Overview
EN ISO 5764:2009 - Milk: Determination of freezing point (thermistor cryoscope method) specifies a reference method for measuring the freezing point of milk using a thermistor cryoscope. It applies to raw bovine milk, heat‑treated whole/reduced‑fat/skimmed bovine milk, and raw ovine and caprine milk. The freezing point is commonly used as an indicator in milk quality control - notably to detect the presence of extraneous water - although calculating the exact amount of added water is outside the standard’s scope.
Key topics and technical requirements
- Principle: A milk sample is supercooled, crystallization is induced and the sample warms to a stable temperature plateau. The plateau temperature corresponds to the freezing point. The plateau is defined as a temperature rise ≤ 0.5 m°C over the previous 20 s.
- Instrument: Uses a thermistor cryoscope (plateau‑timed instrument). The standard is a reference method; routine fixed‑time thermistor cryoscope variants are addressed in Annex C.
- Calibration: Calibration is carried out with sodium chloride (NaCl) standard solutions. The instrument is adjusted to give correct readings for two NaCl standards using the same procedure as for milk samples.
- Reagents & preparation:
- Water meeting ISO 3696 (grade 2).
- Finely ground NaCl, dried (specified drying conditions) and prepared into standard solutions (stored near 5 °C).
- Sampling & sample condition: Procedures for representative sampling are included. Results from samples whose titratable acidity exceeds 20 ml of 0.1 mol/l NaOH per 10 g non‑fat solids are not representative of the original milk.
- Quality aspects: The standard contains precision information (interlaboratory trials, repeatability, reproducibility) and prescribes calculation and expression of results.
Applications and who uses it
- Dairy laboratories conducting official reference testing of milk freezing point.
- Quality control teams in dairy processing plants for adulteration detection (extraneous water).
- Regulatory bodies and milk payment systems that require standardized, traceable freezing point measurements.
- Research groups studying milk composition and processing effects (note: sterilization and vacuum pasteurization can affect freezing point).
Related standards
- ISO 3696 - Water for analytical laboratory use (referenced for reagent quality).
- ISO 6091 - Dried milk - Determination of titratable acidity (reference method).
- Annexes in EN ISO 5764:2009 provide guidance for routine methods (Annex C), interlaboratory data (Annexes A/B) and reference value adjustments (Annex D).
Keywords: EN ISO 5764:2009, thermistor cryoscope, freezing point of milk, reference method, milk quality, extraneous water detection, calibration, sodium chloride standards, titratable acidity.