Overview
ISO 17997-1:2004 (IDF 29-1:2004) defines a reference (indirect) method for determining the casein‑nitrogen content of bovine milk. The method precipitates casein at acid pH, measures the remaining (non‑casein) nitrogen, and calculates casein‑nitrogen as the difference from total nitrogen. It is a laboratory reference method intended for accurate, comparable results and can be modified for other milk species or liquid dairy products.
Key topics and technical requirements
- Principle: Determine total nitrogen (Kjeldahl methods per ISO 8968‑1 or ISO 8968‑2), precipitate casein by adding acetic acid and sodium acetate (final pH ≈ 4.6), filter, and measure nitrogen in the filtrate. Casein‑N = total N − non‑casein N.
- Reagents & volumes: Example reagents specified include acetic acid solution (1.75 mol/L) and sodium acetate solution (1.00 mol/L); a typical test portion is ~10 g milk diluted with 75 mL water, with 1 mL of each reagent added following the procedure.
- Apparatus: Standard laboratory glassware (ISO 1042, ISO 648), Kjeldahl digestion/distillation equipment, water bath (38–40 °C), nitrogen‑free filter paper, analytical balance and pipettes.
- Sample handling: Warm to 38–40 °C to melt fat, mix gently, and complete sample preparation steps within 30 minutes to limit proteolytic breakdown of casein.
- Calculations: Non‑casein nitrogen is calculated from titrant volume, molarity and sample mass (including a standard multiplicative correction factor 0.994 for typical bovine milk). Protein and casein contents are derived using the conversion factor 6.38.
- Quality & precision: Method precision and reproducibility were evaluated in interlaboratory studies (ISO 5725 principles). A blank test and careful titration records are required.
- Safety: Standard warns of hazardous materials and operations; users must follow appropriate safety and regulatory rules.
Applications and users
- Dairy testing and quality control laboratories validating casein content in raw milk or liquid dairy products.
- Regulatory agencies and standards bodies requiring a reference method for compliance and enforcement.
- Research laboratories studying milk composition, casein stability, proteolysis and heat‑treatment effects.
- Producers and instrument manufacturers using reference results to calibrate rapid methods (e.g., infrared milk analyzers).
Related standards
Keywords: ISO 17997-1:2004, casein‑nitrogen, milk analysis, indirect method, reference method, Kjeldahl, non‑casein nitrogen, dairy laboratory, milk testing.
Open preview (PDF)Milk — Determination of casein-nitrogen content — Part 1: Indirect method (Reference method)
