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This standard replaces GB 5413.38-2010 National Food Safety Standard - Determination of Freezing Point in Raw Milk.
The following technical changes have been made with respect to GB 5413.38-2010:
——Theory is modified;
——"Reagents and Materials" is modified;
——"Standard solution C" is added in sodium chloride standard solution in "Reagents and Materials";
——"Calibration C" and "quality control calibration" are added in "apparatus calibration" in "Analytical Procedure".
National Food Safety Standard
Determination of Freezing Point in Raw Milk
1 Scope
This standard specifies the determination of freezing point in raw milk with thermistor cryoscope.
This standard is applicable to the determination of freezing point in raw milk.
2 Theory
Raw milk sample is cooled to moderate temperature; when the tested milk sample is cooled to -3℃, by releasing heat instantaneously, the sample crystallizes, then after the sample temperature achieves a balance and the temperature rising does not exceed 0.5℃ within 20s, the temperature is the freezing point of sample.
3 Reagents and Materials
Unless otherwise specified, reagents of analytically-pure or above and Grade II water (defined in GB/T 6682) are adopted for the purposes of this method.
3.1 Reagents
3.1.1 Ethylene glycol (C2H6O2).
3.1.2 Sodium chloride (NaCl).
3.2 Reagent preparation
3.2.1 Sodium chloride (NaCl): after being grinded, sodium chloride is placed in drying oven and dried at 130℃±2℃ for more than 24h, and then cooled to room temperature in dryer.
3.2.2 Coolant: measure 330mL of ethylene glycol (3.1.1), put it into a 1,000mL volumetric flask, then bring the volume with water to the scale and shake well; the volume fraction of the solution is 33%.
3.3 Sodium chloride standard solution
3.3.1 Standard solution A: weigh 6.731g of sodium chloride (3.2.1), dissolve it into 1,000g±0.1g of water. Store standard solution in polyethylene plastic bottles of 250mL or less separately and put the bottles in about 5℃ refrigerator for cold storage for a storage life of two months. The freezing point of the solution is -400m℃.
3.3.2 Standard solution B: weigh 9.422g of sodium chloride (3.2.1), dissolve it into 1,000g±0.1g of water. Store standard solution in polyethylene plastic bottles of 250mL or less separately and put the bottles in about 5℃ refrigerator for cold storage for a storage life of two months. The freezing point of the solution is -557m℃.
3.3.3 Standard solution C: weigh 10.161g of sodium chloride (3.2.1), dissolve it into 1,000g±0.1g of water. Store standard solution in polyethylene plastic bottles of 250mL or less separately and put the bottles in about 5℃ refrigerator for cold storage for a storage life of two months. The freezing point of the solution is -600m℃.
4 Apparatuses
4.1 Analytical balance: with a sensitivity of 0.000 1g.
4.2 Thermistor cryoscope: detection device, cooling device, metal stirring rod, crystallization device (see Figure 1) and temperature demonstrator.
a) Detection device and cooling device
Temperature sensor is glass probe in diameter of 1.60mm±0.4mm and the resistance is between 3Ω and 30kΩ at 0℃. The material and diameter of sensor's rotating shaft shall ensure that the heat transfers to sample within 2.5×10-3J/s. When the probe is at the measuring position, the top of thermistor shall be placed at the central axis of sample tube and shall have equal distance to inner wall and to tube bottom (see Figure 1). The measurement resolution of temperature sensor and corresponding electronic circuit is 1m℃ at -600m℃~400m℃. Cooling device shall keep the temperature of coolant at -7℃±0.5℃ invariably.
In: mm
Key:
1——Push rod;
2——Sample tube;
3——Metal stirring rod;
4——Thermistor probe.
Figure 1 Detection Device of Thermistor Cryoscope
When the apparatus works normally, the linear error of the circulation system at any point within -600m℃ and -400m℃ shall not exceed 1m℃.
b) Metal stirring rod
Corrosion resistant, used for stirring sample in the cooling process.
The vibration amplitude of metal stirring rod shall be adjusted according to the installation position of corresponding apparatus. During normal stirring, the metal rod shall not impact the glass sensor or sample tube wall.
c) Crystallization device
When the test sample reaches -3.0℃, start the mechanical vibration device and increase the vibration amplitude of metal stirring rod within 1s~2s at the time of crystallization to make the rod impact sample tube wall.
4.3 Drying oven: its temperature is controlled at 130℃±2℃.
4.4 Sample tube: borosilicate glass, 50.5mm±0.2mm in length, 16.0mm±0.2mm in external diameter and 13.7mm±0.3mm in internal diameter.
4.5 Weighing bottle.
4.6 Volumetric flask: 1,000mL, complying with the requirements of Grade A specified in GB/T 12806-2011.
4.7 Dryer: with silica gel hygrometer inside.
4.8 Pipette: 1mL~5mL.
4.9 Polyethylene bottle: with capacity of 250mL or less.
Foreword i
1 Scope
2 Theory
3 Reagents and Materials
4 Apparatuses
5 Analytical Procedures
6 Expression of Analytical Result
7 Accuracy
8 Others
