Required Practical - Testing for Food Chemicals
Biology: AQA GCSE Higher, topic 2.2.12
Testing for Carbohydrates, Lipids and Proteins
The objective of this experiment is to use qualitative reagents to identify the presence of food chemicals such as sugars, starch and proteins. The tests only tell if a substance is present, not how much.
Test for sugars (Benedict’s test)
- Add Benedict’s solution to the food sample.
- Heat gently in a water bath for a few minutes.
- Observe the colour change.
- A red-brown precipitate forms if reducing sugars (like glucose) are present.
- Hazards:
- Benedict’s solution is an irritant.
- Wear safety goggles, avoid contact with skin and eyes.
Test for starch (Iodine test)
- Add a few drops of iodine solution to the food sample.
- For example, peel a potato and drop iodine onto it.
- Observe the colour change.
- If the food turns to a blue-black colour, there is starch present.
- Hazards:
- Iodine is an irritant.
- Wear goggles, avoid contact with skin and eyes.
Test for proteins (Biuret test)
- Add 1 cm3 of Biuret solution A to the food sample and mix.
- Add 1 cm3 of Biuret solution B, shake or allow layers to form.
- Observe the colour change.
- A purple colour or a ring between layers means protein is present.
- Hazards:
- Biuret A is corrosive; Biuret B is an irritant.
- Wear goggles and handle carefully.
Test yourself on Required Practical - Testing for Food Chemicals
What reagent is used to test a food sample for starch?
A student peels a potato and adds iodine solution to it. The potato turns blue-black. What does this show?
What does a red-brown precipitate in Benedict's test tell you about a food sample?
A student adds Benedict's solution to two sugar solutions of different concentrations and heats both gently. Both turn red-brown. What can be concluded from this result?
A student heats a food sample with Benedict's solution and the mixture stays blue. What does this show about the sample?