Overview
ISO 1871:2009 - "Food and feed products - General guidelines for the determination of nitrogen by the Kjeldahl method" provides practical, laboratory-focused guidance for measuring organic nitrogen in food and feed matrices using the classical Kjeldahl method. It covers the full analytical workflow (digestion, alkalinization/distillation, titration) and clarifies critical control points, safety considerations and minimum performance objectives. Note: nitrogen from nitrates and nitrites is not included in this measurement principle.
Key topics and technical requirements
- Principle: Conversion of organic nitrogen to ammonium sulfate by digestion with concentrated sulfuric acid plus catalysts; release of ammonia by addition of sodium hydroxide; capture in boric acid and titration with standardized acid to calculate nitrogen.
- Reagents: Analytical-grade sulfuric acid, catalysts (commonly copper sulfate ± titanium oxide; potassium sulfate to raise boiling point), boric acid/indicator, H2O2 (optional, with care), concentrated NaOH, ammonium salts for controls.
- Apparatus: Analytical balance (0.001 g), digestion units, steam-distillation and titration systems, anti-foaming agents and appropriate fume extraction.
- Operating method: Guidance on test portion selection, acid volumes, digestion temperature/time, alkalinization and distillation procedures, endpoint detection (colorimetric or potentiometric).
- Control tests & performance criteria:
- Blank test and digestion control (using tryptophan, acetanilide or lysine hydrochloride) - digestion recovery should be 98–101%.
- Distillation–titration control (using ammonium sulfate or chloride) - recovery should be 99–101%.
- Results expression: Nitrogen % (or g/100 ml) calculated from titrant volumes and titre; formulae provided in the standard.
- Safety: Warnings about hazardous reagents and the need for ventilation, safe handling of catalysts (selenium-containing), and care when adding oxidizers like H2O2.
Applications and users
ISO 1871:2009 is intended for:
- Food and feed testing laboratories performing nitrogen determination / Kjeldahl nitrogen analyses.
- Quality assurance, regulatory compliance and laboratory method validation teams.
- Manufacturers, contract labs and research groups needing standardized guidance for Kjeldahl workflows across diverse matrices (meat, dairy, cereals, feeds).
Nitrogen results are frequently used to calculate crude protein content (via appropriate conversion factors) for labeling, nutritional assessment and regulatory reporting.
Related standards
ISO 1871 provides general guidelines and complements matrix-specific standards, e.g.:
Keywords: ISO 1871:2009, Kjeldahl method, nitrogen determination, food and feed analysis, digestion, distillation, titration, sulfuric acid, laboratory QA.
