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:

  • Use qualitative reagents to test for the presence of key biological molecules in a range of foods

  • 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 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

  5. Proceed with the food tests

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

  Analysing food test results

• Write a conclusion to state which food groups are present in the food samples you tested and an explanation as to how you know this