This standard specifies the method for determining protein. This standard is applicable to the determination of protein in infant formula and milk powder. GB/T 5413.1-1997 Determination of protein in infant formula and milk powder GB/T5413.1-1997 Standard download decompression password: www.bzxz.net

GB/T5413.1—1997
This standard only modifies the text format of Chapter A.3 “Semi-micro Kjeldahl nitrogen determination method” in GB5413-85, and the content is not changed. This series of standards replaces GB5413-85 from the date of implementation. This standard is proposed by the China Light Industry General Association.
This standard is under the jurisdiction of the National Dairy Standardization Center. The responsible drafting unit of this standard is the National Dairy Quality Supervision and Inspection Center. The participating drafting units of this standard are: Food Hygiene Supervision and Inspection Institute of the Ministry of Health, Zhejiang Light Industry Research Institute, Harbin Morinaga Dairy Co., Ltd., and Nestlé (China) Investment Services Co., Ltd. The main drafters of this standard are Wang Yun, Huang Min, and Li Yuxian. 221
National Standard of the People's Republic of China
Infant formula foods and milk powder
Determination of protein
Milk powder and formula foods for infant and young children-Determination of protein
This standard specifies the method for the determination of protein. This standard applies to the determination of protein in infant formula and milk powder. 2 Method Summary
GB/T 5413. 1--1997
Replaces GB5413--85
When heated, sulfuric acid decomposes into sulfurous anhydride, water and oxygen. Organic matter is oxidized to carbon dioxide and water, while the ammonia nitrogen of protein reacts with excess sulfuric acid to convert into ammonium sulfate, which is distilled in an alkaline solution. The distilled ammonia is absorbed with boric acid and then titrated with an acid standard solution.
3 Reagents
All reagents, if the specifications are not specified, refer to analytical grade, and all experimental water, if no other requirements are specified, refer to grade tertiary water. 3.1 Concentrated sulfuric acid.
3.2 Potassium sulfate.
3.3 Copper sulfate.
3.4 ​​Hydrogen peroxide solution: volume fraction is 30%. 3.5 Boric acid solution: c(H,BO,) is 30g/L. Take 30g of boric acid and dissolve it in 1L of water. 3.6 Methyl red-bromocresol green mixed indicator: Use 95% ethanol by volume to prepare 1g/L ethanol solution of bromocresol green and methyl red. When using, mix them at a ratio of 5:1: 1g/L bromocresol green: 1g/L methyl red. 3.7 Sulfuric acid standard solution: c(H+) is 0.0500mol/L. Take 3mL of concentrated sulfuric acid and add it to 15mL of water. After cooling, wash it into a 1000mL volumetric flask and make up to volume.
3.8 Sodium hydroxide solution, mass ratio is 400/1000. Weigh 400g of sodium hydroxide, dissolve it in 1000mL of water, and transfer it into a reagent bottle after cooling.
4 Instruments
Common laboratory instruments and:
4.1 Kjeldahl flask: 500mL or 250nL.
4.2 Nitrogen determination steam distiller.
4.3 Burette: 25mL.
4.4 Erlenmeyer flask: 250mL.
Approved by the State Administration of Technical Supervision on May 28, 1997 222
Implemented on September 1, 1998
5 Operating stepsbzxz.net
5.1 Sample preparation
GB/T 5413.1-1997
Put all the samples into a clean and dry container about twice the volume of the sample, cover the container immediately, and rotate and shake repeatedly to mix the sample thoroughly.
5.2 Determination
5.2.1 Weigh 2g of sample, accurate to 0.2mg, and put it into a Kjeldahl flask (4.1), add 10g potassium sulfate (3.2) and 1g copper sulfate (3.3), measure 20mL concentrated sulfuric acid (3.1), slowly add it into the Kjeldahl flask, and mix. Note
1 When adding samples and reagents, avoid adhering to the bottleneck. 2 The role of adding potassium sulfate: increase the boiling point of sulfuric acid (338℃), and increase the reaction rate. 10g potassium sulfate will raise the boiling point to 400℃, but too much potassium sulfate will cause the boiling point to be too high, and the generated ammonium bisulfate will decompose at 513℃. 3 The role of adding copper sulfate: act as a catalyst to accelerate the oxidation. 5.2.2 Put a small funnel on the mouth of the Kjeldahl flask, heat it with a low fire (be careful that the foam in the bottle will rush out and affect the results). When the foaming in the bottle stops, slightly increase the heat. At the same time, add 10 mL of hydrogen peroxide solution (3.4) several times (but the flask must be cooled for several minutes before adding). When the color of the contents of the flask gradually turns into a transparent light green, continue digesting for 0.5 to 1 hour (if there are carbonized particles on the wall of the Kjeldahl flask, shake it or wait for the contents in the flask to cool for several minutes, then rinse it with hydrogen peroxide solution and continue digesting until it becomes transparent). Then remove it and let it cool. 5.2.3 Carefully transfer the clarified digestion solution to a 100 mL volumetric flask, wash the Kjeldahl flask three times with water, add the washing solution to the above volumetric flask, dilute to the scale after cooling and shake it.
5.2.4 Pipette 25 mL of digestion solution into the nitrogen determination distiller, place a 250 mL conical flask containing 50 mL of boric acid solution (3.5) and 3 drops of methyl red-cresol green mixed indicator (3.6) at the lower end of the condenser, and make the glass tube at the lower end of the condenser below the liquid surface. Slowly add 25mL of sodium hydroxide solution into the distillation flask (the solution should be strongly alkaline), quickly plug the stopper, and then pass steam to distill. When the liquid level reaches 150mL, take out the glass tube at the lower end of the condenser, rinse the lower end of the condenser with distilled water, and collect the washing liquid in the acid solution. Let the glass tube curtain on the wall of the conical flask, and the liquid outlet is above the 200mL scale line. Continue distillation until the liquid level reaches 200mL. Note: Pay attention to the distillation situation during distillation to prevent the liquid in the bottle from bubbling out and entering the receiving bottle. If the fire is too weak and the pressure in the distillation flask is reduced, the liquid in the receiving bottle will flow back, causing the experiment to fail.
5.2.5 Titrate with sulfuric acid standard solution (3.7) until the solution appears wine red, and record the volume of sulfuric acid standard solution used. At the same time, perform a blank test and correct it in the results. 6 Expression of analysis results
Protein content in sample (g/100g) = (V) × c(H+) × 2 × 0.014 × F, X 100........( 1 )
m×100
V—the volume of sulfuric acid standard solution consumed during titration, mL; where:
V. The volume of sulfuric acid standard solution consumed in the blank test, mL; c(H+)—-the concentration of H+ in sulfuric acid standard solution, mol/L; m—the mass of the sample, g;
0.014—the molar mass of nitrogen atoms, kg/mol; F
The coefficient for converting nitrogen into protein. The coefficient for milk powder is 6.38, the coefficient for pure cereal (formula) food is 5.90, and the coefficient for infant cereal (formula) food containing milk is 6.25.
Note: The blank test is performed without adding sample, and the operation steps are the same as those for the sample. 223
7 Allowable difference
GB/T5413.1—1997
The difference between two measured values ​​of the same sample shall not exceed 1.5% of the average value. 224
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