Overview
ISO 17997-2:2004 (IDF 29-2:2004) - "Milk - Determination of casein‑nitrogen content - Part 2: Direct method" specifies a routine laboratory procedure for directly measuring the casein‑nitrogen content of bovine milk. The method precipitates casein at acidic pH, separates the precipitate, and determines its nitrogen by Kjeldahl or block‑digestion techniques. The standard is intended for routine dairy testing and can be modified for milk from other species or for liquid dairy products.
Key topics and technical requirements
- Principle: Precipitation of casein by adding acetic acid and sodium acetate to reach an approximate pH of 4.6, filtration of the casein precipitate, and determination of nitrogen in the precipitate following ISO 8968‑1 or ISO 8968‑2.
- Reagents & buffers: Defined concentrations for the acetic acid solution (1.75 mol/L), sodium acetate solution (1 mol/L), and a buffer prepared from them.
- Apparatus: Standard laboratory glassware, water bath (38–40 °C), bottle‑top dispensers, nitrogen‑free filter paper, analytical balance (0.1 mg), and equipment for Kjeldahl or block digestion.
- Sample preparation: Warm and gently mix milk to melt fat, weigh ~5 g test portions, add preheated water, and control timing (complete key steps within 30 minutes to limit proteolysis).
- Procedure highlights:
- Add fixed volumes of acetic acid and sodium acetate, allow precipitate formation and settling.
- Filter, rinse with buffer solution, fold filter paper with precipitate and digest using Kjeldahl reagents.
- Run a blank alongside samples.
- Calculation: Casein‑nitrogen is calculated from titration volumes; casein content is obtained by multiplying nitrogen by 6.38 (protein factor). The standard provides the calculation formula and guidance on expressing results to appropriate decimal places.
- Precision: Interlaboratory precision data referenced (ISO 5725). Repeatability limits: not more than 0.007% nitrogen or 0.04% casein in more than 5% of cases.
Applications and users
ISO 17997-2:2004 is used for:
- Routine dairy laboratory quality control (raw milk testing, production QA).
- Regulatory and compliance testing where casein or protein fractions are required.
- Research and method comparison when assessing casein degradation, milk composition, or comparing heat‑treated vs. raw milks.
Primary users include dairy chemists, quality assurance managers, testing laboratories, standardization bodies, and academic researchers.
Note: The standard warns of hazardous reagents and recommends appropriate safety practices. It also notes that storage and heat treatment affect measured casein‑nitrogen (breakdown or whey‑protein denaturation).
Related standards
- ISO 17997‑1 / IDF 29‑1 - Indirect (reference) method
- ISO 8968‑1 / IDF 20‑1 and ISO 8968‑2 / IDF 20‑2 - Determination of nitrogen (Kjeldahl / block digestion)
- ISO 707 - Recommended sampling of milk
- ISO 5725 - Precision of measurement methods
Keywords: ISO 17997-2:2004, casein-nitrogen, milk testing, direct method, IDF 29-2, Kjeldahl, dairy laboratory, milk composition.