Translated English of Chinese Standard: GB5009.5-2016 www.chinesestandard.net Sales@ChineseStandard.net GB NATIONAL STANDARD OF THE PEOPLE S REPUBLIC OF CHINA GB 5009.5-2016 National food safety standard Determination of protein in foods Issued on: December 23, 2016 Implemented on: June 23, 2017 Issued by: National Health and Family Planning Commission of the PRC; State Food and Drug Administration. www.chinesestandard.net Page 1 of 15
Table of contents Foreword... 3 1 Scope... 4 2 Principles... 4 3 Reagents and materials... 4 4 Instruments and equipment... 5 5 Analytical procedures... 6 6 Analysis results expression... 8 7 Precision... 9 8 Principles... 9 9 Reagents and materials... 9 10 Instruments and equipment... 10 11 Analytical procedures... 11 12 Analysis results expression... 12 13 Precision... 13 14 Principle... 13 15 Instruments and equipment... 13 16 Analytical procedures... 13 17 Analysis results expression... 14 18 Precision... 14 19 Others... 14 Appendix A Conversion factor of nitrogen in common foods into protein... 15 www.chinesestandard.net Page 2 of 15
National Food Safety Standard Determination of protein in foods 1 Scope This standard specifies the determination of protein in foods. The method I and the method II of this standard are applicable to the determination of protein in various foods. The method III is applicable to the determination of grain, milk powder, rice flour, protein powder and other solid sample containing protein of more than 10 g/100 g. This standard does not apply to the determination of foods containing inorganic nitrogenous substances, organic non-protein nitrogenous substances. Method I: Kjeldahl method 2 Principles The protein in the food is decomposed under catalytic heating conditions, AND the resulting ammonia is combined with sulfuric acid to form ammonium sulfate. Alkalization distillation is used to make the ammonia free, which is absorbed with boric acid AND then titrated by sulfuric acid or hydrochloric acid standard titration solution; in accordance with the consumption of acid to calculate the nitrogen content, which is multiplied by the conversion factor to obtain the protein content. 3 Reagents and materials 3.1 Reagent Unless otherwise stated, the reagents used in this method are of analytical pure AND the water is level III water as specified in GB/T 6682. 3.1.1 Copper sulfate (CuSO4 5H2O). 3.1.2 Potassium Sulfate (K2SO4). 3.1.3 Sulfuric acid (H2SO4). 3.1.4 Boric acid (H3BO3). www.chinesestandard.net Page 4 of 15
when all the contents are carbonized AND the foaming is completely stopped, STRENGTHEN the fire and KEEP the liquid in the bottle slightly boiling; after the liquid appears blue-green and clear and transparent, CONTINUE heating for 0.5 h ~ 1 h. TAKE it off and LET it cool down; carefully ADD 20 ml of water; after letting it cool down, MOVE it into a 100 ml volumetric flask; USE a small amount of water to wash the nitrogen bottle; INCLUDE the washing liquid into the volumetric flask; then ADD water to the mark; MIX it uniformly and PREPARE for use. Meanwhile, MAKE the reagent blank test. 5.1.2 Determination: INSTALL the nitrogen distillation device as shown in Figure 1; ADD water into the water vapor generator to 2/3; ADD a few glass beads; ADD a few drops of methyl red ethanol solution and a few milliliters of sulfuric acid, to keep the water acidic; HEAT to boil the water in the water vapor generator and MAINTAIN it boiling. 5.1.3 ADD 10.0 ml of boric acid solution and 1 drop ~ 2 drops of A mixed indicator or B mixed indicator to the collection bottle; INSERT the lower end of the condensate pipe in below the liquid level; based on the nitrogen content in the sample, accurately ABSORB 2.0 ml ~ 10.0 ml of sample treatment solution from the small glass cup and INJECT it into the reaction chamber; USE 10 ml of water to wash the small glass cup and make it flow into the reaction chamber; PLUG the rod-shaped glass plug. POUR the 10.0 ml sodium hydroxide solution into the small glass cup; RAISE the glass plug to let it flow slowly into the reaction chamber; immediately COVER tightly the plug; USE water sealing. FIX the screw clamp; START distillation. After distillation for 10 min, MOVE the distillate collection bottle, to make the liquid level be away from the lower end of the condensate pipe; DISTILL it again for 1 min. Then USE a small amount of water to rinse the lower end outside of the condensate pipe; TAKE off the distillate collection bottle. USE the sulfuric acid or hydrochloric acid standard titration solution to titrate it to the end point as soon as possible; if using the A mixed indicator solution, the end point color is grey blue; if using the B mixed indicator solution, the end point color is light gray red. Meanwhile, MAKE the reagent blank test. 5.2 Automatic Kjeldahl method WEIGH 0.2 g ~ 2 g of fully mixed solid sample, 2 g ~ 5 g of semi-solid sample or 10 g ~ 25 g of liquid sample (equivalent to about 30 mg ~ 40 mg nitrogen), accurate to 0.001 g; PLACE it into the digestion pipe; then ADD 0.4 g of copper sulfate, 6 g of potassium sulfate and 20 ml of sulfuric acid into the digestion furnace for digestion. When the furnace temperature reaches to 420 C, CONTINUE digestion for 1 h, at that time the liquid in the digestion pipe is in green transparent state; TAKE it out and LET it cool down; ADD 50 ml of water; in the automatic Kjeldahl nitrogen instrument (before use, ADD sodium www.chinesestandard.net Page 7 of 15
