What colour change shows the presence of proteins?

The presence of proteins is confirmed when the Biuret test changes the solution from blue to violet or purple. This colour change occurs because copper sulphate reacts with the peptide bonds in proteins in the presence of sodium hydroxide.

If the food sample does not contain proteins, the solution remains blue and no violet or purple colour appears. This colour change is a reliable indicator used in school science laboratories to identify protein-rich foods such as milk, eggs, pulses, and beans.