🍎 Freezing point of milk — thermistor cryoscope (detection of the addition of water)
In short
A portion of 2. 5 ml of milk is supercooled in a thermistor cryoscope, freezing is initiated and the highest temperature of the plateau of the freezing curve is read off (to the nearest 0. The test is performed according to PN-EN ISO 5764:2010; Portion of the sample: 2.5 ± 0.1 ml in a test tube 50.8 × 16 mm (13.5 mm internally). The procedure comprises 8 steps (about 12 h in total); it is used for: Detection and estimation of the addition of water to raw milk at collection and in dairy plants, Quality control of pasteurised, UHT and sterilised drinking milk, A settlement parameter in milk collection alongside the content of fat and protein.
At a glance
- Standard: PN-EN ISO 5764:2010
- Category: Food chemistry
- Procedure steps: 8
- Total time of stages: 12 h
- Portion of the sample: 2.5 ± 0.1 ml in a test tube 50.8 × 16 mm (13.5 mm internally)
- Resolution of the reading: 0.001 °C in the range 0 to −1 °C; the scale division not larger than 0.001 °C
- Calibration: Two NaCl solutions covering the expected freezing point, a difference ≥ 0.100 °C; a working check for example with a solution of −0.512 °C (8.646 g NaCl/l), at least once an hour in continuous work
Overview
The standard specifies a method of determining the freezing point of whole, partly skimmed and skimmed milk — raw, pasteurised, UHT, homogenised or sterilised. The freezing point may serve to estimate the proportion of extraneous water in milk; the calculation of the amount of water added is however made difficult by daily and seasonal variations, and the results of samples with an acidity exceeding 0.18 g of lactic acid per 100 ml are not representative of the original milk. The description is based on the preview of ISO 5764:1987; the current edition PN-EN ISO 5764:2010 was not available to us for inspection.
The freezing temperature of milk is a value strictly correlated with the osmotic pressure and depends on the concentration of the dissolved substances: on lactose in 55 %, on chlorides in 25 %, and in 20 % on calcium, potassium, magnesium and phosphates; the fat globules, the casein micelles and the whey proteins have no significant influence on it. An addition of 1 % of water raises the freezing temperature by 0.006 °C, and for that reason the cryoscopic test is a rapid and cheap method of detecting the watering of milk at collection, and in many countries it affects the price of milk alongside the content of fat and protein.
A thermistor cryoscope consists of a thermostatically controlled cooling bath (a suitable liquid is a 33 % solution of ethylene glycol), a thermistor probe with a measuring system, a stirrer for the sample and a device initiating the freezing; the instrument looks for the first plateau of the freezing curve, that is the stretch on which the temperature remains constant within 0.001 °C for at least 20 s. Manual instruments (a Wheatstone bridge with a galvanometer) and automatic ones are available; instruments with a fixed measuring time may be used for routine screening tests.
Method principle
A sample of milk (2.5 ± 0.1 ml) in a glass test tube of the dimensions 50.8 × 16 mm (internal diameter 13.5 mm) is cooled in the cryoscope to the supercooling temperature given by the manufacturer (to the nearest 0.1 °C), after which freezing is initiated — for example by increasing for 1–2 s the amplitude of the vibrations of the stirring wire so that it strikes the wall of the test tube. The temperature of the sample rises abruptly to a value which for a certain time remains practically constant; the freezing point corresponds to the highest temperature reached in that period, and it is that value that is recorded. The instrument is calibrated with two standard solutions of sodium chloride covering the expected freezing point (the difference between them preferably not less than 0.100 °C), for example 6.859 g NaCl/l (−0.408 °C) and 10.155 g/l (−0.600 °C); after the calibration the cryoscope indicates the freezing point of the milk without corrections. The result is the mean of two determinations rounded to the third decimal place; repeatability 0.004 °C, reproducibility 0.006 °C. At the same time the titratable acidity of the milk is determined (ISO 6091).
Applications
- Detection and estimation of the addition of water to raw milk at collection and in dairy plants
- Quality control of pasteurised, UHT and sterilised drinking milk
- A settlement parameter in milk collection alongside the content of fat and protein
- The reference method for indirect methods of measuring the freezing point (for example infrared analysers)
Key parameters
| Parameter | Value |
|---|---|
| Portion of the sample | 2.5 ± 0.1 ml in a test tube 50.8 × 16 mm (13.5 mm internally) |
| Resolution of the reading | 0.001 °C in the range 0 to −1 °C; the scale division not larger than 0.001 °C |
| Calibration | Two NaCl solutions covering the expected freezing point, a difference ≥ 0.100 °C; a working check for example with a solution of −0.512 °C (8.646 g NaCl/l), at least once an hour in continuous work |
| Criterion for recalibration | A deviation of the mean of two readings of the standard > 0.002 °C |
| Precision | Repeatability 0.004 °C; reproducibility 0.006 °C (at the 95 % level) |
| Result | The mean of two determinations to 0.001 °C; where the sum is odd, round to the even value |
Standard
- Standard number
- PN-EN ISO 5764:2010
- Title (PL)
- Mleko — Oznaczanie punktu zamarzania — Metoda z zastosowaniem krioskopu termistorowego (Metoda odniesienia)
- Title (EN)
- Milk — Determination of freezing point — Thermistor cryoscope method (Reference method)
Step-by-step procedure
-
Preparation of the cryoscope
Switch the cryoscope on at least 12 h before the calibration; check the level and the stirring of the cooling liquid, the position of the probe (the tip 44.6 ± 0.1 mm below the upper edge of the test tube, set with a template) and the amplitude of the vibrations of the stirring wire (about ±1.5 mm, without striking the probe or the wall).
