Food tests
Food tests
Food tests: use qualitative reagents to test for a range of carbohydrates, lipids, and proteins to include Benedict’s test for sugars; iodine test for starch; and Biuret reagent for protein.
Students are required to perform qualitative tests for the different substances found in foods, including the Benedict’s test for sugars; iodine test for starch; and Biuret test for proteins. The Benedict’s test satisfies the requirement for the safe use of heating equipment in the laboratory.
Equipment (per pair of participants)
• Eye protection
• Food samples
• Pestle and mortar
• Stirring rod
• Filter funnel
• Filter paper
• Test tubes and rack
• Pipettes
• 10ml measuring cylinder
• 250ml beaker
• 250ml conical flask
• Bunsen burner
• Tripod
• Heatproof mat
• Thermometer
• Electronic water bath optional
• Biuret solution A (0.4M Sodium Hydroxide)
• Biuret solution B (0.01M Copper
(ii) Sulfate solution)
• Benedict’s solution
• Iodine solution
• Ethanol
• Deionised water
• CLEAPSS Hazcards: 27C, 40A, 54, 98.
Method
- Food samples should be ground thoroughly in a pestle and mortar with some water. The resulting solution can then be filtered to leave a testable food solution.
- The students must put on eye protection.
- Benedict’s test
Set up a water bath at around 80°C using a 250ml beaker, thermometer and heating equipment. Add a small amount of food solution a test tube. Add a few drops of Benedict’s solution and place the test tube into the water bath for around five minutes. Make a note of any colour change seen, a brick red colour change indicates the presence of sugar. This is then repeated for other food samples. - Iodine test
Add a small amount of food solution a test tube. Add a few drops of Iodine solution and make a note of any colour change seen. A dark blue/black colour indicates starch is present. Repeat for further food samples. - Testing for lipids
Add a small amount of food solution to a test tube. Add a few drops of Ethanol (highly flammable – keep away from naked flames). Shake the test tube gently and then add a few drops of water to the tube. Record any observations, if an emulsion is formed, this indicates the presence of lipids in the food sample. Students could also rub a piece of each food onto a filter paper and hold the filter paper up to the light. If the filter paper becomes slightly translucent, the food contains lipids. Repeat for further food samples. - Two-Step Biuret Test for Proteins
Add a small amount of food solution to a test tube. Add 1ml of Biuret Solution A and 1ml of Biuret Solution B to the test tube and shake gently to mix the contents. Record any observations of a colour change, the solution will show a purple colour change if proteins are present. Repeat for further food samples.
Video produced by @revisechemistrywithmrb practical GCSE Chemistry tutorials by a specialist Chemistry teacher with over 25 years experience teaching Chemistry and Biology.
Technician tips
Set-up: This could be completed as a circus of activities, or students could be allocated a single food on which to perform the different food tests.
• Biuret test: two-step test performed using 0.4M Sodium Hydroxide solution and 0.01M Copper Sulfate solution.
• 0.4M Sodium Hydroxide is made by dissolving 4g of Sodium Hydroxide pellets in 250mls of water, taking care to observe the health and safety points in the CLEAPSS recipe guidance.
• 0.01M Copper Sulfate solution is made by dissolving 0.62g of Copper (ii) Sulfate 5-water crystals in 250mls of water.
• Food samples: a range of different varieties of foods should be provided which will give positive and negative results for the variety of tests. Meat, cheese, bread, biscuits, oil, apple should give ample choices.
• If students make solutions from the food samples to test by adding water to the food, grinding in the pestle and mortar and filtering, this will prevent students from unnecessarily filling the test tubes with food, making them more difficult to clean.
• Providing labelled washing up bowls for the various components should also help with cleaning up after a food test practical session. The labels could be an A4 laminated sheet which can be re-used for future food test practical sessions. Provide separate labelled washing up bowls for the pestle and mortars and the glassware. This will prevent or reduce glassware breakages that could be caused by the heavy pestle and mortars.
• An example of a positive test could also be provided using glucose solution for the Benedict’s test, protein solution (e.g., egg white albumen) for the Biuret test, oil for the lipid test and starch solution for the Iodine test.
• Great care should be taken when using Ethanol during the test for lipids as it is highly flammable. Naked flames from the Benedict’s test should be extinguished before distributing the Ethanol.
To do this practical, you will need
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The experiment involves using qualitative reagents to test food including Benedict’s test for sugar, iodine test for starch and biuret test for proteins. To do this practical students will need stirring rods, filter papers, test tube racks, disposable pipettes and more. Here at Philip Harris, we provide essential biology equipment you need to carry out experiments safely and efficiently in your school or science lab. Explore more biology resources like genetics and evolution including expert tips and advice at Philip Harris.
