Lesson 4 of 8 / Biological molecules
Testing for proteins and fats
How do you distinguish a colour change from an emulsion?
I can describe biuret and ethanol emulsion tests, including valid observations and controls.
I can describe and carry out the four specified food tests.Syllabus
Syllabus K224 / K225, Topic 3(b). Describe and carry out the four specified food tests.
About 4 min
Make a guess. You are not marked.
You shake a crushed peanut with water and the water turns cloudy white. Is that a positive test for fat?
Show the answer
Fat must first be dissolved in ethanol.
The emulsion test mixes the sample with ethanol, then pours the clear ethanol into water. Cloudy water alone proves little.
The key idea
Biuret solution gives a violet or purple colour with protein. The ethanol emulsion test gives a cloudy white emulsion when fat is present.
An emulsion contains tiny droplets of one liquid dispersed in another. In the fat test, the white cloudiness is different from a blue-to-purple colour change.
Explore the idea
Match the procedure to the observation
Method: Add biuret solution to a fresh sample portion and mix according to the laboratory method.
Protein: The result is not yet available. Use the full procedure, including heating where required.
Illustrative result, not a simulated experiment. Use a known positive control and a suitable negative control. A negative result means the target was not detected under the test conditions.
Explanation
To test for protein, add biuret solution to the food extract and mix. A violet or purple colour shows protein. If there is no protein, the solution stays blue.
To test for fat, first shake the sample with ethanol so that any fat dissolves. Then pour the clear ethanol into water. If fat is there, a cloudy white emulsion forms, because tiny drops of fat spread through the water. If there is no fat, it stays clear.
The order of the steps matters. A food sample that makes water cloudy is not a positive fat test. You must use the clear ethanol, and compare it with a control of ethanol and water.
Use clean equipment and a fresh portion of food for each test, so that one food does not contaminate another. Ethanol catches fire easily, so keep it away from flames.
Step by step
- 1
Select a fresh sample portion
Do not add every reagent to the same tube. Earlier reagents can interfere with later observations.
- 2
Follow the sequence
For fat, extract with ethanol before adding the extract to water. No heating is required for this test.
- 3
Describe what you see
Protein gives a violet or purple colour; fat gives a cloudy white emulsion.
Worked example
Comparing unknown samples
Sample A becomes violet with biuret solution. Sample B gives a white emulsion after ethanol extraction and addition to water. What does each observation support?
One way to explain it
The violet colour supports the presence of protein in A. The white emulsion supports the presence of fat in B. These results do not establish that either sample contains only that molecule group.
Why this answer works
- Identify the reagent and treatment.
- Use the exact observable change.
- Distinguish presence from exclusive composition.
Watch out for this
A student says: "A positive fat test is a white precipitate formed by Benedict's solution." What is wrong with this?
Show the answer
The fat test produces a cloudy white emulsion after ethanol extraction and addition to water. Benedict's solution is used with heating to test for reducing sugars.
Check your understanding
Which sequence is appropriate for the ethanol emulsion test?
Show the answer
Mix the sample with ethanol, then add the ethanol extract to water.
A cloudy white emulsion indicates fat. Keep ethanol away from flames and heat sources.
Exam-style questions
Check your understanding
A student adds biuret solution to egg white, milk, glucose solution and cooking oil. Which pair would turn violet?
Show the answer
Egg white and milk
Both contain protein, which gives a violet colour with biuret solution.
Quick recall
How do you distinguish a colour change from an emulsion?