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Measuring protein quantity

Measuring protein quantity - Nitrogen to protein conversion factor


EUVEPRO is concerned about proposals to move away from using the widely accepted nitrogen to protein conversion factor (PCF) of N x 6.25 factor for soya protein determination, and instead, to replace this with the factor N x 5.71.

The change to a PCF of 5.71 would have a significant negative impact on the perception of soya as a nutritious and high-quality protein, it results in an almost 10% reduction in the calculated protein content of soya products without any change in the composition of the product.  This would have serious repercussions on, for example, isolated soya protein as a food ingredient, since it would no longer meet certain product requirements, may disappear from ingredients lists, could result in expensive formula changes, and create significant extra costs to manufacturers due to the resulting changes to the food labels.
The Codex Guidelines on Nutrition Labelling CAC/GL 2-19851, state that protein is equal to “Total Kjeldahl Nitrogen x 6.25 unless a different factor is given in a Codex standard or in the Codex method of analysis for that food.” 

Isolated soya protein (ISP) is the most commonly used soya protein source in infant formula and follow-up formula.  In addition, soya protein concentrate (SPC) and soya protein flour (SPF) are also used in many different food products.  The Codex Standard for Soy Protein Products CODEX STAN 175-19892 (which includes standards for ISP, SPC, and SPF) lists the appropriate nitrogen to protein conversion factor as 6.25.  The Codex Recommended Methods of Analysis and Sampling CODEX STAN 234-19993 also lists 6.25 as the nitrogen to protein conversion factor for “Soy protein products”.
The analytical science organizations, AOCS4-7, AOAC8, AACC9-12, and ISO13 list the appropriate nitrogen to protein conversion factor for analytical methods suitable for soya products as 6.25.

The country labelling regulations of a number of countries, including Argentina14, Brazil15, China16, European Union17, India18, Japan19, Korea20, Mexico21, Malaysia22, South Africa23, United States24, list 6.25 as the appropriate nitrogen to protein conversion factor for ISP and SPC.
The 5.71 nitrogen to protein conversion factor that is proposed for soya is based on a 1931 paper25, which does not account for all relevant proteins that are present in soya beans.  Since the publication of this paper, subsequent research detailing the composition of soya beans and improvements in analytical testing have shown that the proposed 5.71 conversion factor was based on a faulty premise. Jones, in the 1931 paper, justifies the 5.71 factor for soya beans by stating, incorrectly, that the major protein in soya beans is glycinin (11S), which is composed of 17.5% nitrogen.  He therefore designated a conversion factor for soya protein of 5.71 (100 divided by 17.5 = 5.71)25.  Glycinin (11S), however, represents only 31-52% of total protein in soya beans26-28.  There are many other proteins in soya beans, including beta-conglycinin (7S), which represents around 35% of the total protein. If similarly inappropriate logic were applied, and only 7S protein was considered in the calculation, the nitrogen to protein conversion factor for soya would range from 6.29-6.4527,28.
The ratios of 11S to 7S proteins in soya beans are subject to variation depending on, for example, soya bean variety, or differences in seasonal growing conditions26-28, which does make it difficult to assign a “precise” nitrogen to protein conversion factor for soya or indeed, for other foodstuffs that are subject to natural variability. However, based on typical ranges for 7S and 11S proteins in soya, a PCF of N x 6.25 would appear to be the most appropriate for soya protein determination.

References:

1. Codex Guidelines on Nutrition Labelling CAC/GL 2-1985
2. Codex Standard for Soy Protein Products CODEX STAN 175-1989
3. Codex Alimentarius STAN 234-1999 Recommended Methods of Analysis and Sampling.
4. AOCS Official Method Ba 4d-90, Reapproved 2009.  Nitrogen-amonia-protein Modified Kjeldahl Method Titanium Dioxide + Copper Sulfate Catalyst.
5. AOCS Official Method Ba 4e-93, Reapproved 2009.  Generic Combustion Method for Determination of Crude Protein.
6. AOCS Official Method Ba 4f-00, Reapproved 2009.  Combustion Method for Determination of Crude Protein in Soybean Meal.
7. AOCS Official Method Ba 4a-38, Reapproved 2009.  Nitrogen-amonia-protein Modified Kjeldahl Method.
8. AOAC Official Method 992.23, 18th ed, 2005.  Crude Protein in Cereal Grains and Oilseeds:  Generic Combustion Method; First Action 1992.
9. AACC International Method 46-10.01, Final approval April 13, 1961; Reapproval November 3, 1999.  Crude Protein—Improved Kjeldahl Method.
10. AACC International Method 46-11.02, Final approval October 8, 1976; Reapproval November 3, 1999.  Crude Protein—Improved Kjeldahl Method, Copper Catalyst Modification.
11. AACC International Method 46-16.01, First approval October 12, 1988; Reapproval November 3, 1999.  Crude Protein—Improved Kjeldahl Method, Copper-Titanium Dioxide Catalyst Modification.
12. AACC International Method 46-30.01, Final approval November 8, 1995; Reapproval November 3, 1999.  Crude Protein—Combustion Method.
13. ISO 16634-1:2008, Approved November 1, 2008.  Food Products:  Determination of the Total Nitrogen Content by Combustion According to the Dumas Principle and Calculation of the Crude Protein Content.  Part 1:  Oilseeds and Animal Feeding Stuffs.
14. Argentina Laws for the Labeling and Advertising of Food:  Resolution in Conjuction with SPRyRS 149/2005 y SAGPyA 683/2005.
15. Brazil National Health Surveillance Agency (ANVISA).  Resolution - RDC No. 268, September 22, 2005.
16. China Ministry of Health “GB5009.5 Determination of Protein in Food”.
17. Regulation (EU) No 1169/2011.
18. Lab Manual 3.  Manual of Methods of Analysis of Foods, Cereal and Cereal Products. Directorate General of Health Services, Ministry of Health and Family Welfare, Government of India, 2005.
19. Japanese Agricultural Standard for Vegetable Protein and Seasoned Vegetable Protein, Standard accessed August 28, 2009.
20. Nitrogen Conversion Factors for Protein Calculation, Korea Food Code.
21. Mexico NOM-051-SCFI/SSA1-2010 General Specifications for Labeling of Foods and Non-alcoholic Beverages.
22. Malaysian Guide to Nutrition Labelling & Claims, December 2010
23. South Africa Regulations No. 146:  Labelling and Advertising to Foodstuffs (2010).
24. Title 21 United States Code of Federal Regulations, Part 101.9.
25. Jones, DB (1931, slightly revised 1941)  Factors for Converting Percentages of Nitrogen in Foods and Feeds into Percentages of Protein.  US Department of Agriculture Circular 183.
26. Murphy, PA and Resurreccion, AP (1984)  Varietal and Environmental Differences in Soybean Glycinin and ß-Conglycinin Content.  Journal of Agricultural Food Chemistry 32:  911-15.
27. Roberts, RC and Briggs, DR (1965) Isolation and Characterization of the 7S Component of Soybean Globulins.  Cereal Chem  42:71.
28. Koshiyama, I (1968) Chromatographic and sedimentation behavior of a purified 7S protein in soybean globulin.  Cereal Chem  45:405.