Calculation of Nitrogen-to-Protein Conversion Factors: A Review with a Focus on Soy Protein

Protein content in foods and feeds is measured indirectly by nitrogen determination, requiring a nitrogen-to-protein conversion factor (NPCF). Historically, 6.25 was applied to all proteins based on two assumptions: (1) all proteins had a nitrogen content of 16% (100/16 = 6.25) and (2) all nitrogen was derived from protein. Amino acid analyses revealed that a conversion factor of 6.25 overestimated the protein content of most foods due to variations in amino acid profiles and nonprotein nitrogen. The lack of standardization of methods to determine food-specific NPCF has resulted in continued use of 6.25 and inconsistent development of new factors. This review summarizes efforts made to derive NPCF for various foods. Soy protein has been the subject of considerable debate since publication of the "Jones factors" in 1931, which based the soy conversion factor of 5.71 on the nitrogen content of glycinin. Nonetheless, 6.25 is still used by the soy industry, analytic associations, and government regulatory bodies. Given the impact to the valuation of soy protein products and food relabeling costs, changes to soy alone appear discriminatory in the absence of standardized methods for developing NPCF for all proteins. Proposed solutions include (1) developing a consensus method to determine NPCF applicable to all foods, (2) applying standardized NPCF to measure protein in all foods and assessing the economic, nutritional, and regulatory consequences equitably, and (3) developing standardized direct methods to measure protein, such as amino acid analyses, in foods intended as single sources in vulnerable populations where more accurate measurement and reducing any risk of adulteration with nonprotein nitrogen are essential.