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"GB5009.
5-2010 National Food Safety Standard Determination of Protein in Foods" stipulates three methods: Kjeldahl method, spectrophotometric method and combustion method
.
This training uses the Kjeldahl method (according to the first method in GB5009.
1.
Purpose requirements
(1) Understand the concept of protein and protein coefficient
.
(2) Master the basic principles and calculation methods of Kjeldahl method, and understand the role of copper sulfate and potassium sulfate in this method
.
(3) Be proficient in connecting and using equipment such as nitrogen determination device, and master the operation essentials of Kjeldahl method
.
(4) Will use Kjeldahl method to determine protein content in food
.
2.
Principles of training
The sample is heated and digested with concentrated sulfuric acid and catalyst to decompose the protein, and the generated ammonia is combined with sulfuric acid to form ammonium sulfate
.
Alkali is added to alkalize the digestion solution to free the ammonia, and then the ammonia is distilled out by steam distillation, which is absorbed by boric acid to form ammonium borate
3.
Training supplies
(1) Balance: Sensitivity is 1mg
.
(2) Fixed nitrogen distillation device: as shown in Figure 3-6
.
(3) Automatic Kjeldahl nitrogen analyzer
.
(4) Copper sulfate (CuSO 4 ·5H 2 O)
.
(5) Potassium sulfate (K 2 SO 4 )
.
(6) Sulfuric acid (H 2 SO 4 density is 1.
84 g/L)
.
(7) Boric acid (H 3 BO 3 )
.
(8) Methyl red indicator (C 15 H 15 N 3 O 2 )
.
(9) Bromocresol green indicator (C 21 H 14 Br 4 O 5 S)
.
(10) Methylene blue indicator (C 16 H 18 CIN 3 S·3H 2 O)
.
Figure 3-6 Kjeldahl nitrogen determination digestion device diagram
(11) Sodium hydroxide (NaOH)
.
(12) 95% ethanol (C 2 H 5 OH)
.
(13) Boric acid solution (20g/L): Weigh 20g of boric acid, add water to dissolve and dilute to 1000mL
.
(14) Sodium hydroxide solution (400g/L): Weigh 40g of sodium hydroxide and add water to dissolve it, let it cool, and dilute to 100mL
.
(15) Sulfuric acid standard titration solution (0.
0500mol/L) or hydrochloric acid standard titration solution (0.
(16) Methyl red ethanol solution (1g/L): Weigh 0.
1g of methyl red, dissolve it in 95% ethanol, and dilute to 100 mL with 95% ethanol
.
(17) Methylene blue ethanol solution (1g/L): Weigh 0.
1g of methylene blue, dissolve it in 95% ethanol, and dilute to 100 mL with 95% ethanol
.
(18) Bromocresol green ethanol solution (1g/L): Weigh 0.
1g of bromocresol green, dissolve it in 95% ethanol, and dilute to 100 mL with 95% ethanol
.
(19) Mixing indicator solution: 2 parts of methyl red ethanol solution (16) and 1 part of methylene blue ethanol solution (17) are mixed before use
.
It is also possible to mix 1 part of methyl red ethanol solution (16) with 5 parts of bromocresol green ethanol solution (18) before use