26 THE STORY OF MILK 



digestibility. Pasteurizers with ^'holding devices" are 

 therefore largely taking the place of the ''continuous" 

 sterilizing machines, which allow only momentary ex- 

 posure to the highest temperature. 



Whatever method is used it is essential that after the 

 heating is finished the milk should be cooled as quickly 

 as possible to a temperature sufficiently low to prevent 

 development of the germs that have not been entirely 

 destroyed by the heat and w^hich, if the milk is left 

 for any length of time at a temperature favorable for 

 bacterial life — anywhere between 65° and 110°, — will 

 begin to grow again. Cool the milk to below 60° and 

 if possible to 50° or 40°. 



Sometimes a second pasteurization is practiced, the 

 milk being purposely allowed to stand at 70° to 80" 

 for 12 to 24 hours after the first pasteurization for the 

 germs which escaped destruction to develop into full- 

 grown bacteria. They are then killed by the second 

 heating before the milk is finally chilled to stop any 

 further growth. 



Pasteurization can be done by placing the milk in 

 any tin or enamelled or glass vessel, set in another vessel 

 containing water (a double boiler). Heat until the 

 milk has reached the desired temperature, hold it at 

 that temperature the necessary time, and then place 

 the vessel in cold running water or in ice water until 

 the milk is thoroughly chilled. It is not advisable to 

 place the hot milk in the ice box as cooling in air is too 

 slow. Not until it is thoroughly cooled in water is it 

 safe to put it in the ice box to keep it cool. 



In the seventies Prof. N. J. Fjord, in Denmark, ap- 

 plied to milk the process which had been developed by 

 Louis Pasteur in France to give keeping quality to wine 



