Overview
ISO 5983-1:2005 - Animal feeding stuffs: Determination of nitrogen content and calculation of crude protein content - Part 1: Kjeldahl method specifies a standardized laboratory procedure to measure nitrogen content in animal feed and to calculate crude protein (by convention multiplying nitrogen by 6.25). The document defines reagents, apparatus, sampling/preparation references, digestion (sulfuric acid + catalyst + potassium sulfate), ammonia distillation, titration and result calculation. It is widely used for feed quality control, regulatory compliance and research.
Key topics and requirements
- Principle: Organic matter is digested with sulfuric acid and catalyst; ammonia is liberated by alkalization, distilled and titrated to determine nitrogen.
- Reagents and apparatus: Specifies analytical-grade chemicals (e.g., sulfuric acid c = 18 mol·L−1), catalysts (CuO or CuSO4·5H2O), potassium sulfate, NaOH solution, collecting liquids (sulfuric or boric acid), indicators and digestion/distillation equipment.
- Test portion & sample prep: Guidance on sample mass (air-dry 0.5–2.0 g; wet/inhomogeneous 2.5–5.0 g) and reference to ISO 6498 for sample preparation.
- Procedure: Detailed steps for digestion, dissolution of sulfates, ammonia distillation, titration (automatic pH endpoint recommended) and blank/check tests using acetanilide or tryptophan.
- Calculation & reporting: Formulas for nitrogen (g/kg) depending on collecting liquid and calculation of crude protein (multiply nitrogen by 6.25). Report results to nearest 0.01 g/kg or 0.1%.
- Precision: Repeatability and reproducibility limits are provided (equations relating limits to crude protein content) and interlaboratory data summarized in Annex A.
- Limitations: Does not measure oxidized or heterocyclic nitrogen forms and does not distinguish protein nitrogen from non-protein nitrogen - use other methods when non-protein nitrogen quantification is required.
Applications
- Routine feed laboratory testing for nutritional labelling and quality control
- Regulatory compliance testing for animal feeding stuffs
- Feed formulation validation in production and research
- Reference method for interlaboratory comparisons and proficiency testing
Who should use this standard
- Feed testing laboratories and analysts
- Quality assurance teams at feed manufacturers
- Regulatory agencies and inspection labs
- Academic and industrial researchers in animal nutrition
Related standards
- ISO 5983-2:2005 - Block digestion / steam distillation alternative method
- ISO 6498 - Preparation of test samples (referenced)
- ISO 1871 - General directions for the Kjeldahl method (referenced)
ISO 5983-1:2005 is the established ISO reference for Kjeldahl nitrogen determination in animal feeds and remains essential for accurate crude protein assessment when following standardized, traceable laboratory practice.
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