Nitrogen-to-protein conversion factors for some cereal products in Japan

To evaluate a practical method of determining more accurately conversion factors for calculating the protein contents of foods from the total nitrogen content, 19 cereal products found in Japan were analyzed for total nitrogen, amino acid nitrogen, and amide nitrogen, and then the nitrogen-to-protein conversion factors were calculated. The average conversion factors were 5.75 for rice, 5.81 for wheat, and 5.95 for others. These values, corresponding to the proportion of the amino acid residue to amino acid nitrogen recovered from 20 amino acids, were lower than the currently applied factors to these foods, except for wheat flour and amaranth. The use of this factor for estimating the protein content results in a considerable difference from the estimate based on amino acid residue concentrations, due to the wide variations in amino acid composition and to the presence of a significant level of nonprotein nitrogen. The distribution of the protein nitrogen recovered from the amino acids to total nitrogen averaged 93%. Adjusted conversion factors corresponding to the proportion of the amino acid residue to total nitrogen averaged 5.26 for rice, 5.47 for wheat, and 5.54 for other cereal products. Protein contents estimated using these factors are in good agreement with the contents defined as amino acid residues.