Testing for Biological Molecules
Biology: Cambridge International GCSE Supplement, topic 4.1.3
Testing for Food Substances
Reagents can be used to test for the presence of various food substances. The first step is to grind up the food and add distilled water to dissolve some of the food. You can then test for the food substances:
Starch
- To test for starch, add iodine solution.
- It will turn blue-black if starch is present.
Reducing sugar
- To test for reducing sugar, add Benedict’s reagent and heat for about two minutes.
- It will turn any of green, yellow, orange or red depending on the concentration of reducing sugar present.
- The colour depends on the concentration.
Proteins
- To test for proteins, add Biuret solution.
- It will turn mauve or purple if proteins are present.
Fats and oils
- To test for fats and oils, add ethanol and water and shake.
- If lipids are present, a white emulsion (cloudy liquid) will form.
Vitamin C
- To test for Vitamin C, add DCPIP solution drop-by-drop.
- If vitamin C is present, the solution will turn from blue to clear.
Test yourself on Testing for Biological Molecules
What colour does iodine solution turn when starch is present?
What colour does Biuret solution turn when protein is present?