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This standard replaces the pH determination specified in GB/T 5009.45-2003 Method for Analysis of Hygienic Standard of Fish and Other Aquatic Products and GB/T 10786-2006 Analytical Methods of Canned Food as well as GB/T 9695.5-2008 Meat and Meat Products - Measurement of pH.
The following main changes have been made with respect to the replaced standards:
——This standard name is revised as "National Food Safety Standard - Determination of Foods pH Value";
——This standard integrates the determination methods of food pH specified in GB/T 5009.45-2003 Method for Analysis of Hygienic Standard of Fish and Other Aquatic Products, GB/T10786-2006 Analytical Methods of Canned Food and GB/T 9695.5-2008 Meat and Meat Products - Measurement of pH.
National Food Safety Standard - Determination of pH Value in Foods
1 Scope
This standard specifies the determination method of pH in meat and meat products, aquatic products including ostracean (oyster and ostrica) and canned food.
It is applicable to pH testing of homogenization product in meat and meat products, pH nondestructive testing of carcass, dressed carcass and lean meat, pH determination of aquatic products including ostracean (oyster, ostrica) and the pH determination of canned food.
2 Principle
Glass electrode is used as indicator electrode and calomel electrode or silver-silver chloride electrode is used as reference electrode; if the hydrogen ion concentration in specimen or specimen solution is changed, the electromotive force between indicator electrode and reference electrode will be changed and DC potential (namely potential difference) will be generated; the DC potential will be input to A/D converter via pre-amplifier and the pH may be measured.
3 Reagents and Materials
Unless otherwise specified, analytically-pure reagents and Class-III water (defined in GB/T 6682) are adopted for the purpose of this method. Water used for preparing buffer solution shall be newly-boiled, or CO2-free (the carbon dioxide is eliminated with nitrogen free from carbon dioxide).
3.1 Reagent
3.1.1 Potassium hydrogen phthalate [KHC6H4(COO)2].
3.1.2 Potassium dihydrogen phosphate (KH2PO4).
3.1.3 Disodium hydrogen phosphate (Na2HPO4).
3.1.4 Potassium bitartrate (KHC4H4O6).
3.1.5 Diammonium hydrogen citrate (Na2HC6H5O7).
3.1.6 Citric acid monohydrate (C5H8O7·H2O).
3.1.7 Sodium hydroxide (NaOH).
3.1.8 Potassium chloride (KCl).
3.1.9 Iodoacetic acid (C2H3IO2).
3.1.10 Ethyl ether (C4H10O).
3.1.11 Ethanol (C2H6O).
3.2 Reagent preparation
3.2.1 Buffer solution with pH of 3.57 (20℃): saturated water solution prepared with potassium bitartrate at 25℃ and the pH of this solution is 3.56 at 25℃ and 3.55 at 30℃. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
3.2.2 Buffer solution with pH of 4.00 (20℃): dry potassium hydrogen phthalate to constant weight at 110℃~130℃ and cool to room temperature in dryer. Add 10.211g (to the nearest 0.001g) of potassium hydrogen phthalate into 800ml water, dissolve it, and then scale the volume with water to 1 000mL. The pH of this solution is 4.00 at 0℃~10℃ and 4.01 at 30℃. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
3.2.3 Buffer solution with pH of 5.00 (20℃): prepare disodium monohydrogen citrate to 0.1mol/L solution. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
3.2.4 Buffer solution with pH of 5.45 (20℃): add 7.010g (to the nearest 0.001g) of citric acid monohydrate into 500mL of water, dissolve it and then add into 375mL of 1.0mol/L sodium hydroxide solution (3.2.6) and scale the volume with water to 1 000mL. The pH of this solution is 5.42 at 10℃ and 5.48 at 30℃. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
3.2.5 Buffer solution with pH of 6.88 (20℃): dry the anhydrous potassium dihydrogen phosphate and anhydrous sodium hydrogen phosphate to constant weight at 110℃~130℃ and cool to room temperature in dryer. Weigh 3.402g and 3.549g of above-mentioned potassium dihydrogen phosphate and disodium hydrogen phosphate accordingly, to the nearest 0.001g, then dissolve them in water and scale the volume with water to 1 000mL. The pH of this solution is 6.98 at 0℃, 6.92 at 10℃ and 6.85 at 30℃. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
Generally, the above-mentioned buffer solutions may be preserved for 2~3 months, but if such phenomena as turbidity, mould or sedimentation occur, the buffer solutions shall not be used.
3.2.6 Sodium hydroxide solution (1.0mol/L): dissolve 40g of sodium hydroxide in water and dilute with water to 1 000mL. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
3.2.7 Potassium chloride solution (0.1mol/L): put 7.5g of potassium chloride into a 1000-mL volumetric flask, dissolve it with water and then dilute with water to the scale(if the to-be-tested specimen is in state of prior to stiffness, add 925mg/L iodoacetic acid solution whose pH has been adjusted to 7.0 with sodium hydroxide solution) to prevent glycolysis. Or standard solution approved and awarded with reference material certificate by the nation shall be adopted.
4 Apparatuses
4.1 Mechanical equipment: used for the homogenization of the specimen and including high-speed rotating cutting machine or mincer with pore size of its perforated plate not greater than 4mm.
4.2 pH meter: with accuracy of 0.01. The instrument shall be provided with temperature compensation system; if not, it shall be used below 20℃ and be able to prevent the influence of induced current from outside.
4.3 Composite electrode: assembled with glass indicator electrode and Ag/AgCl or Hg/Hg2Cl2 reference electrode.
4.4 Homogenizer: the rotating speed may up to 20 000r/min.
4.5 Magnetic stirrer
5 Analysis Procedures
5.1 Specimen preparation
5.1.1 Meat and meat products
5.1.1.1 Sampling
Sampling methods are detailed in GB/T 9695.19.
The received sample in the laboratory shall be representative and not be damaged or changed during transportation and storage; representative sample is taken and it is dissolved with 1~2 different water gradient(s) based on the actual situation.
5.1.1.2 Non-homogenization specimen
Representative pH testing point is selected in the specimen and then it is continuously operated according to 5.2.
5.1.1.3 Homogenization specimen
Keep the specimen homogeneous with mechanical equipment (4.1). Pay attention that the temperature of specimen shall not exceed 25℃. If mincer is used, the specimen shall pass it for at least two times, put the specimen into sealed vessel to prevent metamorphism and compositional variation. Specimen shall be analyzed as soon as possible and not greater than 24h after homogenization.
5.1.2 Ostracean (oyster and ostrica) in aquatic products
Weigh 10g of specimen (to the nearest 0.01g), then add cooled water which is newly-boiled to 100mL and shake well, filter or centrifuge it when after immersing for 30min, then take about 50mL of filtrating in a 100-mL beaker.
5.1.3 Canned food
5.1.3.1 Mix the liquid products uniformly for standby; as for products with separate solid phase and liquid phase, the well-mixed liquid phase shall be taken for standby.
5.1.3.2 As for dense or semi-dense products and products from which it is difficult to separate juice [e.g. syrup, jam, fruit (vegetable) pulp, jelly, etc.], grind a part of the sample in a mixing machine or mortar; if the sample obtained is still too dense, add equivalent amount of newly-boiled water and mix uniformly for standby.
Foreword i
1 Scope
2 Principle
3 Reagents and Materials
4 Apparatuses
5 Analysis Procedures
6 Expression of Analysis Results
7 Precision
