DAIRYING 13 



success in developing them depends largely on the care used in 

 all the various steps of the process and the prevention of con- 

 tamination of the starter during the making. The starter should 

 be carefully covered and protected from dust and currents of 

 air; such protection is one of the most essential elements of 

 success in starter-making, as the slightest exposure after heat- 

 ing, to the air, drops of water, or the use of a cover or stirrer, 

 which has not been sterilized immediately before using it, may 

 introduce injurious bacteria into the starter, and by contamina- 

 tion impair its efficiency. 



538. Developing the Pure Cultures. The commercial cul- 

 tures, if active bacteria are present, will soon begin to grow in 

 skim milk when held at 80 degrees Fahrenheit. If maintained 

 until the milk begins to thicken, this will require from 18 to 36 

 hours. This sour milk may then be added to a larger quantity 

 of skim milk which has been previously heated to 185 degrees 

 Fahrenheit. This second transfer, or second generation of the 

 starter, may then be held at any temperature between 50 degrees 

 Fahrenheit and 90 degrees Fahrenheit, which will promote the 

 rapidity of growth desired. This will depend on the time when 

 it is needed for the cream. The souring process goes on rapidly 

 at 90 degress Fahrenheit, but is nearly checked at 50 degrees 

 Fahrenheit. The temperature of the starter must, therefore, be 

 regulated to accommodate the cream in which it is to be used; 

 warming or cooling the starter to any temperature between 

 the points mentioned that may be deemed necessary to hasten 

 or to check the souring process. 



Natural and Commercial Starters. 



The starters now in use for ripening or souring cream may 

 be divided into two general classes. 



First: The Natural Starters. 



539. This name is sometimes given to sour whole milk, 

 skim milk, butter milk, or thin cream which has soured without 

 the addition of an artificial pure culture of any kind. The fer- 



