110 Milk and Its Products 



but almost any others, will readily live and grow 

 should they gain access to milk. Nearly all forms 

 of bacteria are sensitive to conditions of tempera- 

 ture. The range of temperature in which they 

 thrive the best and grow most rapidly is rather 

 narrow, though there is a considerable range above 

 or below, in which they will still grow and develop. 

 The temperature at which any given germ will grow 

 most rapidly is called the optimum temperature, and 

 the optimum temperature varies widely with the 

 various classes of organisms, though by far the 

 larger ^number of bacteria find their optimum point 

 between 75 to 100 F., and a higher temperature 

 not only stops their growth, but if sufficiently high 

 kills them outright. A temperature of 135 F. kills a 

 large number of germs. Very few are able to live 

 above a temperature of 180, and none can with- 

 stand the temperature of boiling water, 212, for 

 more than a few minutes. If heat is accompanied 

 by moisture it is much more effective, so that heat 

 applied in the form of live steam is the best means 

 of destroying the life, of these germs. Under the 

 influence of cold the germs become inactive, and 

 some kinds are killed by a sufficient degree of cold, 

 but very many kinds are able to withstand any 

 degree of cold possible to be produced for long pe- 

 riods of time. 



Under certain conditions bacteria are able to as- 

 sume an inactive condition, or spore form. In do- 

 ing this, the protoplasm shrinks into a hard, glisten- 

 ing mass, and contracts toward one end of the cell, 



