Overview
EN ISO 20541:2008 (ISO 20541 / IDF 197) specifies a validated laboratory method for the determination of nitrate content in milk and milk products. The procedure combines enzymatic reduction of nitrate to nitrite, followed by the Griess reaction and quantification by molecular‑absorption spectrometry (absorbance measured at ~540 nm). The method is applicable when the measured concentration in the sample solution (blank subtracted) is greater than 0.2 mg/L.
Key topics and technical requirements
- Scope & matrices: Whole, partly skimmed and skimmed milk; dried milk; hard, semi‑hard and soft cheeses; processed and whey cheeses; caseins, caseinates; dried whey; milk protein concentrates.
- Analytical principle: Nitrate is enzymatically reduced to nitrite (nitrate reductase). Both unreduced (native nitrite) and reduced samples are reacted with sulfanilamide and N‑(1‑naphthyl)ethylenediamine (Griess reagents) to form an azo dye; absorbance measured against sodium nitrite calibration standards to derive nitrite and nitrate contents.
- Sample preparation: Fat and proteins removed either by Carrez precipitation and filtration or by centrifugal ultrafiltration (membrane cones) - ultrafiltration is preferred for powders and concentrates to avoid turbidity.
- Reagents & water quality: Use analytical‑grade reagents free of nitrate/nitrite; water must comply with ISO 3696 (grade 3 or better). Enzyme/coenzyme solutions require freshly double‑distilled or equivalent water.
- Calibration & blanks: Calibration with sodium nitrite solutions; matrix blank accounts for endogenous nitrite. Results expressed as mg NO3‑ per kg product.
- Validation: The standard includes sections on precision, repeatability and reproducibility and reports interlaboratory trial data.
Practical applications and users
- Quality control and routine testing in dairy laboratories and milk processing plants to monitor nitrate contamination or processing residues.
- Regulatory and food safety laboratories performing compliance testing and surveillance.
- Research laboratories studying nitrate sources, transfer from feed to milk, or processing effects on nitrate/nitrite levels.
- Useful for ingredient specification checks (whey powders, milk protein concentrates) and product safety assessments.
Related standards (normative references)
Standards referenced in EN ISO 20541:2008 include:
- ISO 3696 - Water for analytical laboratory use
- ISO 1042, 835, 648 - Laboratory glassware (volumetric and pipettes)
- ISO 565 - Test sieves
Keywords: EN ISO 20541, ISO 20541, IDF 197, nitrate determination, milk and milk products, Griess reaction, enzymatic reduction, molecular‑absorption spectrometry, dairy testing, nitrate content.