Core Practical: Food Tests (Edexcel GCSE Biology Combined Science): Revision Notes
📚 Revision Notes
Core Practical: Food Tests
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Aim
To investigate the use of various chemical reagents to identify the presence of starch, reducing sugars, proteins, and fats (lipids) in food samples.
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Risks and Control Measures
- Potassium hydroxide is harmful to skin and eyes, so wear eye protection and wash off any splashes immediately.
- Take care with hot water in the water bath to avoid scalding. Use heatproof gloves if necessary.
- Do not taste any of the food samples.
- Handle chemicals carefully to prevent irritation.
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Tools and Equipment
- Food samples (e.g., beef, vegetable oil, rice, breakfast cereal)
- Iodine solution (for starch test)
- Benedict's solution (for reducing sugars test)
- Biuret reagent (for protein test)
- Potassium hydroxide
- Copper sulphate solution
- Ethanol (for lipid test)
- Test tubes and rack
- Water bath (set at 95°C)
- Dropper and spatulas
- Distilled water
- Goggles and protective equipment
- Heatproof gloves
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Methods:
Iodine Test for Starch
- Place one spatula of the food sample or 1 cm³ if the sample is liquid onto a dish.
- Using a dropper, add a few drops of iodine solution to the food sample.
- Observe and record any colour change.
- Positive Result: If starch is present, the iodine solution will turn blue-black.
Benedict's Test for Reducing Sugars
- Place two spatulas of the food sample or 1 cm³ of a liquid sample into a test tube.
- Add 1 cm³ of water and stir to mix.
- Add an equal volume of Benedict's solution.
- Place the test tube in a water bath at 95°C for a few minutes.
- Observe and record the colour of the solution.
- Positive Result: If reducing sugars are present, the Benedict's solution will change from blue to cloudy orange or brick red.
Sugars - Benedicts reagant test results
Biuret Test for Proteins
- Place one to two spatulas of the food sample or 1 cm³ of liquid sample into a test tube.
- Add 1 cm³ of water and stir to mix.
- Add an equal volume of potassium hydroxide solution and stir.
- Add two drops of copper sulphate solution and stir for two minutes.
- Observe and record the colour of the solution.
- Positive Result: If proteins are present, the solution will turn mauve or purple.
Proteins - biuret test results
Emulsion Test for Lipids
- Place two spatulas of the food sample or 1 cm³ of a liquid sample into a test tube.
- Add 2 cm³ of ethanol and shake vigorously.
- Allow the mixture to settle.
- Pour the liquid from the top into a test tube half-filled with distilled water.
- Observe and record whether the water becomes cloudy or remains clear.
- Positive Result: If lipids are present, a milky-white emulsion will form.
Lipids - emulsion test results
Results
| Food | Iodine Test (Starch) | Benedict's Test (Reducing Sugars) | Biuret's Test (Proteins) | Emulsion Test (Lipids) |
|---|---|---|---|---|
| Beef | Orange-brown | Blue | Purple | Milky white |
| Vegetable Oil | Orange-brown | Blue | Blue | Milky white |
| Rice | Blue-black | Blue | Blue | Colourless |
| Breakfast Cereal | Blue-black | Orange | Blue | Milky white |
Conclusion
The tests show the presence of starch, reducing sugars, proteins, and lipids in different food samples:
- Beef contains proteins and lipids.
- Vegetable oil contains lipids but no proteins or starch.
- Rice contains starch but no reducing sugars, proteins, or lipids.
- Breakfast cereal contains starch, reducing sugars, and lipids.