Cereal Chem 38:467 - 478.  |  VIEW ARTICLE

Determination of the Nitrogen Content of Cereal Grain by Colorimetric Methods.

A. C. Jennings. Copyright 1961 by the American Association of Cereal Chemists, Inc. 

Two modifications of colorimetric methods previously published are presented for the determination of cereal nitrogen content. In method 1, the nitrogen content of white-grained varieties of wheat and barley is determined by a modification of the biuret method (Pinckney). In this method, extraction of the protein and color development occur simultaneously in an alkaline copper tartrate solution. The use of potassium sodium tartrate in place of glycerol in the biuret reagent results in a 13% increase in sensitivity in this method. Those varieties of barley with blue-or black-colored aleurone layers may be conveniently analyzed by method 2 which involves treatment of the extract prepared by the biuret extraction procedure with Folin- Ciocalteu phenol reagent (Lowry et al.). The results from both methods are as reproducible as those obtained with the Kjeldahl method of nitrogen determination. (Pinckney: Cereal Chem. 26: 423; 1949. Lowry et al.: J. Biol. Chem. 193: 265; 1951.) The percentage of the total nitrogen content actually extracted into solution in the extraction procedure common to both methods increases as the total nitrogen content increases. Plotting the extracted nitrogen instead of the total nitrogen against the colorimeter reading increases the degree of correlation to only a small extent. The amount of brown-colored material extracted by alkaline solutions from barley and wheat is much greater in the case of barley. The variation in the amount of this extracted brown material is great enough in the case of barley to lessen the degree of correlation between Kjeldahl nitrogen content and the color produced in method 1. The accuracy of method 2 is not affected by this extraneous brown color. The main factor causing imperfect correlation between Kjeldahl nitrogen content and the color produced in both methods is the variation in the absorbance coefficients of the extracted proteins.