Overview
EN ISO 14637:2006 (ISO 14637:2004 / IDF 195:2004) is the international reference method for the determination of urea content in milk. It specifies an enzymatic method using the difference in pH produced when urease acts on milk urea. The standard is intended as a laboratory reference procedure for accurate quantification of urea (expressed in mg/L) and is widely used in dairy testing, quality control and method validation.
Key topics and technical requirements
- Principle: Urease hydrolyses urea into ammonia and carbon dioxide. At pH 6.7, ammonia hydrolysis releases hydroxyl ions while CO2 liberates protons; the net pH change is proportional to urea concentration and is measured with a differential pH analyser.
- Reagents: Defined reagents include a pH 6.7 buffer, urease enzyme solution (target activity 2 100 U/mL ± 300 U/mL), urea standard solution, and “zero milk” (treated milk blank). Cleaning, regenerating and strong regenerating solutions for electrode maintenance are specified.
- Apparatus: Typical equipment includes an analytical balance, positive-displacement micropipettes, controlled water baths, and a differential pH apparatus with peristaltic pumps, mixing chamber and two flow-through pH electrodes.
- Procedure elements: The standard covers blank determinations, calibration and calibration checks, the measurement procedure, electrode cleaning and regeneration, and maintenance routines.
- Quality and performance: Instructions for sampling (reference to ISO 707), preparation of test samples, calculation and expression of results, and documented precision data (repeatability and reproducibility from interlaboratory tests) are included.
- Storage and stability: Typical reagent storage guidance is given (for example, many solutions can be kept for 6 months at 4 °C; cleaning solutions for 1 year at room temperature).
Applications and users
- Dairy laboratories and manufacturers performing milk urea determination for product quality, nutritional assessment and fraud detection.
- Regulatory and conformity assessment bodies needing a reference method for verification and method validation.
- Research laboratories studying nitrogen compounds in milk and developing or validating routine screening methods against a reference procedure.
- Providers of laboratory equipment and sensor manufacturers validating differential pH analysers and electrode systems.
Related standards
- ISO 707 (milk sampling) is referenced for sampling practice.
- EN ISO 14637 is published jointly by ISO and IDF and is compatible with related dairy analysis standards used for method validation and routine testing.
Keywords: EN ISO 14637:2006, milk urea determination, enzymatic method, differential pH, urease, dairy testing, reference method, urea content, quality control.
