Required Practical - Food Testing
Biology: Edexcel IGCSE, topic 2.2.3
Testing for Carbohydrates, Lipids and Proteins
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.
Glucose
- To test for small sugars like glucose, add Benedict’s reagent and heat for about two minutes.
- It will turn any of green, yellow or red if sugar is 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.
Lipids
- To test for lipids, add ethanol and water and shake.
- If lipids are present, a white emulsion (cloudy liquid) will form.
Calorimetry
- We can also test the calorie (energy) content of foods:
- Burn a known mass of the food under a boiling tube filled with a known volume of water.
- Calculate the change in temperature of the water.
Test yourself on Required Practical - Food Testing
Select the correct pairs of reagents and substances being tested?
What colour does iodine solution turn when starch is present?
What colour does Biuret solution turn when protein is present?