Food Test for Protein (Biuret Test)

Aim

To test a food sample for the presence of protein using the Biuret test, which reacts with the peptide bonds in a protein and needs no heating.

What it tests for

The Biuret test detects the presence of protein in food by reacting with peptide bonds, the bonds that join amino acids together in a protein chain. Copper(II) ions in the reagent form a coloured complex with peptide bonds, so this test does not require heating.

Variables

• Manipulated variable: the type of food sample tested, for example egg white, milk, bean extract, or glucose solution.
• Responding variable: whether the mixture turns purple or violet, which shows the presence or absence of protein.
• Controlled variables: the volume of food sample, the volume of sodium hydroxide solution, and the number of drops of copper(II) sulfate solution added each time.

Materials and apparatus

• Food samples to be tested
• Sodium hydroxide solution
• Dilute copper(II) sulfate solution
• Distilled water
• Test tubes and a test-tube rack
• A dropper or teat pipette
• A spatula, and a mortar and pestle for solid foods

Reagent and method

1. Prepare a food extract or use a liquid food sample directly, placed in a clean test tube.
2. Add an equal volume of sodium hydroxide solution to the food sample and mix gently.
3. Add a few drops of dilute copper(II) sulfate solution to the mixture.
4. Mix gently by shaking or swirling the test tube.
5. Observe and record the colour of the mixture at room temperature.

Positive result

If protein is present, the mixture changes from blue to purple or violet. If no protein is present, the mixture remains blue, the colour of the copper(II) sulfate solution.

Expected results

A food that contains protein turns purple or violet, while a food without protein stays blue. Typical observations are shown below.

Inference and conclusion

A colour change from blue to purple or violet shows that the food contains protein, because copper(II) ions form a coloured complex with the peptide bonds that join the amino acids. If the mixture stays blue, no protein is present.

The Biuret test therefore identifies protein directly and at room temperature, without the heating needed for the Benedict's test.

Safety

Common mistakes

Paper 3-style questions

Question 1 (hypothesis). A student expects egg white to contain protein. State a hypothesis and the observation that would support it.

Model answer. Hypothesis: egg white contains protein. It is supported if, after adding sodium hydroxide and a few drops of copper(II) sulfate, the mixture turns purple or violet.

Question 2 (variables and fair test). The student tests egg white, milk and glucose solution. State the manipulated and responding variables and one controlled variable.

Model answer. The manipulated variable is the type of food sample; the responding variable is the colour of the mixture, blue or purple. A controlled variable is the volume of sodium hydroxide and the number of drops of copper(II) sulfate used, so the test is fair.

Question 3 (recording and inference). Egg white and milk turned purple, but the glucose solution stayed blue. Record the results and give the inference.

Model answer. A table with the columns 'Food sample' and 'Final colour' records purple for egg white and milk and blue for glucose. The inference is that egg white and milk contain protein while the glucose solution does not, because only the peptide bonds in protein give the purple Biuret colour.

Source:SRC-DSKP-EN

Frequently asked questions