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LESSON XVI 

 COAGULATION OF MILK 



1. Prepare a solution of rennet by extracting the fourth stomach of the 

 sucking calf with glycerin. Clark's rennet, which is sold for making junkets, 

 will do equally well. 



2. Prepare a solution of pure caseinogen in the following way : — Saturate 

 milk with magnesium sulphate by shaking it with excess of the powdered 

 salt ; or the caseinogen may be more readily precipitated by half saturation 

 with anunonium sulphate — that is, by adding to the milk an equal voliune 

 of saturated solution of ammonium sulphate. Filter. The caseinogen and 

 fat remain together on the filter. Save the filtrate, and label it A. Wash 

 the precipitate on the filter with saturated solution of magnesium sulphate 

 or half-saturated anmionium sulphate solution, as the case may be, until the 

 washings contain no albumin. Add water to the precipitate. The caseinogen 

 dissolves, the fat being insoluble. In this way a solution of caseinogen in 

 weak magnesium or ammonium sulphate is obtained. So far the operations 

 should be performed beforehand by the demonstrator. 



3. To this solution add acetic acid. The caseinogen is precipitated ; 

 collect it on a filter ; wash the acid away with distilled water. Dissolve the 

 precipitate in lime water by grinding it up in a mortar with the lime water ; 

 filter, and an opalescent solution of caseinogen is obtained. 



4. To a portion of this solution add a few drops of rennet extract. Put 

 it in the water-bath at 40° C, and if the caseinogen has been thoroughly 

 washed to firee it from calcimn salts no coagulation will occur. The calciim[i 

 contained in the lime water subsequently added is not efifective in promoting 

 coagulation. 



5. Treat another portion in the same way, adding, however, a few drops 

 of 0*5-per-cent. phosphoric acid as well as the rennet. Warm to 40° C. 

 Coagulation — that is, formation of casein from caseinogen — usually occurs in 

 a few minutes. The addition of the phosphoric acid causes the formation of 

 calcium phosphate, which is effective in promoting coagulation. 



6. Examine the filtrate A (see above). Saturate a portion with sodium 

 chloride. A small amount of precipitate of a proteid comes down. This is 

 the so-called lacto-globulin. This contains only a trace of true globulin : it 

 is mostly caseinogen previously left in solution, together with calcium 

 sulphate. 



7. Heat another portion of A to 77®, acidifying faintly with a few drops 

 of 2-per-cent. acetic acid. Lactalbumin is coagulated at this temperature. 



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