Required practical: food testsSpec 4.2.2.1
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.