Overview
ISO 14637:2004 (IDF 195:2004) - "Milk - Determination of urea content - Enzymatic method using difference in pH (Reference method)" defines a laboratory reference method for measuring urea in milk. The procedure uses urease to hydrolyse urea into ammonia and carbon dioxide, and quantifies urea by measuring the resulting differential pH change with a flow‑through differential pH analyser. This International Standard was prepared jointly by ISO, IDF and AOAC and is intended for reliable, reproducible determination of urea content expressed in mg/L.
Key topics and technical requirements
- Principle: Urease-catalysed hydrolysis of urea at pH ~6.7 produces pH change proportional to urea concentration; measured as the difference in pH between two electrodes.
- Reagents: Specifications for buffer (pH 6.7), urease enzyme solution (activity specified), urea standard solution, and cleaning/regenerating solutions are included, with recommended storage conditions.
- Apparatus: Requirements for a differential pH apparatus (peristaltic pumps, mixing chamber, two glass capillary flow‑through electrodes), micropipettes, water bath and analytical balance.
- Procedure: Stepwise instructions for sample preparation, blank determination, calibration (including calculation of pH difference and slope), routine checks (stability, contamination), and electrode cleaning/regeneration.
- Quality control: Calibration checks, blank limits (mpH ranges), and electrode maintenance procedures to ensure measurement accuracy.
- Precision and reporting: The standard contains results from interlaboratory tests and guidance on repeatability, reproducibility and required content of the test report.
Applications and practical value
- Dairy quality control: Routine and reference testing of milk for urea content to monitor animal health, feeding practices and milk adulteration.
- Regulatory and reference laboratories: Use as an established reference method for method validation, interlaboratory comparisons and official control.
- Research and method development: Basis for developing rapid screening methods and validating alternative analytical techniques against an accepted reference.
Who should use ISO 14637:2004
- Dairy laboratory technicians and managers
- National and regional food control authorities
- Instrument manufacturers (differential pH analysers)
- Researchers validating urea measurement procedures
Related standards
- ISO 707 - recommended sampling method for milk (sampling is outside the scope of ISO 14637 but referenced)
- Associated IDF/AOAC documents cited within the standard for collaborative context
Keywords: ISO 14637:2004, milk urea determination, enzymatic method, urease, differential pH analyser, reference method, dairy testing, urea in milk.
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