Required Practical: Food Tests (AQA GCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
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Food Tests
Aim: To use qualitative reagents to test for a range of carbohydrates, lipids and proteins. To include: Benedict’s test for sugars, Iodine test for starch, the emulsion test for lipids and the Biuret reagent for protein
You will:
Use qualitative reagents to test for the presence of key biological molecules in a range of foods
Safely use appropriate heating devices and techniques including the use of a Bunsen burden and a water bath
A qualitative food test indicates if a substance is present or absent in a sample (although it doesn’t tell you how much is present)
Observations are essential in this practical; you are looking for colour changes in particular which can indicate if a substance is present or absent:
Food test colour changes table
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 do this:
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
It is important that you carry out the tests methodically, recording your observations carefully
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 is also an irritant to eyes (wear goggles)
Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash hands immediately
Ethanol is highly flammable; keep it away from the Bunsen burner used in the Benedict’s test (you should turn the Bunsen off completely)
And of course, the Bunsen itself is a hazard!
Be prepared to explain what molecules are or are not present in a food sample – make sure you know the positive and negative results for each test
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