AQA GCSE Biology · Organisation
Food tests required practical
Use qualitative reagents to test a range of foods for starch, reducing sugars, protein and lipids.
Each nutrient has its own test and its own colour change. You prepare a food solution, then add the right reagent and watch for the change.
Equipment
- Pestle and mortar, distilled water and filter paper
- Test tubes, rack and pipettes
- Iodine solution
- Benedict’s solution and a water bath at about 75 °C
- Biuret reagent
- Ethanol (or Sudan III stain)
Variables
- Independent you change
- The food being tested
- Dependent you measure
- The colour change (or cloudiness) of the reagent
- Control you keep the same
- Same volume of food solution
- Same volume of reagent
- Same heating time for the Benedict’s test
Method
- 1
Grind the food with distilled water using a pestle and mortar, then filter it to get a food solution.
- 2
Starch: add a few drops of iodine solution. Orange-brown turning blue-black means starch is present.
- 3
Reducing sugars: add Benedict’s solution and heat in a water bath at about 75 °C for 5 minutes. Blue turning green, yellow or brick-red means sugar is present.
- 4
Protein: add Biuret reagent. Blue turning purple (lilac) means protein is present.
- 5
Lipids: shake the food with ethanol, then pour into water. A cloudy white emulsion means lipids are present.
- 6
Record the colour before and after each test in a table.
Worked example
Results for four foods.
| Food | Iodine | Benedict’s | Biuret | Ethanol |
|---|---|---|---|---|
| Potato | Blue-black | Blue | Blue | Clear |
| Glucose drink | Orange-brown | Brick-red | Blue | Clear |
| Egg white | Orange-brown | Blue | Purple | Clear |
| Milk | Orange-brown | Green-yellow | Purple | Cloudy white |
Conclusion
Potato contains starch, the glucose drink contains a lot of reducing sugar, egg white contains protein, and milk contains some sugar (lactose), protein and lipids. The Benedict’s colour also gives a rough idea of how much sugar: green is a little, brick-red is a lot.
Risks
- Ethanol is highly flammableKeep it away from flames; use a water bath, not a Bunsen.
- Biuret reagent contains an alkali that can harm eyes and skinWear eye protection and rinse off any splashes.
- Hot water bathUse test tube holders to move tubes.
Common mistakes
- Forgetting to heat the Benedict’s test. It only changes colour when heated in a hot water bath.
- Mixing up the colours: iodine goes blue-black for starch, Biuret goes purple for protein.
- Saying Benedict’s tests for all sugars. It tests for reducing sugars such as glucose; sucrose gives a negative result.
Better results
- Use the same volume of food solution and reagent each time so colours can be compared.
- Use a colour chart or colorimeter to make the Benedict’s results less subjective.
- Rinse the pestle, mortar and pipettes between foods so one food doesn’t contaminate the next.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Organisation flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.