Biochemical Tests: Proteins
- The Biuret test can be carried out quickly and easily in a lab to determine if a sample contains proteins
- Biuret ‘reagent’ contains an alkali and copper (II) sulfate which react in the presence of peptide bonds
Apparatus
- Test tubes
- Test tube rack
- Food solution
- Control solution (containing no proteins e.g. distilled water)
- Sodium hydroxide
- Copper (II) sulfate solution
- Pipette
- White tile
- Gloves
Method
- Add sodium hydroxide to the food solution sample to make the solution alkaline
- Add a few drops of copper (II) sulfate solution (which is blue) to the sample
- Biuret ‘reagent’ contains an alkali and copper (II) sulfate
- Repeat steps 1 and 2 using the control solution
- Compare the colours of the control solution and the food sample solution
Results
- If a colour change is observed from blue to lilac/mauve, then protein is present.
- The colour change can be very subtle, it’s wise to hold the test tubes up against a white tile when making observations
- If no colour change is observed, no protein is present
- For this test to work, there must be at least two peptide bonds present in any protein molecules, so if the sample contains amino acids or dipeptides, the result will be negative
A positive result from the Biuret test will produce a colour change from blue to mauve/lilac
Limitations
- The Biuret test is qualitative - it does not give a quantitative value as to the amount of protein present in a sample
- If the sample contains amino acids or dipeptides, the result will be negative (due to lack of peptide bonds)
