Food tests
GCSE Biology (8461) · Required practical 4 — method, variables, the marks examiners report students losing.
Use qualitative reagents to test a food sample for reducing sugars, starch, protein and lipids.
Apparatus
- Food sample ground up with distilled water and filtered
- Benedict's solution and a water bath at about 75 degrees C
- Iodine solution
- Biuret solution (or potassium hydroxide + copper sulfate)
- Ethanol and distilled water for the lipid (emulsion) test
- Test tubes, test-tube rack, pipettes and a spotting tile
Method
- 1Reducing sugars (Benedict's test): add Benedict's solution to the sample and heat in a water bath at about 75 degrees C for a few minutes.
- 2Starch (iodine test): add a few drops of iodine solution to the sample on a spotting tile or in a tube.
- 3Protein (Biuret test): add Biuret solution to the sample at room temperature and mix.
- 4Lipids (emulsion test): mix the sample with ethanol, then pour the mixture into water.
- 5Record the colour change for each test and compare with a distilled-water control.
Results & processing
- Benedict's: blue to green/yellow/orange/brick-red as the amount of reducing sugar increases (brick-red = high concentration).
- Iodine: orange-brown to blue-black if starch is present.
- Biuret: blue to purple/lilac if protein is present.
- Emulsion test: a cloudy white emulsion forms if lipid is present.
Where students lose marks
Confusing the Benedict's and iodine colour changes.
Fix: Benedict's goes blue to brick-red for sugar (needs heating); iodine goes orange-brown to blue-black for starch (no heating).
Not using a control.
Fix: Test distilled water alongside so you know the reagent's own colour and can judge a real positive.
Saying Benedict's shows the exact sugar amount.
Fix: It is qualitative/semi-quantitative — the colour indicates roughly how much reducing sugar, not an exact value.
Improve the method
- Use the same volume of sample and reagent for each test so colours can be compared.
- Heat Benedict's for the same time in a temperature-controlled water bath.
- Use a colour standard or colorimeter to judge the Benedict's result more objectively.
Try it — exam-style
A student adds Benedict's solution to a food sample and heats it. The solution turns brick-red. State what this shows.
Name the reagent used to test for protein and state the positive colour change.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on food tests?
Food-test marks are pure recall of the reagents and colour changes — I drill them until they stick, and your first lesson is free.