National Food Safety Standard
Determination of Acidity in Foods
食品安全国家标准 食品酸度的测定
1 Scope
This standard specifies the methods for determination of acidity in raw milk and milk products, starch and derived products and grain and produce.
In this standard, Method I is applicable to the determination of acidity in raw milk and milk products, starch and derived products, and grain and produce; Method II is applicable to the determination of acidity in milk powder; Method III is applicable to the determination of acidity in milk and other milk products.
Method I Phenolphthalein Indicator Method
2 Principle
After the specimen is treated, titrate it to neutral with 0.1000mol/L sodium hydroxide standard solution with phenolphthalein as indicator and then determine the acidity in specimen through calculation according to the volume of consumed sodium hydroxide solution.
3 Reagent 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.
3.1 Reagents
3.1.1 Sodium hydroxide(NaOH).
3.1.2 Cobalt sulfate hepathydrate (CoSO4·7H2O).
3.1.3 Phenolphthalein.
3.1.4 95% ethanol.
3.1.5 Ethyl ether.
3.1.6 Nitrogen: with the purity of 98%.
3.1.7 Trichloromethane (CHCl3).
3.2 Reagent preparation
3.2.1 Sodium hydroxide standard solution(0.1000mol/L)
Weigh 0.75g the working reference reagent, i.e. the potassium hydrogen phthalate, dried in 105~110℃ electric dry oven, dissolve it with 50mL carbon dioxide-free water, add two drops of phenolphthalein indicating liquid (10g/L), and titrate the solution to turn pink with the well-prepared sodium hydroxide solution and maintain for 30s. Meanwhile, conduct the blank test.
Note: the carbon dioxide (CO2) is limited in washing bottle or drying tube so that its concentration won’t be affected due to the absorption of CO2by NaOH in the dropper. An enclosed system may be formed via the burette containing sodium hydroxide solution connected with the washing bottle containing 10% sodium hydroxide solution, or by connecting the end of burette containing fresh sodium hydroxide or calcium oxide to avoid such solution from absorbing carbon dioxide (CO2).
3.2.2 Reference solution
Dissolve 3g cobalt sulfate hepathydrate in water and scale the volume to 100mL.
3.2.3 Phenolphthalein indicating liquid
Weigh 0.5g phenolphthalein and dissolve it into 75mL ethanol with volume fraction of 95%, add 20mL water, and add sodium hydroxide volumetric solution (3.2.1) by drops until the solution appears reddish and then scale the volume with water to 100mL.
3.2.4 Neutral ethanol - ethyl ether solution mixture
Take the same volume of ethanol and ethyl ether, mix them mixture, add three drops of phenolphthalein indicating liquid and titrate the mixed solution with sodium hydroxide solution (0.1mol/L) to turn reddish.
3.2.5 Carbon dioxide-free distilled water
Boil the water for 15min to expel carbon dioxide, cool and close it.
4 Instruments and Apparatus
4.1 Analytical balance: with sensibility of 0.001g.
4.2 Alkali burette: with volume of 10mL and minimum scale of 0.05mL.
4.3 Alkali burette: with volume of 25mL and minimum scale of 0.1mL.
4.4 Water bath.
4.5 Conical flask: 100mL, 150mL and 250mL.
4.6 Ground conical flask with stopper: 250mL.
4.7 Crusher: enable more than 95% of the crushed sample to pass through CQ16 sieve [hole diameter: 0.425mm (40-mesh)]; the inside discharge hopper shall be free from heat during sample crushing.
4.8 Oscillator: reciprocating type, with oscillation frequency of 100 times/min.
4.9 Medium-speed qualitative filter paper.
4.10 Pipette :10mL and 20mL.
4.11 Measuring cylinder:50mL and 250mL.
4.12 Glass funnel and funnel stand
5 Analysis Procedures
5.1 Milk powder
5.1.1 Specimen preparation
Transfer all samples to a clean and dry container (with seal cover) with about two times the volume of the samples, tighten the cover immediately, rotate and shake the container repeatedly to make the samples thoroughly mixed. The samples shall avoid the exposure to air as possible in this operation process.
5.1.2 Determination
Weigh 4g sample (accurate to 0.01g) in a 250mL conical flask. Measure 96mL water at about 20℃ (3.2.5) with measuring cylinder to re-dissolve the sample, mix and keep it still for 20min.
Add 2.0mL reference solution in a conical flask containing 96mL water at about 20℃ and rotate gently to make it mix, thus obtaining standard reference color. If many similar products are determined, then such reference solution may be used for the whole determination process, however, the time shall not exceed 2h.
Add 2.0mL phenolphthalein indicating liquid in another conical flask containing sample solution, rotate gently to make it mix. Add sodium hydroxide solution by drops in the conical flask with 25mL alkali burette, rotate the flask while dripping till the color is similar to that of reference solution and won't fade away in 5s; the whole titration process shall be completed in 45s. In the process of titration, blow nitrogen into the conical flask to prevent the solution from absorbing the carbon dioxide in the air. Record the volume of the consumed sodium hydroxide solution (V1), accurate to 0.05mL and calculate by substituting in Formula (1).
5.1.3 Blank titration
Conduct blank test with 96mL water and read the volume (V0) of the consumed sodium hydroxide standard solution. The volume of sodium hydroxide consumed for the blank test shall not be less than zero, otherwise the satisfactory distilled water shall be prepared again and used.
