10 MODERK DAIRY PRACTICE. 



a cold of 166 F. ( 110 C.) for an hour, a temperature 

 which only few organisms can stand ; the cholera bacteria 

 can stand a cold of 14F. ( 10 C.) for the same length of 

 time without being destroyed; and typhus bacteria have 

 been known to remain alive for 100 days frozen in the ice. 

 A special account is given later on of the influence of cold 

 on certain fermentation bacteria. 



It is, however, not only cold but also high heat which 

 arrests the development and multiplication of bacteria. 

 Temperatures between 39 and 122 F. (4 and 50 C.) are 

 most favorable to their development. At 140-158 F. (60- 

 70 C. ) their vegetative forms are usually killed, provided 

 this temperature is allowed to act for a sufficiently long 

 time, while the spores can stand a much higher heat. 

 The optimum temperature, i.e., the most favorable tem- 

 perature, for the development of the pathogenic bacteria 

 (the disease bacteria) coincides nearly with the body tem- 

 perature of animals; for the greater portion of the other 

 bacteria and especially for the fermentation bacteria it is 

 from 59-77 F. (15-25 C.). There are, however, num- 

 erous exceptions to these rules. There are thus, e.g., some 

 soil and water bacteria which only develop at 122-158 F. 

 (50-70 C.). I have further found bacteria in milk that 

 were not only alive but multiplied at 140-149 F. (60-65 

 C.). At lower temperatures the development of these bac- 

 teria was practically suspended. 



Chemical Reaction Necessary for Baetsria, The develop- 

 ment of bacteria also depends on the chemical reaction of 

 the nutritive substratum. An alkaline or at least a neutral 

 reaction is an essential condition for a large majority. 

 Some few bacteria can, on the other hand, thrive in an 

 acid substratum, (e.g., lactic-acid bacteria). An acid re- 



