MARKET MILK 165 



Clarification of milk. 



The production of milk which is entirely free from extraneous 

 dirt requires expensive methods, and such milk cannot be sold 

 at ordinary market prices. The increasing demand of con- 

 sumers and health officers for a supply of market milk which 

 is free from dirt and at the usual market price has led to special 

 efforts to remove all visible dirt from ordinary milk. The 

 ordinary centrifugal separators have been used for this pur- 

 pose by passing the milk through the machine and allowing 

 the cream and skim-milk to run into one vat where they were 

 again thoroughly mixed ; but it was soon found that this pro- 

 cess injured the keeping quality of the milk, and the method 

 has not come into general use in spite of the fact that this is a 

 very efficient means of removing any foreign material from the 

 milk. More recently a modified form of the centrifugal sepa- 

 rator has been developed, especially for removing foreign 

 matter from milk. These machines are known as milk clari- 

 fiers and are now in quite general use. Their efficiency is based 

 on the principle of centrifugal force, throwing the heavy parti- 

 cles of dirt to the outer edge of the hollow bowl, where they are 

 deposited as a layer of slime, together with considerable num- 

 bers of bacteria and leucocytes. This process of treating milk 

 is very useful for the removal of dirt, but it does not improve 

 its keeping qualities. 

 Cooling of milk. 



In handling milk, no one factor is more important than the 

 temperature at which it is held. Even when produced under 

 the very best sanitary conditions, it will contain a greater or 

 less number of bacteria, and these will multiply with astonish- 

 ing rapidity if the milk is allowed to remain warm. To pre- 

 vent their rapid growth, milk should be cooled as soon as pos- 

 sible after it is drawn from the cow, to a temperature of 50° F. 

 or below. The lower the temperature, the more slowly the 

 bacteria will develop and the better will be the quality of the 



