Overview
ISO 937:2023 - "Meat and meat products - Determination of nitrogen content - Reference method" specifies a reference Kjeldahl-based method for measuring nitrogen content in meat and meat products. The standard defines the principle, required reagents and apparatus, sample preparation, digestion, distillation and titration steps, and quality checks needed for accurate and traceable results. The second edition (2023) updates the original 1978 method and adds procedures for automatic Kjeldahl meters in addition to traditional apparatus.
Key Topics and Requirements
- Scope and principle: Conversion of organic nitrogen to ammonium ions by sulfuric acid digestion with a copper(II) sulfate catalyst, followed by alkalization, distillation of ammonia into boric acid and titration to quantify nitrogen.
- Sampling & sample prep: Start from a representative laboratory sample (recommended ≥200 g), homogenize without exceeding 25 °C, analyze homogenate within 24 h.
- Reagents: Specified analytical-grade chemicals (e.g., CuSO4·5H2O, K2SO4, concentrated H2SO4, boric acid 40 g·L‑1, NaOH solution ≈33 g/100 g), drying and purity notes for standards and controls.
- Apparatus: Mechanical homogenizer, Kjeldahl flasks, digestion and distillation systems (traditional steam or ordinary distillation), analytical balance (0.001 g), Class A burette.
- Procedure variants: Detailed steps for both traditional Kjeldahl apparatus and automatic Kjeldahl meters, including digestion conditions, distillate volumes (minimum 150 ml), indicators (methyl red/methylene blue or methyl red/bromocresol green) and titration with accurately known HCl or H2SO4 solutions.
- Quality control: Blank and recovery tests, duplicate determinations from the same sample, precision data and reporting requirements are included.
- Safety & handling: Emphasis on preventing sample loss, controlling foaming for high-fat samples, and avoiding contamination from nitrogen-containing preservatives.
Applications and Users
ISO 937:2023 is intended for:
- Food testing laboratories performing nitrogen analysis of fresh, processed or cured meat products.
- Meat processors and quality assurance teams monitoring raw material and product composition.
- Regulatory and inspection bodies verifying compliance with labeling or regulatory requirements.
- Research institutions studying meat composition or validating analytical methods.
The method provides a traceable reference for laboratories implementing routine nitrogen or protein-related analyses (protein estimation using appropriate conversion factors where applicable).
Related Standards
- ISO 385 - Laboratory glassware - Burettes (referenced for titration equipment)
- CAC/GL 50-2004 - Recommended sampling methods for foods (sampling guidance)
- Prepared by ISO/TC 34 (Food products), Subcommittee SC 6 (Meat, poultry, fish, eggs)
Keywords: ISO 937:2023, Kjeldahl, nitrogen determination, meat and meat products, reference method, nitrogen content, automatic Kjeldahl, digestion and distillation.
