Food Tests — AQA GCSE Biology
Food tests use indicators that change colour to show which nutrients are present in a sample.
The four tests
| Nutrient | Reagent | Method | Positive result |
|---|---|---|---|
| Starch | iodine solution | add a few drops | blue-black |
| Reducing sugars | Benedict's solution | add and heat in a water bath | green → yellow → brick-red |
| Protein | Biuret reagent | add to the sample | purple/lilac |
| Lipids (fats) | ethanol (emulsion test) / Sudan III | mix with ethanol then add water | cloudy white emulsion / red layer |
Preparing the sample
- Break up the food and mix it with distilled water to make a solution or suspension.
- Test a separate portion of the sample for each nutrient.
How to get reliable results
- Use a water bath (not a naked flame) for the Benedict's test — it's safer and more even.
- Keep amounts and timings consistent.
- Note that Benedict's gives a range of colours depending on how much sugar is present (a rough measure of concentration).
Safety
- Wear eye protection — Biuret contains sodium hydroxide (corrosive).
- Ethanol is flammable — keep away from naked flames.
Exam tips
- Memorise each reagent and its positive colour change.
- The Benedict's test needs heating; the others do not.
- Benedict's colour indicates how much sugar is present.
- Learn the safety points (eye protection, ethanol flammable).