Overview - ISO 20541:2008 (IDF 197:2008)
ISO 20541:2008 specifies a validated laboratory method for the determination of nitrate content in milk and milk products. The procedure uses enzymatic reduction of nitrate to nitrite (nitrate reductase), followed by the Griess reaction to form a coloured azo dye that is quantified by molecular-absorption spectrometry (spectrophotometry). The method is applicable to whole, partly skimmed, skimmed and dried milk, a wide range of cheeses, processed cheese, whey cheese, caseins, caseinates, dried whey and milk protein concentrates. It is suitable for sample solutions giving a measured concentration (blank subtracted) greater than 0.2 mg/L.
Key topics and technical requirements
- Principle: Nitrate → enzymatic reduction → nitrite; nitrite reacts in the Griess reaction to give a coloured azo dye whose absorbance is measured (540 nm or Hg 546 nm).
- Sample preparation: dispersion in warm water and removal of fat/protein by either Carrez precipitation or centrifugal ultrafiltration (membrane cones), depending on matrix.
- Reagents and enzymes: sodium nitrite and potassium nitrate standards for calibration; nitrate reductase, NADPH/FAD coenzymes; sulfanilamide and N-(1‑naphthyl)ethylenediamine for the Griess reaction; standard laboratory buffers (potassium phosphate).
- Calibration and blanks: calibration with nitrite/nitrate standards and use of a matrix blank to account for endogenous nitrite.
- Performance: the standard includes precision data from interlaboratory trials, and specifies requirements for repeatability and reproducibility of results.
- Instrumental: molecular-absorption spectrometer (visible range), properly maintained glassware and reagents free of nitrate/nitrite contamination.
Practical applications and users
This standard is intended for:
- Dairy industry quality-control laboratories verifying nitrate levels in raw materials and finished products.
- Food safety and regulatory laboratories monitoring compliance with food safety limits.
- Research institutions studying nitrate/nitrite behavior in dairy matrices.
- Third-party testing labs offering compositional and contaminant analysis services.
Benefits:
- Robust, matrix-adapted method for diverse milk products
- Clear procedure for sample cleanup and enzymatic reduction
- Traceable calibration enabling quantitative results and interlaboratory comparability
Related standards
Normative references cited within ISO 20541 include ISO 565 (test sieves), ISO 648/835 (pipettes), ISO 1042 (volumetric flasks) and ISO 3696 (water for analytical use). ISO 20541 is published jointly by ISO and IDF (International Dairy Federation).
Keywords: ISO 20541:2008, nitrate determination, milk and milk products, Griess reaction, enzymatic reduction, spectrophotometry, nitrate in milk, nitrate reductase, Carrez reagents, ultrafiltration.