-
Calibration
Choose two standard NaCl solutions covering the expected freezing point (a difference ≥ 0.100 °C). Pour 2.5 ml of one solution into a clean dry test tube and carry out a measurement; adjust the calibration knobs until the reading equals the freezing point of the standard. Repeat with the second solution and alternately, until successive readings of both without adjustment give correct values.
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The milk sample
Take the milk according to ISO 707; test it immediately or keep it below 5 °C (up to 12 weeks at −18 °C). Remove foreign bodies and solidified fat, if necessary filter through an inert filter; mix gently. The standard solutions and the milk should be at the same temperature.
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Working check
Before every series measure the freezing point of a standard (for example −0.512 °C) until two successive readings do not differ by more than 0.001 °C; if the mean departs from the value of the standard by more than 0.002 °C — recalibrate. In continuous work the check at least once an hour.
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Measurement
Measure out 2.5 ± 0.1 ml of milk into a clean dry test tube, put it into the cryoscope; the sample is cooled and the freezing is initiated within 0.1 °C of the temperature given by the manufacturer. Record the highest temperature of the plateau after the initiation of the freezing.
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Premature freezing
If the freezing began before the set temperature, repeat with a new portion; if it happens again, heat a portion of the sample to about 45 °C for 5 min (melting of the crystallised fat), cool it to 20 ± 2 °C and test it immediately.
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Repeat determination
Take the test tube out, rinse it, dry the probe and the wire with soft lint-free tissue and carry out a second determination on a new portion of milk. If the results differ by more than 0.004 °C, discard them and carry out two further determinations.
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Result and test report
Give the mean of two results to the third decimal place (where the sum is odd, round to the even value), the titratable acidity, the identification of the sample with the conditions of storage and the preservative, the date of receipt and of testing and any observations that might call the result into question. After the work leave an empty test tube in the socket, so that the probe stays cool.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Thermistor cryoscope | An immersion cooling bath (a difference of temperature in the liquid ≤ 0.2 °C) or a flow-through one, stability ±0.5 °C; a glass thermistor of a diameter ≤ 1.8 mm, a time constant < 2 s; a stirring wire of 1–1.5 mm of an inert metal | — |
| Measuring system | A Wheatstone bridge with a galvanometer and a scale (manual) or a digital readout with a discrimination ≥ 0.001 °C and a linearity of ±0.001 °C in the range −0.400 to −0.600 °C | — |
| Measuring test tubes | Glass, 50.8 ± 0.1 mm long, 16 ± 0.1 mm external diameter, 13.5 ± 0.1 mm internal | — |
| Analytical balance and class A volumetric flasks of 1000 ml | An accuracy of 0.1 mg — for preparing the standard solutions | — |
| Drying oven at 130 ± 1 °C or a furnace at 300 ± 25 °C | For drying the sodium chloride; a desiccator | — |
| Stabilised power supply | Variations of voltage ≤ 1 % with variations of the mains of 6 % | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sodium chloride | 7647-14-5 | Of analytical grade, finely ground, dried for 5 h at 300 ± 25 °C or at least 24 h at 130 ± 1 °C; standard solutions from 6.859 g/l (−0.408 °C) to 10.155 g/l (−0.600 °C) in distilled water, kept in polyethylene bottles of ≤ 250 ml at about 5 °C, for not longer than two months (without a fungicide) |
| Water | — | Distilled in apparatus of borosilicate glass, boiled and cooled to 20 ± 2 °C immediately before use |
| Ethylene glycol (1,2-ethanediol) | 107-21-1 | An aqueous solution of 33 % (v/v) as the cooling liquid of the bath |
Health and safety (OHS)
- The cooling liquid based on ethylene glycol is harmful if swallowed — top it up in gloves, do not confuse it with water
- The drying of sodium chloride at 300 °C — tongs and thermal gloves, cool it in a desiccator
- The instrument is mains-powered with heating and cooling elements — do not flood the casing with milk
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 5764:2010 — “Milk — Determination of freezing point — Thermistor cryoscope method (Reference method)”.
How does this method work?
A sample of milk (2.5 ± 0.
What is the measuring range and accuracy?
Portion of the sample: 2.5 ± 0.1 ml in a test tube 50.8 × 16 mm (13.5 mm internally); Resolution of the reading: 0.001 °C in the range 0 to −1 °C; the scale division not larger than 0.001 °C; Calibration: Two NaCl solutions covering the expected freezing point, a difference ≥ 0.100 °C; a working check for example with a solution of −0.512 °C (8.646 g NaCl/l), at least once an hour in continuous work; Criterion for recalibration: A deviation of the mean of two readings of the standard > 0.002 °C.
How long does the test take?
The times given for the individual stages add up to approximately 12 h. The procedure comprises 8 steps.
What equipment is required?
Thermistor cryoscope, Measuring system, Measuring test tubes, Analytical balance and class A volumetric flasks of 1000 ml, Drying oven at 130 ± 1 °C or a furnace at 300 ± 25 °C, Stabilised power supply.
Where is this test used?
Detection and estimation of the addition of water to raw milk at collection and in dairy plants; Quality control of pasteurised, UHT and sterilised drinking milk; A settlement parameter in milk collection alongside the content of fat and protein; The reference method for indirect methods of measuring the freezing point (for example infrared analysers).
What safety precautions apply?
The cooling liquid based on ethylene glycol is harmful if swallowed — top it up in gloves, do not confuse it with water; The drying of sodium chloride at 300 °C — tongs and thermal gloves, cool it in a desiccator; The instrument is mains-powered with heating and cooling elements — do not flood the casing with milk.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 5764.