a substance is present or absent:

Food test colour changes

Preparing a food sample

• Food samples must be prepared before carrying out the tests (especially for solid foods)

• To do this:

  1. Break up the food using a pestle and mortar

  2. Transfer to a test tube and add distilled water

  3. Mix the food with the water by stirring with a glass rod

  4. Filter the mixture using a funnel and filter paper, collecting the solution

Benedict's test for sugars

1. Prepare a food sample

2. Set up a water bath at 80-90 °C

   • This can be achieved using a Bunsen burner and a beaker, or with an electronic water bath

3. Add food sample to a test tube with a few drops of Benedict's solution

4. Place the test tube in the water bath until it reaches the same temperature as the surrounding water

5. Note down the resulting colour

   • A change from blue to brick-red indicates the presence of sugar

Iodine test for starch

1. Prepare a food sample

2. Add food sample to a test tube with a few drops of Iodine

3. Note down the resulting colour

   • A change from yellow-brown to blue-black indicates the presence of starch

Emulsion test for lipids

1. Prepare a food sample

2. Add food sample to a test tube with a few drops of ethanol

3. Pour into a tube of distilled water

4. Shake the test tube gently

5. Note down the resulting appearance

   • The formation of a milky/cloudy emulsion indicates the presence of lipids

Biuret test for proteins

1. Prepare a food sample

2. Add food sample to a test tube with a few drops of Biuret solution

3. Note down the resulting colour

   • A change from blue to purple/lilac indicates the presence of protein

Use these images

Important hazards

• The main hazards in this practical are:

  • Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so goggles must be worn

  • Iodine is also an irritant to eyes (wear goggles)

  • Sodium hydroxide in Biuret solution is corrosive, so if any chemicals get onto the skin it must be washed off immediately

  • Ethanol is highly flammable so it must be kept away from the Bunsen burner used in the Benedict’s test

  • The Bunsen should be turned off completely when not in use