A positive Biuret test shows a pale to intense violet color when there are two or more peptide bonds in a compound. A negative result, or lack of violet color, indicates either a lack of protein or the presence of free amino acids.
The Biuret test does not reveal protein specificity, it simply detects the presence of two carbonyl groups attached to a nitrogen or carbon atom. Additional tests are needed to determine the type of protein present. The more intense the violet color is, the more peptide bonds are present, indicating a longer polypeptide chain. Fewer peptide bonds and shorter polypeptide chains yield a lighter violet to pale pinkish color.