The Biuret reagent is used to test for the presence of protein in foods. It may be used qualitatively to detect the presence or absence of protein, as well as quantitatively to measure the total concentration of protein using spectrometric methods.
The Biuret reagent contains:
- Hydrated copper sulfate
- Potassium hydroxide solution
- Potassium sodium tartrate
When using the Biuret reagent to test for protein, positive and negative control samples should be prepared as follows:
Positive control: egg whites
Negative control: distilled water
Results are interpreted as follows:
No color change (solution remains blue): proteins not present
Violet color: proteins are present
Pink color: peptides are present
The color changes are the result of copper ions forming a violet-colored chelate complex with peptide bonds. Peptide bonds connect amino acids in proteins. The higher the concentration of peptide bonds in the sample, the greater the intensity of the violet color.
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