CARE OF MILK ON THE FARM 341 



the operation. When nearly all the milk has run out of the 

 pipette, the last drop is forced out with a puff of the breath. 



Adding the acid. — The temperature of the milk when the acid 

 is added should be between sixty degrees and seventy degrees F., 

 and the acid should be at about the same temperature. Seven- 

 teen and one-half cubic centimeters of the acid is measured out, 

 and, with the bottle held at an angle, carefully poured down the 

 side, the l)ottle being turned slowly at the same time so that any 

 milk adliering to the neck will be washed doAvn. 



Mixing the acid 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, wath the mouth of 

 the bottle held away from the operator. When once connnenced 

 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 

 subsequently dissolved. As the solution progresses the color 

 changes first to a light yellow, then to dark yellow, then through 

 various shades of violet to brown and finally to dark brown, if 

 the acid is of the proper strength and the milk and acid are at 

 the right temperature when united. Too strong or too warm acid 

 produces a dense black. Common errors of beginners are failure 

 to mix the acid thoroughly with the milk and to continue the 

 shaking until the solution is complete. A good plan is to shake 

 the bottle for a minute or so after the solution is apparently 

 complete. 



Centrifuging the bottles. — The bottles are now placed in the 

 sockets of the centrifuge, taking care that they are equally dis- 

 tributed about the wheel or disk so that the equilibrium of the 

 latter is not disturbed. An even number of bottles should always 

 be whirled. Should an odd number of tests be made a test bottle 

 filled with water may be used to balance the machine. When the 

 l)ottles are in place, the tester is covered in order to keep the 

 bottles from getting cold and to protect the operator from flying 

 glass and acid should anj^ of the bottles break. The tester is 

 now set in motion and the bottles whirled four to five minutes 

 at proper speed. This will be sufficient to bring practically all 

 the fat to the surface. In cold weather, if a hand tester is used, 

 it may be necessary to pour hot water into the jacket of the tester 

 to keep the bottles warm. 



Speed of centrifuge. — Farrington and AVoll have calculated 

 the proper speed of testers with wheels of different diameters to 

 be as follows: 



