240 CITY MILK SUPPLY 



and the heavy particles are thrown to the outside wall making the skim- 

 milk, while the lighter particles, the butterfat, remain in the center 

 forming the cream. Bowls are run at a high rate of speed, those of 

 small diameter turning fastest; some make 16,000 r.p.m. Within the 

 bowl are discs or leaves over which the milk spreads in thin films as it 

 enters so that it is quickly given a centrifugal motion and the separation 

 of the cream accelerated accordingly. In some types of separators a 

 revolving flange gives a rotary impulse to the incoming milk with 

 the same result. The length of time the milk remains in the bowl is 

 determined by the obstructions therein. There are a number of things 

 that effect the efficiency of the skimming process, viz. : 



1. The speed of the bowl. 



2. The steadiness of the bowl. 



3. The temperature of the milk. 



4. Rate of skimming. 



5. Richness of the cream. 



6. Acidity of the cream. 



7. Character of the milk, due to breed of cow, stage of lactation and the richness 

 of the milk. 



The richness of the cream is controlled by the cream screw in the 

 separator bowl ; turning the screw toward the center of the bowl throttles 

 the outlet and increases the richness of the cream, and turning it away 

 from the center opens the outlet and makes the cream thinner. 



With the increase of speed more skim-milk is forced out the openings, 

 making the cream richer; the effect of the speed is more marked when the 

 cream screw is set to deliver a high percentage of fat. The rate of inflow 

 of the milk is affected by the speed of the bowl because the higher the 

 speed the greater the tendency to produce a partial vacuum in the revolv- 

 ing bowl. As a rule when the separator is run at three-fourths its normal 

 speed the loss in skim-milk is two to three times as great as when it is 

 run at the indicated rate. Machines usually leave the factory adjusted 

 so that 10 per cent, of the milk put in comes out as cream. 



The separator should run smoothly for all vibration or jarring in- 

 creases the loss in fat. For this reason the separator should set level and 

 be mounted on a rigid foundation. An unbalanced bowl or loose or 

 dirty bearings cause the separator to run unevenly. 



The temperature of the milk at skimming is important; it should be 

 between 90 and 100F. The effect of reducing the temperature of the 

 milk below this is to increase the percentage of butterfat in the skim- 

 milk. Between 70 and 80F. there is an appreciable loss of butterfat 

 and at lower temperatures it is serious. The loss is caused by the greater 

 viscosity of the cream at low temperatures which makes it pass slowly 

 through the outlet of the bowl. The milk can be so cold that the 

 separator will not deliver the cream. 



