Overview
ISO 5764:2009 - Milk - Determination of freezing point - Thermistor cryoscope method (Reference method) specifies a reference laboratory method for measuring the freezing point of milk using a thermistor cryoscope. It covers raw bovine milk, heat‑treated whole, reduced‑fat and skimmed bovine milk, and raw ovine and caprine milk. The measured freezing point is used to assess possible extraneous water in milk; however, calculating the exact amount of added water (subject to daily and seasonal variation) is outside the scope of the standard.
Key topics and technical requirements
- Principle: Samples are supercooled, crystallization is induced, and the resulting temperature plateau (when rise ≤ 0.5 m°C over 20 s) defines the freezing point (expressed in millidegrees Celsius).
- Instrument: A thermistor cryoscope with:
- Glass thermistor probe (approx. 1.60 mm ± 0.4 mm diameter; resistance 3 Ω–30 kΩ at 0 °C).
- Read‑out discrimination of 1 m°C or better; circuit linearity to limit error ≤ 1 m°C across the working range.
- Stirring wire to ensure uniform temperature and act as a crystallization initiator.
- Cooling device options: immersion bath, circulation, or cooling block (maintain cooling liquid ≈ −7.0 °C ± 0.5 °C).
- Calibration: Performed against sodium chloride standard solutions (prepared to specified concentrations) using the same plateau procedure as for milk samples. The method requires calibration checks and routine performance verification.
- Reagents & sample handling: Use analytical‑grade water (ISO 3696 grade 2) and dried NaCl for standards. Samples with titratable acidity > 20 ml of 0.1 mol/l NaOH per 10 g non‑fat solids (bovine milk) are considered unrepresentative.
- Precision & validation: The standard includes interlaboratory precision data and guidance for repeatability and reproducibility. Annexes provide trials and guidance for routine methods and adjustments.
Applications and who uses it
- Quality control in dairy processing: Detecting dilution or adulteration by estimating freezing point depression.
- Regulatory testing and enforcement: Reference method for laboratories performing official milk analyses.
- Calibration and validation: Reference procedure to validate routine cryoscopes and lab methods (Annex C provides guidance for routine fixed‑time instruments).
- Research and metrology: Studies on milk composition and instrument performance.
Typical users: dairy laboratory analysts, QC managers, regulatory laboratories, instrument manufacturers, and food scientists.
Related standards and references
- ISO 3696 - Water for analytical laboratory use
- ISO 6091 - Dried milk - Titratable acidity (reference method)
- ISO 707 / IDF 50 - Recommended sampling for milk
Keywords: ISO 5764, thermistor cryoscope, freezing point of milk, extraneous water, cryoscope method, dairy testing, reference method.