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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. [ABSTRACT FROM AUTHOR]

Twenty (JAPANESE) vegetables in Japan were analyzed for total nitrogen, amino acid nitrogen, amide nitrogen, and nonprotein nitrogen and then calculated nitrogen-to-protein conversion factors were calculated. The average conversion factor based on quantitative amino acid composition data was 6.00. However, the use of this factor for estimating protein content led to a considerable difference from estimates based on amino acid residue concentrations, due to the presence of high levels of nonprotein nitrogen. The distribution of protein nitrogen to total nitrogen averaged 73%. Adjusted conversion factors, defined as the proportion of amino acid nitrogen to total nitrogen, averaged 4.39. Protein contents estimated with this factor agreed well with contents determined by amino acid residues. [ABSTRACT FROM AUTHOR]