196 MODERN METHODS OF TESTING MILK 



(b)~ Appearance of filtrate. The filtrate should be 

 quite clear, though this is not always a sure indication 

 that the right amount of acid has been used. Some- 

 times the filtrate may be clear when not quite enough 

 acid has been added, in which case the filtration is 

 usually slow. In case of milks rich in fat, a slight tur- 

 bidity may appear, due to fat-globules in the filtrate. 

 The filtrate should be free from all signs of marked 

 turbidity or anything like milkiness. If such a filtrate 

 appears, a new sample of milk should be taken and the 

 operation repeated from the beginning, more acid be- 

 ing used than before. With a little experience, espe- 

 cially under proper instruction, no difficulty should be 

 found in recognizing quickly when the casein is sepa- 

 rated so as to give satisfactory results. 



(5) Titration with alkali. After filtration is com- 

 pleted, one measures 100 cc. of the filtrate with the 

 pipette into a cup and then from the burette runs into 

 this the standard alkali until a faint, but distinct, pink 

 color remains clearly marked through the solution for 

 half a minute or longer before beginning to fade. The 

 number of cubic centimeters of alkali used is noted and 

 this result is recorded as B. 



The last portions of alkali must be added carefully, 

 a drop at a time, agitating the mixture well after each 

 addition. The exact neutral point is not perfectly 

 sharp on account of the presence of phosphates, and 

 the apppearance of the desired coloration is, therefore, 

 not as sudden and pronounced as might be desired. 

 With experience one should have no difficulty in get- 

 ting within one drop of the correct amount of alkali. 

 The chief precaution to be observed is to have the same 



