BUTTER-MAKING 



233 



whole milk, the physical condition of the milk, and the position 

 of the cream or skim-milk screw. The centrifugal force exerted 

 upon the particles of milk as they pass through the separator 

 is dependent on the velocity and diameter of the bowl; the 

 greater the speed and the larger the bowl, the greater is the 

 centrifugal force and the more completely will the fat globules 

 be separated from the milk-serum. It is also important that 

 the speed be uniform in order to maintain a constant force upon 

 the milk passing through the bowl. This is illustrated by the 

 following results obtained by Guthrie : 1 



Effect of Speed on Percentage of Fat in Cream and Milk 



Separator 



Revolutions of Crank a Minute 



60 



50 



No. 1 

 No. 2 

 No. 3 



Cream 

 %fat 



30.9 

 28.1 



28.8 



Skim-milk 

 %fat 



.024 

 .036 

 .054 



Cream 



%fat 



21.3 



27.6 

 23.0 



Skim-milk 

 %fat 



.034 

 .041 

 .050 



The temperature affects the viscosity and fluidity of the milk ; 

 the warmer the milk, the less the viscosity and the more easily 

 the particles can move, hence the more complete the separation, 

 Milk is usually separated at temperatures between 85° and 

 95° F. At lower temperatures, the milk flows more slowly and 

 gives a richer cream and greater loss of fat in the skim-milk. 

 If the temperature is too low, the machine will become clogged 

 and separation will be at least partly prevented, the temperature 

 at which this will take place differing in different styles of 

 machines. The effect of temperature is shown by the following 

 summary of experiments conducted by Guthrie : 2 



1 C. U. Bui. 360. 2 C . U. Bui. 360. 



