Ice cream - Determination of protein content by Kjeldahl method.
1 instruments and equipment
Kjeldahl flask: 500 ml;
: 50 ml beaker;
Buret: 50 ml;
: 250 ml flask;
Pipet: 50 ml;
Ventilation exhaust hood;
Brandreth platform ( containing iron );
A gas or electric stove.
The 2 reagent
The copper sulfate: chemical pure;
The potassium sulfate: chemical pure;
The concentrated sulfuric acid: chemical pure;
The sodium hydroxide solution: 40%;
The mixed indicator: 1 volume of methylene blue ( 0.05% alcohol) and the 2 volume of the methyl red indicator ( 0.05% alcohol solution )
The hydrochloric acid standard solution: 0.1 mol / l;
The zinc particles;
3 methods of operation
( 1) digestion
Precise weighing 2 to 5 grams of sample into 500 ml Kjeldahl flask, add 3 grams of potassium sulfate and 1 grams of copper sulfate, a few grains of glass beads, in order to prevent the sample liquid in the digestion of boiling spill. 20 ml of concentrated sulfuric acid slowly to join the Kjeldahl flask.
Digestion ( must be enclosed with fan carried within the channel, to digest of harmful gases exclude ), first with a small fire heating and pay attention to prevent the Kjeldahl flask boiling spill within, and affect the correctness of inspection ( as long as the sample liquid is a little splash, it is necessary to sample digestion, therefore, inspection personnel shall not leave ). When the Kjeldahl flask in after the disappearance of the bubble, which can fire heating. As found in bottle stuck with a carbon particle, after cooling for bottle blowing distilled water, shaking the carbon decomposition, heating is continued, until the bottle material is completely transparent to the green liquid.
Adding concentrated sulfuric acid to make samples of organic compounds by oxidation, carbon dioxide and sulfur dioxide generated from the sample in the escape, in a sample of nitrogen into ammonia, and excessive sulfuric acid to produce ammonium sulfate. During digestion due to ammonium sulfate high boiling point can be retained in the sulfuric acid solution, the distillation, adding excess sodium hydroxide and ammonium sulfate reacts with ammonia escape. Adding copper sulfate and potassium sulfate is not involved in the chemical reaction, only a catalytic effect (increasing the sample liquid boiling point, accelerate the speed of chemical reactions ).
( 2) distillation stage
The liquid sample digestion is complete, you can enter the distillation stage. To good digestion sample liquid is added with a few grains of zinc particles (to prevent the sample fluid by boiling and splashing ), 40% sodium hydroxide solution 60 ml, no ammonia neutral distilled water 200 milliliter, plug rubber plug in distillation rack, rubber plug connection nitrogen pipe. One will distillation device installed with fire after heating, which is distilled liquid collecting bottle bottle to accept, accept the Central Plains has 50 ml, 0.1 mol / L standard hydrochloric acid solution, and add a few drops of methyl red indicator. Distillation first with small fire with fire, so that the sample liquid boiling state. Distillation, to master the flow rate of cooling water, inspection personnel shall not leave.
Distillation end point judgment method:
The bottle is not less than 150 ml of distilled liquor;
The Kjeldahl flasks are rolled;
( 3) the titration phase
In 0.1 mol / L standard hydrochloric acid solution titration, drops to the sample fluid is light yellow transparent liquid.
4 calculation method
( V0-V1 ) CHCl x 0.014
Protein % = -------------------- x 6.38 x 100
V0: when the titration to 0.1 mol / L HCl milliliter number;
V1: blank determination for 0.1 mol / L hydrochloric acid milliliter number;
CHCl: the concentration of a solution of hydrochloric acid;
0.014: n a number;
6.38: protein conversion coefficient;
M: sample quality;
In order to ensure the accuracy of the results, should be making blank experiment, i.e. not adding sample titration.