Lesson 4 of 7 / Biological molecules
Testing for proteins and fats
How do you distinguish a colour change from an emulsion?
In this lesson: Describe biuret and ethanol emulsion tests, including valid observations and controls.
About 4 min
The key ideaBiuret 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
For protein, add biuret solution to a suitable food extract and mix according to the laboratory method. A violet or purple colour indicates protein. A negative result remains blue. The reagent reacts with peptide bonds, which join amino acids in polypeptides.
For fat, mix the sample with ethanol so that any fat dissolves. Then add the ethanol extract to water. If fat is present, a cloudy white emulsion forms because tiny fat droplets are dispersed in the water. A negative result remains clear.
The sequence matters. Simply adding water to a cloudy food sample does not establish a positive fat test. Use an appropriate clear ethanol extract and compare with an ethanol-and-water negative control. A known oil sample can provide a positive control.
Use fresh portions of the sample and clean apparatus for different tests. Contamination from another food can give a misleading result. Ethanol is flammable, so keep it away from naked flames and heat sources. Perform these tests under the instructed laboratory conditions.
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.
Is this true? "A positive fat test is a white precipitate formed by Benedict's solution."
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.