Evaluation and Elimination of the Interference by Starch in the Biuret Determination of Wheat Protein

In the biuret determination of wheat protein, starch, color substances and lipids extractable with an alkaline solution such as biuret reagent were found to interfere with the biuret method. Comparative evaluation of their effects revealed that starch, a major component of wheat, has the most significant effect on the biuret method. In the presence of starch, the correlation between Kjeldahl protein and the optical density of biuret was poor. It was found that several organic solvents effectively retarded solubilization of wheat starch in the biuret reaction mixture. By treatment of the wheat samples with these solvents prior to the biuret procedure, the effect of starch was completely eliminated and the optical density of biuret became closely correlated with Kjeldahl protein.