How will you test for protein in a food sample class 6?

December 13, 2025 •

4 min read

Proteins are vital macromolecules found in all living organisms, playing a crucial role in building and repairing tissues. For students, learning how to test for protein in a food sample class 6 is a fundamental biology experiment that uses a simple chemical reaction to produce a distinct color change.

Quick Summary

This guide explains the simple Biuret test to detect proteins in a food sample for Class 6 students. It outlines the required materials, a step-by-step procedure, safety rules, and how to observe and interpret the color-change results from the experiment.

Key Points

Test Principle: The Biuret test works by using copper(II) ions to react with the peptide bonds in proteins, creating a violet-colored complex in an alkaline medium.
Key Reagents: The test requires a sodium hydroxide (caustic soda) solution and a copper sulfate solution.
Positive Result: The appearance of a violet or purple color indicates that protein is present in the food sample.
Negative Result: If the solution remains blue, it means the food sample does not contain protein.
Safety First: All chemicals must be handled with care, wearing protective equipment like lab aprons and goggles under adult supervision.
Sample Preparation: Solid food items need to be crushed and mixed with water to create a solution before testing.
Color Intensity: The vibrancy of the violet color can give a rough idea of the amount of protein, with a deeper color suggesting more protein.

Introduction to Proteins and the Biuret Test

Proteins are large biological molecules, or macromolecules, consisting of one or more long chains of amino acid residues. They perform a vast array of functions within organisms, including catalyzing metabolic reactions, DNA replication, responding to stimuli, and transporting molecules. The Biuret test is a reliable chemical method used to identify the presence of protein in a given food sample. It is a qualitative test, meaning it tells you if protein is present or not, but not the exact amount.

The Chemical Principle of the Biuret Test

This test is based on a reaction that occurs in an alkaline medium, where copper(II) ions ($Cu^{2+}$) from copper sulfate react with the peptide bonds present in proteins. The nitrogen atoms in the peptide bonds coordinate with the copper(II) ions to form a characteristic violet-colored complex. For this reaction to occur, there must be at least two peptide bonds present, which is why free amino acids do not give a positive result. The intensity of the violet color is proportional to the number of peptide bonds present, indicating the amount of protein.

Materials and Reagents Required

To conduct this experiment, gather the following materials. Always perform this test under the supervision of a teacher or an adult.

Test Tubes: Clean and dry test tubes.
Droppers: For adding reagents safely.
Test Tube Stand: To hold the test tubes upright.
Mortar and Pestle: For grinding solid food samples.
Distilled Water: For preparing food solutions.
Food Samples: Examples include milk, a paste made from pulses (e.g., lentils), egg white solution, and a non-protein sample like sugar solution for comparison.
Sodium Hydroxide (NaOH) Solution: A 10% solution, also known as caustic soda.
Copper Sulfate (CuSO₄) Solution: A 1% solution.

Step-by-Step Procedure

Follow these steps carefully to test for protein in a food sample:

Prepare the Sample: Take a small quantity of the food item to be tested. If it is a solid, like lentils or paneer, grind it into a paste using a mortar and pestle.
Add to Test Tube: Place a small amount of the prepared food sample into a clean and dry test tube.
Add Water: Add about 10 drops of distilled water to the food sample and shake the test tube gently to mix the contents thoroughly.
Add Sodium Hydroxide: Using a dropper, add 10 drops of sodium hydroxide solution to the test tube. Shake well to mix everything evenly.
Add Copper Sulfate: Next, add 2 drops of copper sulfate solution to the test tube. Shake the test tube gently to ensure the reagents mix completely.
Observe the Color Change: Place the test tube in a test tube stand and wait for about 5 minutes. Observe the color of the mixture.
Interpret the Results: A change in the color of the mixture to violet or purple indicates the presence of protein in the food sample. If no protein is present, the solution will remain blue (the color of the copper sulfate solution).

Safety Precautions for School Experiments

Working with chemical reagents requires strict adherence to safety protocols. When performing this experiment, students should:

Always wear lab aprons and safety goggles to protect skin and eyes from chemical splashes.
Handle sodium hydroxide and copper sulfate solutions with care, as they can be corrosive. Use droppers and avoid direct contact with skin.
Ensure the work area is clean and tidy before and after the experiment.
Dispose of all chemical waste according to the teacher's instructions.

Comparison of Biuret Test Results

To clearly understand the results, a comparison is often made using controls. A positive control (with a known protein source like egg white) should turn violet, while a negative control (with only water or sugar solution) should remain blue.


Sample Tested	Observation	Conclusion
Egg White (Positive Control)	Turns violet/purple	Protein is present
Milk	Turns violet/purple	Protein is present
Sugar Solution (Negative Control)	Stays blue	Protein is absent
Paste of Pulses	Turns violet/purple	Protein is present
Water	Stays blue	Protein is absent

Conclusion

The Biuret test is a simple, effective, and standard method for qualitatively detecting the presence of protein in a food sample. By following a safe and systematic procedure, students can successfully observe the distinct violet color change that confirms the presence of peptide bonds. This hands-on experiment not only demonstrates a key chemical reaction but also reinforces the importance of proteins in our diet. It is a fundamental activity for Class 6 science students to understand the composition of the food they consume.

For more detailed information on protein tests, you can consult reliable educational resources online such as Vedantu.

Frequently Asked Questions

The Biuret test is a chemical test used to detect the presence of peptide bonds in a substance, and is therefore used to identify proteins.

A positive result in the Biuret test is indicated by the solution turning violet or purple after adding the reagents.

Sodium hydroxide is added to create an alkaline (basic) medium, which is necessary for the reaction between the copper(II) ions and the peptide bonds to occur.

If a food sample contains no protein, the solution will remain the same blue color as the copper sulfate solution after the test is completed.

No, solid food items must be ground into a paste and mixed with water to create a solution, which allows the reagents to react effectively with any protein present.

Safety precautions include wearing lab aprons and safety goggles, handling chemicals carefully with droppers, and performing the experiment under the supervision of a teacher or adult.

The violet color is formed when copper(II) ions ($Cu^{2+}$) from the copper sulfate solution form a chelate complex with the peptide bonds of the protein.