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Test for Proteins

Test for Proteins

The Biuret Test for proteins using involves testing for the presence of the peptide bond. Biuret reagent is a copper-based reagent that turns purple when bound to protein in an alkaline solution (see figure below). The more peptide bonds present, the greater the intensity of the purple colour, indicating a higher protein concentration.

The presence of protein can also be detected using Millon’s reagent. Millon’s reagent reacts with tyrosine amino acids, common to most proteins, and results in the formation of a reddish-brown precipitate when heated.

The table below summarises the major tests and their expected results in the presence and absence of protein.

Test reagentPositive resultNegative result
Biuret reagentViolet/purple colourBlue colour
Millon’s reagentRed-brown colourWhite colour

Investigation: Test for the Presence of Proteins

Aim

To use the Biuret test as well as Millon’s reagent to test for the presence of proteins

Apparatus

  1. bunsen burner and a beaker containing water
  2. or water bath with hot water (50°C)
  3. Dropper or plastic pipette
  4. Test tubes:
    1. two with albumin solution (positive control)
    2. two with sugar water (negative control)
    3. test tubes with samples to be tested for the presence of protein
    4. test tube with Millon’s Reagent
    5. test tube with solution for Biuret test

( NOTE: The Millon’s Reagent and Biuret’s solution in this experiment should be prepared for you by your teacher).

Method

Test for protein using Millon’s reagent

WARNING! Millon’s reagent is highly toxic! Avoid breathing in its fumes.

  1. Using the dropper or pipette, add a few drops of Millon’s Reagent to the test-tube containing albumin.
  2. Using the dropper or pipette, add a few drops of Millon’s Reagent to the test-tube containing sugar water.
  3. Using the dropper or pipette, add a few drops of Millon’s Reagent to the test-tube containing samples of your food to be tested.
  4. Heat the mixtures in boiling water for 5 minutes.
  5. Observe any colour changes.

Test for protein using the Biuret test

  1. Using the dropper or pipette, add a few drops of the Biuret solution to the test-tube containing albumin.
  2. Using the dropper or pipette, add a few drops of the Biuret solution to the test-tube containing sugar water.
  3. Using the dropper or pipette, add a few drops of the Biuret solution to the test-tube containing samples of your food to be tested.
  4. Observe any colour changes.

Image credit: Siyavula

Observations

Record your observations, noting any key differences between the positive control, negative control and experimental samples.

Video Demonstration

See the video below showing Biuret test for proteins.

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