#33 Food test 4 - Biuret test for Proteins

The Biuret Test is done to show the presence of peptide bonds, which are the basis for the formation of proteins. These bonds will make the blue Biuret reagent turn purple. 

Process

• add an equal amount of NaOH to a solution of the food, mix carefully.
• add a few drops of 1% CuSO_4, do not shake the mixture.
• a PURPLE/MAUVE COLOR is a positive result: protein is present.

Explanation

• The reagent used in the Biuret Test is a solution of copper sulfate (CuSO₄) and sodium hydroxide (NaOH).
• The NaOH is there to raise the pH of the solution to alkaline levels; the crucial component is the copper II ion (Cu²⁺) from the CuSO₄.
• When peptide bonds are present in this alkaline solution, the Cu²⁺ions will form a coordination complex with 4 nitrogen atoms from peptide bonds.

• The complex of Cu²⁺ ions and nitrogen atoms make the color of CuSO₄ solution changes from blue to violet.
• This color change is dependent on the number of peptide bonds in the solution, so the more protein, the more intense the change. 

Video