Overview
EN ISO 5983-2:2009 - "Animal feeding stuffs - Determination of nitrogen content and calculation of crude protein content - Part 2: Block digestion and steam distillation method" (CEN adoption of ISO 5983-2:2009) defines a Kjeldahl-based laboratory procedure for measuring Kjeldahl nitrogen and calculating crude protein in animal feeds and pet foods. It is intended as a semi-micro, rapid routine method using block digestion with a copper catalyst and steam distillation into boric acid, suitable for samples with >0.5 % mass fraction Kjeldahl nitrogen.
Key topics and technical requirements
- Method principle: Acid digestion (H2SO4 with K2SO4 and Cu catalyst) converts organic nitrogen to ammonium sulfate; excess NaOH liberates ammonia, which is distilled into boric acid and quantified by titration.
- Digestion and distillation: Uses block digestion apparatus and manual, semi‑automatic or fully automatic steam distillation units.
- Endpoint detection: Titration endpoint may be colorimetric (indicator mix: methyl red + bromocresol green) or potentiometric (pH electrode) for automatic systems.
- Reagents: High‑purity sulfuric acid, Kjeldahl catalyst tablets (K2SO4 + CuSO4·5H2O), H2O2, antifoam, NaOH solution, boric acid trapping solution and standard HCl titrant.
- Calculations: Nitrogen mass fraction is calculated from ammonia recovered; crude protein is obtained by multiplying nitrogen result by the conventional conversion factor 6.25.
- Scope & limitations:
- Applicable to animal feeding stuffs, pet foods and raw materials with >0.5 % Kjeldahl nitrogen.
- Does not measure oxidized nitrogen forms or heterocyclic nitrogen compounds.
- Does not distinguish between protein and non‑protein nitrogen.
- Quality and precision: Standard includes guidance on blanks, recovery checks and reports interlaboratory precision (repeatability and reproducibility) - see annexes for test results.
- Safety: Warns about hazardous reagents and requires appropriate lab safety practices.
Applications and users
- Routine feed testing and quality control in:
- Feed and pet food manufacturers
- Independent analytical laboratories
- Regulatory and inspection bodies verifying label claims
- Research labs studying feed composition and nutrition
- Typical uses include crude protein declaration, raw material evaluation, and batch release testing where rapid, routine nitrogen analysis is needed.
Related standards
- ISO 5983-1 - Kjeldahl method (other part of the series)
- ISO 1871 - General guidelines for Kjeldahl nitrogen determination
- ISO 6498 - Sample preparation guidelines for animal feeding stuffs
- EN adoption: EN ISO 5983-2:2009 (CEN endorsement)
Keywords: EN ISO 5983-2:2009, Kjeldahl, block digestion, steam distillation, crude protein, nitrogen determination, animal feeding stuffs, pet food analysis, boric acid titration, feed testing.