Testing food for nutrients
Simple chemical tests show which nutrients a food contains. A small sample of the food is first ground up and mixed with water to make a solution or suspension. The reagent is then added, and each test gives a clear colour change (or an emulsion) for a positive result. Always compare the final colour with the starting colour of the reagent.
Key idea
Iodine → starch (orange-brown to blue-black). Benedict’s → reducing sugar (blue to brick-red, on heating). Biuret → protein (blue to purple). Ethanol emulsion → fat (cloudy white emulsion).
The four main tests
| Nutrient | Reagent | Positive result |
| Starch | Iodine solution | Orange-brown → blue-black |
| Reducing sugar | Benedict’s solution + heat | Blue → brick-red |
| Protein | Biuret reagent | Blue → purple/violet |
| Fat | Ethanol then water | Cloudy white emulsion |
Worked example
A student adds Benedict’s solution to apple juice and heats it in a water bath. The colour changes from blue to brick-red. Conclusion: the apple juice contains a reducing sugar (such as glucose). If it had stayed blue, no reducing sugar would be present.
The Benedict’s result also shows roughly how much reducing sugar is present: the colour changes from blue through green, yellow and orange to brick-red as the amount increases. Because heating is involved, a water bath is used rather than a naked flame, and eye protection should be worn when handling the reagents.
Remember
- Only Benedict’s test needs heating.
- The Biuret reagent contains sodium hydroxide and copper sulfate.
- If iodine stays orange-brown, no starch is present.