Practical: Food Tests (Edexcel IGCSE Biology (Modular)): Revision Note

Practical: Food Tests

• Food samples can be investigated for the presence of the following biological molecules:

  • Glucose

  • Starch

  • Protein

  • Fat

Preparing a sample

• Before you can carry out any of the food tests described below, you may need to prepare a food sample first (especially for solid foods to be tested)

• To prepare solid food for a food test:

  • Break up the food using a pestle and mortar

  • Transfer to a test tube and add distilled water

  • Mix the food with the water by stirring with a glass rod

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

  • Proceed with the food tests

• Powdered foods also work really well for this experiment:

  • Eg. powdered potato, milk powder, protein powder, powdered egg white and powdered glucose

• There are several safety precautions to be aware of:

  • Wear eye protection

  • Wash any splashes from skin quickly

  • Do not taste any of the food substances

  • Take care to avoid spilling hot water

Test for glucose (a reducing sugar)

• Add a few drops of Benedict's solution to the sample food solution

  • Benedict's solution is a light or bright blue colour

• Heat in a beaker of very hot water for 5 minutes

• Observe if there has been a colour change

• A positive test will show a colour change from a light blue to orange / brick red depending on the concentration of glucose in the food sample

  • If there is a colour change, the darker the colour, the higher the concentration of glucose present

• Safety precautions for this food test:

  • Wear safety goggles

  • Switch off the Bunsen burner after the water has been heated to almost boiling (or alternatively use a water bath)

The Benedict's test for glucose

Test for starch using iodine solution

• Iodine solution is used to test for the presence or absence of starch in a food sample

  • Iodine solution is yellow-brown in colour

• Add several drops of iodine solution to the food sample

• A positive test for starch is a colour change from yellow-brown to blue-black

In the presence of starch, iodine solution will turn from a yellow-brown colour to a blue-black colour

Protein food test

• Add a few drops of Biuret solution to the food sample

  • Biuret solution is a blue colour

• A positive test will show a colour change from blue to violet / lilac / purple

The Biuret test for protein

Test for lipids (fats)

• Mix the food sample with 4cm³ of ethanol

  • Ethanol is a clear, colourless liquid

• Place a bung firmly in the end of the test tube before shaking the tube vigorously

• Allow time for the sample to settle

• Strain the ethanol solution into another test tube

• Add the ethanol solution to an equal volume of cold distilled water (4cm³)

• A positive test will show a cloudy emulsion forming

The ethanol test for lipids

Food Test Results Table

Important hazards

• Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm

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

  • Iodine solution is also an irritant to the eyes

  • Sodium hydroxide in Biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately

  • Ethanol is highly flammable; keep it away from any Bunsen burner

  • The Bunsen burner itself is a hazard due to the open flame. It should be turned off when not in use