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
