GB 5413.38—2010
1
GB 5413.38—2010
Preface
This standard is non-equivalent to the international standard ISO 5764/IDF 108-2002 Milk-Determination of freezing point - Thermistor cryoscope method (Reference method)).
this standard is issued for the first time.
National Food Safety Standard
Determination of freezing point of raw milk
1 Scope
This standard stipulates the detection method of freezing point of raw milk by using a thermistor cryoscope.
This standard is applicable to the determination of freezing point of raw milk.
2 Normative Documents Cited by this Standard
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
3 Terms and Definitions
Freezing Point of Raw Milk
Freezing point of raw milk is the value measured according to the method specified below and expressed in thousandth degrees Celsius (m℃).
4 Principle
A certain quantity of test portion of milk is put in the test tube and be supercooled to freezing point or below in a cold trap, and crystallization is induced by mechanical vibration at -3℃, with a continuous release of heat, which causes the temperature to rise to the highest temperature and the temperature may remain constant for a short time, i.e. arrive at a plateau. Commence the determination. The plateau gives the freezing point of test portion.
5 Reagents
All reagents are analytical grade or above unless otherwise stated.
All water is 1st grade specified in GB/T 6682.
5.1 Sodium Chloride (NaCl). --- Finely ground sodium chloride is dried for at least 24 hours at 130 ± 5 ℃ in the drying oven and cooled to room temperature in a desiccator.
5.2 Ethylene glycol (C2H6O2).
5.3 Calibration solution (Sodium chloride standard solutions).
Select two standard sodium chloride solutions with freezing point close to the expected value of testing milk. The difference in freezing points between the two solutions should not be less than 100 m℃ (see Table 1).
5.3.1 Solution A
Prepare solution A at 20℃～25℃: Accurately weigh 6.731g of NaCl (5.1) to nearest 0.1mg, dissolve in water, transfer to a 1000mL volumetric flask, make to 1000mL with water. The freezing point is - 0.400℃.
5.3.2 Solution B
Prepare solution B at 20℃: Accurately weigh 9.422g of NaCl (5.1) to nearest 0.1mg, dissolve in water, transfer quantitatively to a 1000m L volumetric flask, make to 1000mL with water. The freezing point is – 0.557℃.
5.4 Cooling liquid
Accurately pipette 330mL ethylene glycol (5.2) into a 1000mL volumetric flask, make to 1000mL with water. Shake well to get 33% [v/v] solution.
Table 1 Freezing point of sodium chloride standard solutions （20℃）
NaCl solution (g/L) / NaCl solution (g/kg) / Freezing point (m℃)6.731 / 6.763 / -400.0
6.868 / 6.901 / -408.0
7.587 / 7.625 / -450.0
8.444 / 8.489 / -500.0
8.615 / 8.662 / -510.0
8.650 / 8.697 / -512.0
8.787 / 8.835 / -520.0
8.959 / 9.008 / -530.0
9.130 / 9.181 / -540.0
9.302 / 9.354 / -550.0
9.422 / 9.475 / -557.0
10.161 / 10.220 / -600.0
6 Apparatus
6.1 An analytical balance, weighing to 0.0001g.
6.2 A thermistor cryoscope, with a thermostatically controlled cooling bath (cold trap), a thermistor probe, a stirrer & freezer (see Figure 1) and a temperature measuring and readout device.
6.2.1 Cooling bath, temperature sensor and associated circuit
Temperature sensor is a glass thermistor probe with diameter 1.60mm ± 0.4mm, and resistance is between 3Ω and 30kΩ at 0℃. When the probe is in measuring position in cryoscope, tip of glass bead should lie in axis of test sample tube and the distance from the tip to inner wall shall be the same as that from tip to bottom of the tube (see Figure 1).
Thermistor probe and associated circuit shall provide 1 m℃ or better measuring discrimination within range from -400 m℃ to -600 m℃.
Linearity of circuit such that no error greater than 1 m℃ is introduced at any point within range from -400 m℃ to -600 m℃ when instrument is correctly operated.
Figure 1 Detail of thermistor cryoscope
1. Mandrel 2. Sample tube 3. Stirring wire 4. Thermistor bead (thermistor probe)
6.2.2 Stirrer.---Wire of metal inert to milk, corrosion-resistant, to mix test portions in cooling procedure.
Adjust the amplitude of vibration of stirring wire in accordance with the position of probe. Wire must not strike thermistor bead or wall of tube at any time during normal stirring operation.
6.2.3 Freezer. --- A mechanical vibration set, which can initiate freezing of test portion when the latter is cooled to -3.0℃.
When initiating freezing, increase amplitude of vibration of stirring wire for 1-2 s so that stirring wire strikes wall of test sample tube.
6.3 Test sample tubes.---Borosilicate glass, 50.5 ± 0.2 mm long, 16.0 ± 0.2 mm od, 13.7± 0.3 mm id.
6.4 Weighing bottles.
6.5 Volumetric flasks.---1000mL.
6.6 A drying oven thermostatically controlled at 150 ± 5℃.
6.7 A desiccator.
6.8 Pipettes. ---1mL~5mL.
7. Procedure
7.1 Test portion preparation
Store the test sample at 0-6℃. It would be better to test sample at once when sample is arrived in the laboratory. Allow it to reach ambient temperature before starting determination. It is desirable that standard solutions and milk test portions are at same temperature when used.
7.2 Pre-cooling of apparatus
Switch on the thermistor cryoscope, uncover the cooling bath (cold trap) until thermistor bead rises. Add a certain volume of cooling liquid (5.4) in accordance with the manufacturer’s instructions. Cover the cold trap, and pre-cooling begins. 30 minutes later, commence the determination.
7.3 Instrument calibration checks
7.3.1 Check A
Respectively pipette 2.20mL calibration solution A (5.3.1) into 3 sample tubes. Start the freezer, insert the tubes. Calibration check is done only if the repeated measuring results are within -0.400 ± 0.020℃.
7.3.2 Check B
Respectively pipette 2.20mL calibration solution B (5.3.2) into 3 sample tubes. Start the freezer, insert the tubes. Calibration check is done only if the repeated measuring results are within -0.557 ± 0.020 ℃.
7.4 Determination
Transfer 2.20mL test portion into clean, dry sample tube. Insert sample tube into the measuring hole in the calibrated cryoscope. Current temperature of test portion shall be read from the temperature measurer, which shows a downward trend. When the sample is cooled to -3℃, freezing is initiated by mechanical vibration of stirrer. Temperature of milk after initiation of freezing will rise to a high value and remain constant for some time. At that time, measurement will stop and thermistor bead will rise up. Record this temperature, i.e. the freezing point of raw milk.
Ensure that probe and stirring wire are clean and dry after determination. If necessary, wipe with clean, soft gauze.
If freezing is initiated before -3.0℃, abandon the test and repeat with another test portion of the milk. When the measurement has been completed, remove the tube, rinse with water and dry the thermistor probe and stirring wire.
At least two parallel determinations should be carried out for each of the test sample. If the absolute error of freezing points is within 4 m℃, take the mean result as the final result.
8 Expression of results
If, following the routine calibration check, the calibration is confirmed, take as the result the mean of the two values obtained rounded to the third decimal place.
9 Precision
Repeatability
Difference between values obtained in duplicate determinations carried out in rapid succession by same analyst should not exceed 4 m℃.
10 Others
The limit of detection (LOD) of this standard is 2 m℃.