THE REL|A.TION OF MICROORGANISMS TO MILK 



391 



which will grow with considerable rapidity at temperatures below 10°, 

 especially some of the spore-bearing non-acid forms. If the tempera- 

 ture of the milk is allowed to rise above 10°, the growth of the common 

 species increases rapidly. The influence of temperature upon the 

 development of bacteria may be seen in the following experiment 

 where a given lot of milk was thoroughly mixed and divided into 

 seven portions, which were then held at the temperatures indicated 

 for twelve hours, at the end of which time they were plated for the 

 total germ content. 



Effect op Different Temperatures itpon the Development op Bacteria 



IN Milk 



The fresh milk showed a count of 5,000 per c.c. and curdled in 

 fifty-two hours at a temperature of 21°. The curdling time of these 

 samples was determined by placing them at a constant temperature 

 of 21° at the close of the twelve-hour period and holding them at this 

 temperature until coagulation took place. The difference in time of 

 curdling therefore is due to the maintenance of the special tempera- 

 ture for twelve hours only and not for the entire period up to the time 

 of curdling. 



Pastetteization. — The term pasteurization is used to designate 

 the process of heating milk to a temperature sufficient to destroy 

 a portion of the bacteria and then cooling.it to a temperature which 

 will prevent the rapid development of the organisms that are left. 

 The temperatures commonly used for this purpose vary from 60° to 

 85°. The length of time the milk is exposed to the high temperature 

 may also vary from a few seconds to thirty minutes, depending upon 

 the method employed. The two chief purposes for the pasteuriza- 



