15 



Mixing the add and the milk. The acid is now mixed 

 with the milk by giving a combined rotary motion and 

 gently shaking with the hand grasping the neck of the 

 bottle, with the mouth of the bottle held away from the 

 operator. When once commenced the mixing must not 

 be interrupted until the solution is complete. The first 

 effect of the acid on the milk 

 is a curdling, which is sub- 

 sequently dissolved. As the 

 solution progresses the color 

 changes first to a light yellow, 

 then to dark yellow, then 

 through various shades of vio- 

 let to brown and finally to dark 

 brown, if the acid is of the 

 proper strength and the milk 

 and acid are at the right, tem- 

 perature when united. Too 

 strong or too warm acid pro- 

 duces a dense black. If the 

 milk has been preserved with 

 formaldehyde, a longer time is 

 required to complete the solu- 

 tion, owing to the toughening 

 of the casein by that preserva- 

 tive. Common errors of begin- 

 ners are failure to mix the acid 

 thoroughly with the milk and 

 to continue the shaking tin til 

 the solution is complete. A 

 good plan is to shake the bottle 

 for a minute or so after the so- 

 lution is apparently complete. 

 Although not necessary, it is 

 preferable to centrifuge the bottles immediately, though 

 they may be kept 24 hours if desired, in which case they 

 must be placed in water from 170 to 180 F. for 15 to 20 

 minutes before whirling. 



Centrifuging the bottles. The bottles are now placed 

 in the sockets of the centrifuge, taking care that they are 



8346017 3 



\ 



FIG. 14. The wrong way of 

 adding the milk to the 

 milk bottle. (Farrington 

 and Woll, Testing Milk 

 and Its Products.) 



