K327 / K328 / 2027

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 K327 / K328, 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?

  1. Fat must first be dissolved in ethanol.
  2. Fat would turn the water purple instead.
  3. Cloudy white in water is the sign of 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.

Biuret test: before resultComplete the stated procedure to interpret a result.

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. 1

    Select a fresh sample portion

    Do not add every reagent to the same tube. Earlier reagents can interfere with later observations.

  2. 2

    Follow the sequence

    For fat, extract with ethanol before adding the extract to water. No heating is required for this test.

  3. 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?

  1. Add biuret reagent, then heat strongly over a flame.
  2. Mix the sample with ethanol, then add the ethanol extract to water.
  3. Shake the sample with water and record any cloudiness as a positive result.
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?

  1. Egg white and milk
  2. Glucose solution and milk
  3. Glucose solution and cooking oil
  4. Cooking oil and egg white
Show the answer

Egg white and milk

Both contain protein, which gives a violet colour with biuret solution.

Quick recall

Card 1 of 3. Answer in your head, then check.

How do you distinguish a colour change from an emulsion?

You can return to this lesson any time.