5.2 Milk and other milk products
5.2.1 Reference solution preparation
Add 2.0mL reference solution in a conical flask containing the same volume of corresponding solution and rotate gently to make it mix, thus obtaining standard reference color. If many similar products are determined, then such reference solution may be used for the whole determination process, however, the time shall not exceed 2h.
5.2.2 Pasteurized milk, sterilized milk, raw milk and fermented milk
Weigh 10g (accurate to 0.001g) well-mixed specimen, put into a 150mL conical flask, add 20mL newly-boiled water cooled to ambient temperature, mix uniformly, then add 2.0mL phenolphthalein indicating liquid, titrate with sodium hydroxide solution after mixing, rotate the flask while dripping till the color is similar to that of reference solution and won't fade away in 5s; the whole titration process shall be completed within 45s. In the process of titration, blow nitrogen into the conical flask to prevent the solution from absorbing the carbon dioxide in the air. Record the volume of the consumed standard sodium hydroxide volumetric solution (V2) and calculate by substituting in Formula (2).
5.2.3 Cream
Weigh 10g (accurate to 0.001g) well-mixed specimen, put into a 250mL conical flask, add 30mL neutral ethanol - ethyl ether mixture, mix uniformly, then add 2.0mL phenolphthalein indicating liquid, titrate with sodium hydroxide solution after mixing, rotate the flask while dripping till the color is similar to that of reference solution and won't fade away in 5s; the whole titration process shall be completed within 45s. In the process of titration, blow nitrogen into the conical flask to prevent the solution from absorbing the carbon dioxide in the air. Record the volume of the consumed standard sodium hydroxide volumetric solution (V2) and calculate by substituting in Formula (2).
5.2.4 Condensed milk
Weigh 10g (accurate to 0.001g) well-mixed specimen, put into a 250mL conical flask, dissolve with 60mL newly-boiled water cooled to ambient temperature, mix uniformly, then add 2.0mL phenolphthalein indicating liquid, titrate with sodium hydroxide solution after mixing, rotate the flask while dripping till the color is similar to that of reference solution and won't fade away in 5s; the whole titration process shall be completed within 45s. In the process of titration, blow nitrogen into the conical flask to prevent the solution from absorbing the carbon dioxide in the air. Record the volume of the consumed standard sodium hydroxide volumetric solution (V2) and calculate by substituting in Formula (2).
5.2.5 Casein
Weigh 5g (accurate to 0.001g) ground and well-mixed specimen and put it in a conical flask, add 50mL water (3.2.5), keep for 4~5h at ambient temperature (18~20℃), or heat in water bath to 45℃ and maintain at this temperature for 30min, then add 50mL water (3.2.5), mix uniformly and filter with dry filter paper. Pipet 50mL filtrate and put it into a 50mL conical flask, add 2.0mL phenolphthalein indicating liquid and then titrate with sodium hydroxide solution after mixing uniformly, rotate the flask while dripping till the color is similar to that of reference solution and won't fade away in 5s; the whole titration process shall be completed within 45s. In the process of titration, blow nitrogen into the conical flask to prevent the solution from absorbing the carbon dioxide in the air. Record the volume of the consumed standard sodium hydroxide volumetric solution (V3) and calculate by substituting in Formula (3).
5.2.6 Blank titration
Conduct the blank test with the same volume of water (3.2.5) and read the volume (V0) of the consumed sodium hydroxide standard solution (applicable to 5.2.2, 5.2.4 and 5.2.5). Conduct the blank test with 30mL neutral ethanol - ethyl ether mixture and read the volume (V0) of the consumed sodium hydroxide standard solution (applicable to 5.2.3).
The volume of the sodium hydroxide consumed for the blank test shall not be less than zero, otherwise satisfactory distilled water or neutral ethanol - ethyl ether mixture shall be re-prepared and used.
5.3 Starch and derived products
5.3.1 Sample pretreatment
The sample shall be mixed uniformly.
5.3.2 Sample weighing
Weigh 10g sample (accurate to 0.1g), transfer it into a 250mL-conical flask, add 100mL water, shake and mix uniformly.
5.3.3 Titration
Add 2.0mL reference solution in a conical flask containing 100mL water of about 20℃ and rotate gently to make it mix, thus obtaining standard reference color. If many similar products are determined, then such reference solution may be used for the whole determination process, however, the time shall not exceed 2h. Add 2~3 drops of phenolphthalein indicator in the conical flask containing sample and then titrate with sodium hydroxide solution after mixing uniformly, rotate the flask while dripping till the color is similar to that of reference solution and won't fade away in 5s; the whole titration process shall be completed within 45s. In the process of titration, blow nitrogen into the conical flask to prevent the solution from absorbing the carbon dioxide in the air. Record the volume of the consumed standard sodium hydroxide volumetric solution (V4) and calculate by substituting in Formula (4).
5.3.4 Blank titration
Conduct the blank test with 100mL water (3.2.5) and read the volume of the consumed sodium hydroxide standard solution (V0).
The volume of the sodium hydroxide consumed for the blank test shall not be less than zero, otherwise satisfactory distilled water shall be re-prepared and used.
Foreword I
1 Scope
2 Principle
3 Reagent and Materials
4 Instruments and Apparatus
5 Analysis Procedures
6 Expression of Analysis Result
7 Precision
8 Principle
9 Reagents and Materials
10 Instruments and Apparatus
11 Analysis Steps
12 Expression of Analysis Result
13 Precision
14 Principle
15 Reagents and Materials
16 Instruments and Apparatus
17 Analysis Steps
18 Expression of Analysis Result
19 Precision
