44^ 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. 



