Violet Color Indicates the Presence of Protein in Food

November 16, 2025 •

4 min read

In a well-known chemical test, a positive result for protein in a food sample is shown by a color change to violet or purple. This process, called the Biuret test, relies on a reaction between a copper-based reagent and the peptide bonds that link amino acids together in proteins. This simple visual indicator is a fundamental principle in food science and nutritional analysis.

Quick Summary

The Biuret test uses a copper sulfate solution in an alkaline environment to detect peptide bonds. The formation of a violet or purple complex confirms the presence of protein within a food sample.

Key Points

The Biuret Test Principle: The Biuret test is the most common chemical method for detecting protein, using copper sulfate and an alkali.
Violet Color Confirmation: A change to a violet or purple color in the Biuret test confirms the presence of protein due to a reaction with peptide bonds.
Peptide Bonds are Key: The color change is triggered by the copper ions in the reagent reacting specifically with the peptide bonds of proteins and polypeptides.
Negative Result Color: If no protein or significant protein content is present, the Biuret solution will remain its original blue color.
Other Tests Offer Specificity: While Biuret is general, other tests like the Xanthoproteic (yellow/orange) and Millon's (red) tests detect specific amino acids or protein components.
Intensity is Proportional: The intensity of the violet color can indicate the relative concentration of protein in a sample, with a deeper color suggesting more protein.
Lab Safety is Crucial: When conducting chemical tests for protein, proper safety precautions, including handling reagents carefully, are essential.

The Chemical Reaction Behind the Violet Color

The most common method for identifying protein using a color change is the Biuret test. This test is a qualitative analysis, meaning it determines if protein is present or absent, but not the exact amount. The principle is based on the interaction between copper(II) ions ($Cu^{2+}$) and the peptide bonds that link amino acids together to form proteins.

To conduct the Biuret test, a food sample is first prepared as an aqueous solution. Next, sodium hydroxide (or a similar alkali) is added to create an alkaline medium. Finally, a few drops of copper sulfate solution are added. In the presence of two or more peptide bonds, the copper(II) ions form a violet-colored chelate complex with the nitrogen atoms in the peptide bonds. The appearance of this characteristic violet or purple color is a positive result for protein. If no protein is present, the solution will remain blue, the original color of the copper sulfate reagent.

Other Colorimetric Tests for Proteins

While the Biuret test is a general test for peptide bonds and is widely used for protein detection, other chemical tests exist that rely on different color changes based on specific amino acid side chains. These offer more specialized results:

Xanthoproteic Test: This test identifies amino acids with aromatic rings, such as tyrosine and tryptophan. Concentrated nitric acid is added and heated, causing a yellow color change. When an alkali is added, the color deepens to orange.
Millon's Test: This test is specific for the amino acid tyrosine, which contains a phenol group. A red color or precipitate is formed when the sample is treated with Millon's reagent and heated.
Ninhydrin Test: This test is used for detecting all alpha-amino acids. When a solution of ninhydrin is boiled with the sample, it produces a deep blue or purple color.

The Importance of Visual Identification

Visually identifying the presence of protein is an essential step in many fields, from academic labs to industrial settings. In food manufacturing, a simple color test can be part of a quality control process to ensure ingredients meet specifications. For biological research, it's a quick way to confirm protein extraction from a sample before more complex analysis begins. For at-home food preparation, observing the coagulation of proteins when heating foods like milk or eggs is a simple, chemical-free indicator.

How to Perform a Simple Biuret Test at Home

For those interested in a hands-on approach to protein detection, a simplified Biuret test can be performed with some common materials and precautions. This experiment visually demonstrates the principle of the violet color change.

Materials:

Food sample (e.g., milk, egg white, crushed pulse)
Distilled water
Sodium hydroxide (or potassium hydroxide)
Copper sulfate solution
Test tubes or small, clear glass containers
Dropper

Procedure:

Prepare the Sample: Mix a small portion of your food sample with distilled water to create a liquid solution.
Add Alkali: Add about 2 ml of sodium hydroxide solution to the test tube containing the sample.
Add Reagent: Add a few drops of copper sulfate solution.
Observe: Gently mix and observe the color change after a few minutes. A violet or purple color confirms the presence of protein.

Disclaimer: Always use appropriate safety precautions, including wearing gloves and eye protection, when handling chemical reagents. Dispose of chemicals responsibly.

Comparison of Protein Detection Tests


Test	Principle	Positive Color Change	Detects	Sensitivity	Applications
Biuret	Copper(II) ions chelate with peptide bonds in alkaline solution.	Blue to Violet/Purple	All proteins and polypeptides with 2+ peptide bonds	Low to Moderate	General protein detection in food science, biochemistry, and clinical settings.
Xanthoproteic	Nitration of aromatic rings (in certain amino acids) by nitric acid.	Yellow (turns Orange with alkali)	Amino acids with aromatic rings (tyrosine, tryptophan)	Moderate	Specific detection of proteins containing aromatic amino acids.
Millon's	Formation of a mercury salt with the phenolic group of tyrosine.	Red	Tyrosine and proteins containing tyrosine	Moderate	Detection of proteins rich in tyrosine; Gelatin tests negative.
Ninhydrin	Reaction with free alpha-amino acids.	Blue/Purple	All alpha-amino acids and proteins	High	Detection of amino acids and general protein presence; more sensitive than Biuret.

Conclusion

The color that indicates the presence of protein in food is violet or purple, a result observed in the well-established Biuret chemical test. This color change is a direct result of a reaction between copper ions and the peptide bonds that are the building blocks of all proteins. While other tests exist for more specific protein components, the Biuret test provides a simple, foundational method for general protein detection that is invaluable across laboratories and food industries. From verifying nutritional content to educational experiments, understanding the chemistry behind this vibrant indicator is key to confirming the presence of this essential macronutrient.

Optional Outbound Markdown Link

For more detailed information on protein analysis techniques, including the Biuret test and other colorimetric assays, visit University of Massachusetts Amherst's food science section on protein analysis.

Frequently Asked Questions

The primary test is the Biuret test, which uses a reagent containing copper sulfate and sodium hydroxide. A sample containing protein will change the reagent's color to violet or purple.

The purple color is produced because the copper(II) ions ($Cu^{2+}$) in the Biuret reagent form a coordination complex with the nitrogen atoms in the peptide bonds of a protein in an alkaline solution.

The Biuret test detects any molecule that contains at least two peptide bonds, which includes all proteins and polypeptides. It cannot detect individual amino acids, which lack the required peptide linkage.

If the solution remains blue after performing the Biuret test, it indicates that the sample lacks significant protein content. The blue is the color of the copper sulfate reagent itself.

A yellow color, which turns orange with alkali, can indicate the presence of specific aromatic amino acids like tyrosine and tryptophan. This is the result of the Xanthoproteic test, which uses concentrated nitric acid.

A qualitative test, like the standard Biuret test, only indicates if protein is present or absent. A quantitative test, such as a spectrophotometric adaptation of the Biuret test, measures the actual concentration of the protein by measuring the intensity of the color change.

Yes, a simple Biuret test can be performed at home using accessible chemicals under proper safety measures. The heat coagulation method, where proteins like those in milk and egg whites solidify upon heating, is also a simple, chemical-free option.