Required practical: food testsSpec 4.2.2.1
In short
Food tests use qualitative reagents to show whether a food molecule is present. Benedict's solution, heated in a hot water bath, turns from blue to green, yellow, orange or brick-red with sugars. Iodine turns from orange-brown to blue-black with starch. Biuret reagent turns from blue to purple with protein, and Sudan III gives a red-stained oily layer with lipids.
You can use qualitative reagents to test for a range of carbohydrates, lipids and proteins. Qualitative means the test tells you whether a food molecule is present, not how much.
- Prepare a food sample: grind a small amount of food with water and filter it or let it settle. Test the liquid.
- Add the test reagent to a sample of the food solution.
- Look for the colour change and compare with the table below.
| Food molecule | Test | Positive result |
|---|---|---|
| Sugars | Benedict's test: add Benedict's solution (blue) and heat in a hot water bath | Blue changes to green, yellow, orange or brick-red |
| Starch | Iodine test: add iodine solution (orange-brown) | Orange-brown changes to blue-black |
| Protein | Biuret test: add Biuret reagent (blue) | Blue changes to purple (lilac) |
| Lipids | Sudan III test: add a few drops of Sudan III stain to the sample in water and shake | A red-stained oily layer forms on top of the water |
Some schools use the ethanol emulsion test for lipids instead: shake the sample with ethanol, then pour it into water. A cloudy white emulsion shows that lipid is present.
The more the Benedict's colour moves towards brick-red, the more sugar there is. If the colour stays blue, no sugar is present.
Take care with the hot water bath and wear eye protection. Sudan III and ethanol are flammable, so keep them away from flames. Always state the colour before and after the test.
Say ‘blue-black’, not just ‘black’, for the iodine test, and ‘purple’, not ‘pink’, for Biuret.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026