7 Precision The absolute difference between the two independent determinations obtained under repeated conditions shall not exceed 10% of the arithmetic mean. Method II: spectrophotometry 8 Principles The protein in the food is decomposed under the condition of catalytic heating. The ammonia produced by the decomposition is combined with sulfuric acid to form ammonium sulfate. The reaction occurs with acetylacetone and formaldehyde in the sodium acetate-acetic acid buffer solution at ph 4.8 to form yellow 3, 5-diacetyl-2, 6-dimethyl-1, 4-dihydropyridine compounds. The absorbance value is measured at a wavelength of 400 nm AND compared with the standard series for quantification. The result is multiplied by the conversion factor, which is the protein content. 9 Reagents and materials 9.1 Reagents Unless otherwise stated, the reagents used in this method are of analytical pure AND the water is level III water as specified in GB/T 6682. 9.1.1 Copper sulfate (CuSO4 5H2O). 9.1.2 Potassium sulfate (K2SO4). 9.1.3 Sulfuric acid (H2SO4): excellent grade pure. 9.1.4 Sodium hydroxide (NaOH). 9.1.5 p-nitrophenol (C6H5NO3). 9.1.6 Sodium acetate (CH3COONa 3H2O). 9.1.7 Anhydrous sodium acetate (CH3COONa). 9.1.8 Acetic acid (CH3COOH): excellent grade pure. 9.1.9 37% formaldehyde (HCHO). 9.1.10 Acetylacetone (C5H8O2). www.chinesestandard.net Page 9 of 15
11 Analytical procedures 11.1 Sample digestion WEIGH 0.1 g ~ 0.5 g of fully mixed solid sample (accurate to 0.001 g), 0.2 g ~ 1 g of semi-solid sample (accurate to 0.001 g), or 1 g ~ 5 g of liquid sample (accurate to 0.001 g); TRANSFER it into a 100 ml or 250 ml dry nitrogen bottle; ADD 0.1 g of copper sulfate, 1 g of potassium sulfate and 5 ml of sulfuric acid; after shaking it uniformly, PLACE a small funnel at the bottle mouth; SUPPORT the bottle at an angle of 45 C (translator note: should be 45 ) onto the asbestos gauze with small holes. HEAT it carefully; when all the contents are carbonized AND the foaming is completely stopped, STRENGTHEN the fire and KEEP the liquid in the bottle slightly boiling; after the liquid appears blue-green and clear and transparent, CONTINUE heating for 0.5 h. TAKE it off and LET it cool down; carefully ADD 20 ml of water; after letting it cool down, MOVE it into a 50 ml or 100 ml volumetric flask; USE a small amount of water to wash the nitrogen bottle; INCLUDE the washing liquid into the volumetric flask; then ADD water to the mark; MIX it uniformly and PREPARE for use. Meanwhile, MAKE the reagent blank test using the same method. 11.2 Preparation of sample solution PIPETTE 2.00 ml ~ 5.00 ml of sample or reagent blank digestion into a 50 ml or 100 ml volumetric flask; ADD 1 drop ~ 2 drops of p-nitrophenol indicator solution; after shaking it uniformly, ADD sodium hydroxide solution to neutralize it to yellow; then ADD acetic acid solution until the solution becomes colorless; USE water to dilute it to the mark; MIX it uniformly. 11.3 Drawing of standard curve PIPETTE 0.00 ml, 0.05 ml, 0.10 ml, 0.20 ml, 0.40 ml, 0.60 ml, 0.80 ml and 1.00 ml of ammonia nitrogen standard use solution (equivalent to 0.00 μg, 5.00 μg, 10.0 μg, 20.0 μg, 40.0 μg, 60.0 μg, 80.0 μg and 100.0 μg nitrogen) respectively into 10 ml colorimetric tube. ADD 4.0 ml of sodium acetate-acetic acid buffer solution and 4.0 ml color reagent; ADD water to dilute to the mark; MIX it uniformly. PLACE it in the 100 C water bath to heat if for 15 min. TAKE it out and USE water to cool it to room temperature; TRANSFER it into a 1 cm cuvette; USE the zero tube as reference; MEASURE the absorbance value at the wavelength of 400 nm; based on the standard absorbance value of each point, DRAW the standard curve or CALCULATE the linear regression equation. 11.4 Sample determination www.chinesestandard.net Page 11 of 15
When the protein content is 1 g/100 g, the result is retained with three significant figures; if the protein content is < 1 g/100 g, the result is retained with two significant figures. 13 Precision The absolute difference between the two independent determinations obtained under repeatability shall not exceed 10% of the arithmetic mean. Method III: Combustion method 14 Principle The sample is burnt at a high temperature of 900 C ~ 1200 C. During the combustion process, the mixed gas is generated, AND the carbon and sulfur and other interfering gases and salts are absorbed by the absorption tube, whilst the nitrogen oxides are all reduced to nitrogen, forming the nitrogen gas stream which flows through the thermal conductivity detector (TCD) for detection. 15 Instruments and equipment 15.1 Nitrogen/protein analyzer. 15.2 Balance: sensitivity of 0.1 mg. 16 Analytical procedures As required by the instrument instruction, WEIGH 0.1 g ~ 1.0 g of the fully-mixed sample (accurate to 0.001 g); USE foil to wrap it and PLACE it on the sample tray. After the sample is feeding into the combustion reaction furnace (900 C ~ 1200 C), it is completely burnt in the high purity oxygen ( 99.9%). The product (NOx) in the combustion furnace is transported by carrier carbon dioxide or helium gas to the reduction furnace (800 C), where it is reduced to nitrogen; AND its content is detected. www.chinesestandard.net Page 13 of 15