36 . FIRST LESSONS IN DAIRYING 



starter or a commercial starter from a pure culture. 

 The changes may be retarded by excluding bacteria, 

 by stopping their growth with cold ; for, like other 

 forms of plant life, they do not grow when too cold. 

 Few forms common to milk develop rapidly at a 

 temperature of 50 or below, and practically no 

 development takes place at the freezing tempera- 

 ture. They may be destroyed by subjecting them 

 to heat. Complete destruction i.e. to render sterile 

 requires that milk be heated to the boiling point for 

 twenty minutes on three successive days. Mani- 

 festly, this is not commercially practicable. Pas- 

 teurization is heating the milk or cream sufficiently 

 to destroy the vegetative forms, which include the 

 commoner disease and putrefactive bacteria. In 

 the vegetative form the bacterium is ready for active 

 growth. In the spore form it can resist great ex- 

 tremes of heat and cold and may retain life for a long 

 period of time, ready to grow and multiply rapidly 

 when conditions become favorable. The changes 

 may be hastened by introducing bacteria, or by a 

 favorable temperature. The rapidity of the change 

 will depend on the numbers of bacteria present and 

 whether the temperature is favorable to their rapid 

 growth or not. 



