SCIENCE AND PRACTICE OF CHEESE-MAKING 



mercuric chlorid tablets containing coloring-matter 

 cannot be used. 



(8) Summary of precautions. Assuming that the 

 graduated glassware is accurate and the standardized 

 solutions of, correct strength, the following special 

 points are to be observed with care in making test. 



(a) Preliminary neutralization. In neutral-zing 

 the sample of milk, excess of alkali must be avoided, 

 which can be controlled by the use of a properly pre- 

 pared color-standard. 



(b) Conditions of precipitation. Before precip- 

 itating with acid, have the dilute, neutralized milk at 

 a temperature between 65 and 75 F. Add enough 

 acid to cause the casein to separate promptly in large 

 flakes, leaving the supernatant liquid clear. Shake the 

 mixture vigorously at intervals during the addition 

 of acid ; also after complete precipitation and again 

 after dilution to 200 cc. mark. 



(c) Filtration. Allow most of the liquid to run 

 through the filter before making the final titration with 

 alkali. 



(d) Titration with alkali. In titrating the filtrate 

 with alkali, avoid an excess of alkali. Add the alkali 

 solution cautiously until, after thorough agitation, a 

 faint but distinct pink color remains through the solu- 

 tion half a minute or longer. The same uniform shade 

 and duration of pink color should be obtained as nearly 

 as possible in all cases. 



(e) Acid milk. Milk that is sour or that coag- 

 ulates on heating should not be used. 



(f) Use of preservatives. Milk treated, when 

 fresh, with a small amount of pure powdered mercuric 

 chlorid (corrosive sublimate) and then kept in a cool 

 place gives good results for two or three weeks. 



