THE RELATION OF MICROORGANISMS TO MILK. 



3*9 



lowing experiment where a given lot of milk was thoroughly mixed and 

 divided into six 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 of Different Temperatures upon the Development of Bacteria in Milk. 



Temperature maintained 

 for 12 hours 



Bacteria per c.c. at end 

 of 12 hours 



Hours to curdling 

 at 21 



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 there- 

 fore is due to the maintenance of the special temperature for 12 

 hours only and not for the entire period up to the time of curdling. 



PASTEURIZATION. 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 pasteurization of milk are to 

 increase its keeping quality and to destroy any pathogenic organisms which 

 the milk may contain. The purpose for which the pasteurization is done 

 will determine the method used. In commercial pasteurization, where 

 the chief purpose is to destroy the lactic organisms and thus improve 



