Overview
ISO 2918:1975 - "Meat and meat products - Determination of nitrite content (Reference method)" is a reference analytical procedure published by ISO for quantifying nitrite levels in meat and processed meat. The method is a colorimetric/photometric reference technique based on hot‑water extraction, protein precipitation, diazotization with sulfanilamide and coupling with N‑naphthyl‑ethylenediamine to form a red azo‑complex, and photometric measurement at 538 nm.
Key topics and technical requirements
- Sample preparation: Homogenize a representative test portion (sample handling and minimum representative mass described). Typical test portion described in the standard is about 10 g, with specific instructions for homogenization and storage.
- Extraction and deproteinization: Hot‑water extraction of the test portion, addition of saturated borax, heating (bath), then precipitation of proteins using hexacyanoferrate (Reagent I) and zinc acetate/acetic acid (Reagent II), with subsequent decantation and filtration.
- Color development reagents:
- Sulfanilamide solution (acidified) and
- N‑naphthyl‑1‑ethylenediamine hydrochloride (coupling agent, often abbreviated NED).
These produce a red colored complex in presence of nitrite.
- Measurement: Photometric/colorimetric determination at approximately 538 nm using a spectrophotometer or photoelectric colorimeter in 1 cm cells.
- Calibration and quantification: Prepare fresh sodium nitrite standard solutions the day of use; construct a calibration curve (µg/mL) and derive sample concentration. The standard defines the symbols used in the calculation (c = concentration from calibration curve in µg/mL; m = sample mass in g; V = aliquot volume in mL) and specifies how to express results as mg sodium nitrite/kg.
- Quality criteria: The method requires duplicate determinations and specifies repeatability limits (difference between two determinations by same analyst should not exceed 10% of the mean). Reagents must be analytical grade; some prepared solutions have short shelf lives and cold storage requirements.
Practical applications and users
- Food testing laboratories performing routine or regulatory nitrite monitoring in raw and processed meat.
- Meat processors and QA/QC teams for specification control and product safety verification (curing salts and residual nitrite).
- Regulatory authorities and inspection bodies verifying compliance with permissible nitrite limits.
- Research and development groups studying nitrite behavior in meat processing or developing alternative curing methods.
Related standards
- ISO 3100 - Sampling of meat and meat products (referenced for representative sampling)
- ISO/R 1042, ISO/R 648 - Specifications for volumetric glassware and pipettes used in the method
Keywords: ISO 2918:1975, nitrite determination, meat and meat products, photometric method, sulfanilamide, N‑naphthyl‑ethylenediamine, colorimetric assay, food safety, analytical method, calibration curve.
