The nitrogen-to-protein conversion factor is used to calculate total protein content from Kjeldahl or Dumas nitrogen measurement:
Protein = Total Nitrogen × Conversion Factor
This factor accounts for the fraction of total nitrogen in the seed that belongs to true protein (rather than non‑protein nitrogen, NPN, such as free amino acids, nitrates, nucleic acids, amides). For pulse dry fractionation labs and production QC, selecting the correct factor directly changes reported protein % (dry basis) for feed flour, fine protein concentrate and coarse starch fraction.
Standard conversion factors for common pulses
| Pulse species | Nitrogen-to-protein factor | Notes |
|---|---|---|
| Yellow pea (Pisum sativum) | 5.70 | Most widely adopted for pea flour, pea protein concentrate from dry air classification; replaces the generic 6.25 |
| Faba bean / broad bean (Vicia faba) | 5.70 | Standard for dehulled faba bean flour and dry-fractionated faba protein concentrate |
| Lentil (Lens culinaris) | 5.70 | Used for lentil flour and lentil protein concentrate |
| Chickpea (Cicer arietinum) | 6.25 or 5.70 | Many laboratories use 5.70; older literature uses 6.25; verify with specification / regulatory requirement |
| Mung bean (Vigna radiata) | 5.70 | Dehulled mung bean flour and dry-fractionated mung protein |
| Common bean / navy bean (Phaseolus vulgaris) | 5.70 | White bean, red kidney bean |
The universal default factor 6.25 assumes protein contains 16% nitrogen. This overestimates pulse protein because pulses contain measurable non‑protein nitrogen, so 5.70 is the preferred factor for most whole dehulled pulses and dry-fractionated pulse protein concentrates.
Factors for separated fractions from dry air classification
- Dehulled raw pulse flour (feed to mill)
Factor: 5.70 - Fine protein-rich concentrate (air-classified fines)
Factor: 5.70The protein bodies have the same amino acid nitrogen profile as the parent pulse; NPN concentration does not shift enough in single-pass dry classification to require a new factor.
- Coarse starch-rich fraction (air-classified coarse stream)
Factor: 5.70Coarse fraction contains residual seed protein plus low-level NPN, same seed matrix origin.
When to use 6.25
- Regulatory or customer specification explicitly requires 6.25.
- Comparison to older literature that used 6.25.
- Some whole-seed, hull-included pulse analyses or certain food labelling regions may still specify 6.25.
Important warning: Using 6.25 instead of 5.70 will artificially inflate protein results by ~9%. For example, 4.0% nitrogen ×6.25 =25.0% protein; ×5.70=22.8% protein. This difference is significant for product certificates and commercial sales of pulse protein concentrates.
Exceptions & practical considerations
- Isolated pulse protein isolates (wet extraction)
Some wet-processed pea protein isolates may use 5.70, but some standards specify 6.25 for highly purified isolates. This does not apply to dry-fractionated protein concentrates from air classification. - Non-protein nitrogen impact
Pulses contain NPN including free amino acids, ureides and nucleic acids. The 5.70 factor is calibrated to subtract this inherent NPN bias for pulse seeds. - Consistency rule for your dry fractionation plant
Use the same conversion factor across all streams (feed, fine, coarse) when calculating protein mass balance, enrichment factor and protein separation efficiency. Mixing factors between streams creates invalid mass balance and misleading enrichment numbers.
Regulatory reference
FAO / AOAS recommendations: Legumes (pulses) commonly apply 5.70, while 6.25 is a general default for many other protein sources (dairy, meat, many oilseed meals). Always align your QC method with your target market’s labelling standard.
For most pulse species used in dry fractionation (pea, faba bean, lentil, mung bean, common bean), the recommended nitrogen-to-protein conversion factor is 5.70. The generic 6.25 overstates protein content due to non-protein nitrogen. All streams from air classification (feed flour, protein-rich fines, starch-rich coarse fraction) should use this consistent factor for reliable mass balance and separation efficiency calculations.
