Converting Nitrogen into Protein—Beyond 6.25 and Jones' Factors

Abstract

The protein content in foodstuffs is estimated by multiplying the determined nitrogen content by a nitrogen-to-protein conversion factor. Jones' factors for a series of foodstuffs, including 6.25 as the standard, default conversion factor, have now been used for 75 years. This review provides a brief history of these factors and their underlying paradigm, with an insight into what is meant by "protein." We also review other compelling data on specific conversion factors which may have been overlooked. On the one hand, when 6.25 is used irrespective of the foodstuff, "protein" is simply nitrogen expressed using a different unit and says little about protein (s.s.). On the other hand, conversion factors specific to foodstuffs, such as those provided by Jones, are scientifically flawed. However, the nitrogen:protein ratio does vary according to the foodstuff considered. Therefore, from a scientific point of view, it would be reasonable not to apply current specific factors any longer, but they have continued to be used because scientists fear opening the Pandora's box. But because conversion factors are critical to enabling the simple conversion of determined nitrogen values into protein values and thus accurately evaluating the quantity and the quality of protein in foodstuffs, we propose a set of specific conversion factors for different foodstuffs, together with a default conversion factor (5.6). This would be far more accurate and scientifically sound, and preferable when specifically expressing nitrogen as protein. These factors are of particular importance when "protein" basically means "amino acids," this being the principal nutritional viewpoint.

Life Cycle Assessment (LCA) of Light-Weight Eco-composites

“…The detection limit for persulphate digestion (detection range 10-150mgN/L) and Peterson's modified Lowry assay (detection range above 5mg/L) could cause this error. Converting protein to total N based on equation 17 (the NF of soybean or meal 5.5 was applied) (Mariotti et al, 2008), 0.6 mg N/g TS and 0.77 mg N/g TS for PSBF and MSBF were obtained and thus these trace amounts of N were considered as negligible in the N balance calculation for the LCA model.…”

“…Using this NF, the converted protein from equation 12 is 75.7mg /g TS, which gives a higher value than the Lowry microassay result (42.55mg protein/g TS). This can be partly explained by a non-protein N fraction present in the total N results (Mariotti et al, 2008), such as free amino acids, amides, and other nonpolymeric N constituents . However, the non-protein N fraction in wheat is reported to be negligible, only accounting for 1.5% of total N .…”

“…When converting total N content of WBF to total protein, two Nitrogen Factor (NF) recommended in previous studies were compared -5.7, a recommended NF for wheat grain (Mosse, 1990, Barikmo et al, 2003) and 5.52, a proposed NF for wheat flour (Mariotti et al, 2008, Sarwar et al, 1983. In this research, NF 5.52 was applied to WBF due to wheat flour being the main component of WBF.…”

Desenvolvimento de um sistema de recirculação para estudos sobre digestibilidade em condições de alto desempenho para camarões marinhos: avaliação de ingredientes protéicos alternativos à farinha de peixe em diferentes níveis de inclus

“…Além disto, os aminoácidos não são as únicas fontes de nitrogênio nos alimentos que também podem conter ácidos nucléicos, aminas, uréia, amônia, nitratos, fosfolipídeos, entre outros (Mariotti et al, 2008). Neste sentido, o desenvolvimento de métodos para a determinação da proteína de forma mais direta a partir de análises da proteína verdadeira é fortemente recomendado, uma vez que a proteína bruta é cada vez menos utilizada na nutrição humana.…”

“…Neste sentido, o desenvolvimento de métodos para a determinação da proteína de forma mais direta a partir de análises da proteína verdadeira é fortemente recomendado, uma vez que a proteína bruta é cada vez menos utilizada na nutrição humana. No entanto, na nutrição animal este termo continua em uso a despeito de suas limitações e falta de precisão e o uso de fatores específicos que provavelmente reduziriam o teor de proteína de diversos alimentos traria sérias conseqüências financeiras (Mariotti et al, 2008;Tacon et al, 2009 …”