The Freezing Point of milk is the constant physical-chemical property of milk which is determined only by its water-soluble components such as lactose and salts, which in accordance with the Wigner law are held in milk at an approximately constant concentration.
The Freezing Point of milk varies depending on the breeds of animals and the regions from minus 0,525°C to minus 0,565 °C, and for the combined milk is in the range of from minus 0,530 °C to minus 0,550 °C. The most common/ typical freezing point value of milk which can be used as a comparison value is equal to minus 0,540 °C, according to the State Standard of Russian Federation (GOST № 30562-97 «Milk. Determination of freezing point. Thermistor cryoscope method”) it is minus 0,520°C. Colostrum (foremilk), which composition is a little modified in with respect to normal milk has the lowest freezing point from minus 0,570°C to minus 0,580 °C.
The dependence between the freezing point and the concentration of the water-soluble components of milk allows for the identification of adulteration of milk with water. Regardless of the technological methods (used for milk adulteration) the changes in the concentration of the water-soluble components of milk are observed in the following cases: milk adulteration with water /dilution of milk with water; adulteration of milk by adding neutralizing or inhibiting substances as a result of biochemical transformation of lactose.
The dependence between the freezing point of milk and the amount of added water is given in the table below:
- Freezing Point: 0,533°С - Amount of added water: 0%
- Freezing Point: 0,518°С - Amount of added water: 5,5%
- Freezing Point: 0,505°С - Amount of added water: 7,25%
- Freezing Point: 0,494°С - Amount of added water: 10,3%
- Freezing Point: 0,498°С - Amount of added water: 13,0%
The Freezing point decreases sharply with acidification of milk, when pH decreases from 6,6 to 6,0°C this value changes from minus 0,543 to minus 0,564 °C. When the acidity of milk is more than 24 ° T it is impossible to measure its freezing point, as the accumulation of lactic acid leads to the protein denaturation.
If the mass and composition of the water-soluble components of milk remain constant, it is possible to calculate the percentage of water added to the adulterated sample. In such cases the term "freezing point" is used by which is meant the absolute value of the freezing point multiplied by 100.
In connection with the introduction of the new requirements of the State Standard (GOST № 30562-97 «Natural raw cow milk. Technical conditions”) there is a need of the continuous quality control on the freezing point of milk and identification/ detection of milk adulteration based on the amount of added water.
Currently there are two standardized methods of determination of the freezing point of milk (GOST № 30562-97 «Milk. Determination of freezing point. Thermistor cryoscope method” and GOST № 25101-82 «Milk. Method of the freezing point determination”.
Metrological characteristics of the determination methods are given in the table below:
- Determination method: State Standard ( GOST 30562-97) (ISO 5764-87)
- Convergence of the results: 0,004
- Reproducibility of the results: 0,006
- The limit of permissible absolute measurement error: 0,012 °С
- Determination method: MilkoScanMinor
- Convergence of the results: 0,005
- Reproducibility of the results: 0,008
- The limit of permissible absolute measurement error: 0,020 °С
Thermistor cryoscope method (GOST № 30562-97 ISO 5764087) consists in the following: milk is cooled down to a predetermined temperature, crystallization of water present in the milk sample is stimulated by means of mechanical vibration and after that the temperature is rapidly increased to a plateau, that corresponds to the sample’s freezing point value. This method requires the use of the standard samples corresponding to the freezing point values of the solutions which slows down the measurement process considerably.
Another standardized method is based on the measurement of the decrease of the freezing point of milk in comparison with the freezing point of bidistilled water.
Most cryoscopic instruments works in the following way: the sample is placed in a cooling mixture and cooled down to a predefined temperature (5-7 °C below the freezing point). Subsequently the instant crystallization is stimulated by means of rapid and efficient mixing.
Currently determination of the freezing point of milk is a very urgent and pressing requirement for many milk processing plants and enterprises which has caused activation of the market of the instruments and equipment for the analysis of the given parameter of milk.
The following instruments were used for the comparative tests: milliosmometers (MT-5-02, MT-4) (produced by “Burevestnik”, St. Petersburg), MilkoScanMinor (Foss Electric, Denmark), AKM-98 and manual Beckman device (State Standard 25101-82). The testing was conducted using the same raw milk samples in the same test conditions. In this context comparative testing with the purpose of the milk freezing point measurement and milk adulteration detection were held using standardized instrumental methods and when appropriate equipment (both Russian and imported) based on different methods of measurement (ultrasonic and infrared milk analyzers).
The results of the comparative tests are given in the tables below:
Analyzing the above test results obtained on the instruments of various types, it is clear that more reliable data were obtained on the equipment, specialized for measuring of the freezing point of milk. Express analyzers intended for the measurement of physico-chemical parameters, including the freezing point, cannot cope with quite challenging measurements, as they are based on the mathematical calculation of the mass fraction/ mass content of lactose in milk which is not sufficient for conducting of the measurements of raw milk and identification of raw milk adulteration. MilkoSkanMinor has steadily given the same result/ value, which might be explained by the calibration of the instrument on European raw milk.
The analysis of the comparative tests results of the cryoscopic devices and ultrasonic analyzer AKM-98 ( an ultrasonic milk analyzer which uses the ultrasonic method for milk freezing point determination) shows that the difference of the obtained values is equal to 0,013 - 0,020 °C (0,005 °C corresponds to 1 % of added water) which is 2-3 times higher than the repeatability value which is equal to 0,006 °C according to the State Standard GOST 30562-97 (ISO 5764-87).
Based on the data received it can be concluded that ultrasonic devices and IR analyzers cannot be recommended for the measuring of the freezing point of milk.
Direct method of freezing point measurement (method of sample freezing) is used in Thermoscan Mini Cryoscope produced by Sibagropribor.
Thermoscan Mini Cryoscope produced by Sibagropribor is based on thermistor cryoscope method which is the reference/arbitration method of freezing point determination that allows to receive the most accurate and reliable measurements results of the freezing point of milk and based on this meas. parameter identify the possible adulterations of milk with water or dry substances.
Thermoscan Mini Cryoscope fully complies with the standards of Russian Federation (GOST № 30562-97 «Milk. Determination of freezing point. Thermistor crypscope method”).
The analyzer is included in the State Register of Russian Federation for Measurement Devices. An affordable price and convenience for users make this product irreplaceable for any Dairy laboratory.