Overview
ISO 14891:2002 (IDF 185:2002) specifies a routine Dumas combustion method for determining the total nitrogen content of milk and milk products. The method converts sample nitrogen to molecular nitrogen by high‑temperature combustion (900 °C to 1 200 °C) in an oxygen atmosphere, removes interfering combustion products, and quantifies N2 with a thermal conductivity detector. Results are calculated using a microprocessor and expressed as mass fraction (percentage by mass).
Key topics and technical requirements
- Principle: Destructive combustion (Dumas) converting nitrogen compounds to N2, followed by gas cleanup and thermal conductivity detection.
- Apparatus: Dumas combustion unit with thermal conductivity detector and signal integration; analytical balance (0.0001 g); drying oven (105–110 °C); crucibles or tin capsules; recommended instrument components (combustion and post‑combustion tubes, catalysts, reduction materials).
- Reagents and gases: High‑purity carrier gases (CO2 or He) and O2; absorbents for SO2; CuO and platinum catalyst mixtures; silver wool; copper or tungsten for reduction; drying agents (P2O5 or Mg(ClO4)2); standard nitrogen compounds (e.g., aspartic acid, EDTA) for calibration.
- Sampling & sample prep: Sampling is not included but ISO 707 is recommended. Prepare homogeneous test portions; liquid samples often pre‑dried in boats/crucibles (105–110 °C, ~1 h). Typical test portion: at least 0.2 g (up to 3.5 g for low‑protein samples).
- Calibration & quality control: Use certified pure nitrogen compounds to prepare calibration series (examples: 4–100 mg N steps ≈5 mg; 10–200 mg steps ≈10 mg). Regular comparison with reference materials or the Kjeldahl method is recommended to ensure equivalence and to derive matrix/instrument correction factors.
- Performance: The standard includes precision data (interlaboratory tests, repeatability, reproducibility) and annexes with instrument flowcharts, calibration guidance, and comparisons with Kjeldahl results.
- Operational details: Allow instrument stabilization (~1 h), control oxygen demand or use self‑optimizing oxygen control (residual O2 mass fraction 2–8 %), and follow manufacturer instructions for specific Dumas equipment.
Applications and users
- Practical uses: Routine nitrogen/protein testing for milk, whey, cheese, powdered dairy and other milk products; quality control, production monitoring, nutritional labeling, R&D, and compliance testing.
- Who uses it: Dairy laboratory analysts, QC managers, milk processors, instrumental method developers, accreditation bodies, and regulatory laboratories seeking a rapid alternative to Kjeldahl analysis.
Related standards
- ISO 8968-1 / IDF 20-1: Kjeldahl method for nitrogen determination (reference and comparison method).
- ISO 707: Sampling of milk and milk products (recommended sampling procedures).
- ISO 6732: Sample preparation guidance referenced for homogenization.
Keywords: ISO 14891, Dumas method, nitrogen determination, milk and milk products, combustion analysis, thermal conductivity detector, routine method, IDF 185, Kjeldahl comparison, dairy quality control.
Open preview (PDF)Milk and milk products — Determination of nitrogen content — Routine method using combustion according to the Dumas principle