ILLINOIS STATE DAIRYMEN'S ASSOCIATION. 221 



The species favored rapidly increases and covers up the work 

 of others less favored, but which may continue to grow slowly. 

 At about 90 degrees F. most forms grow with great rapidity, 

 the rate of their multiplication decreasing with the decrease of 

 temperature. Bacteriologists have shown that at 93 F. certain 

 germs may increase in number in four hours more than two hun- 

 dred fold, while at 55 degrees F. their increase is only about 

 eight fold. An experiment is reported in which a difference of 

 18 degrees in the temperature of two samples of milk caused, in 

 fifteen hours, a difference of almost 75,000,000 bacteria per 

 cubic centimeter. This shows very plainly how much the rate 

 of growth of bacteria depends upon temperature. 



At 50 degrees F. bacteria are quite inactive, but at this and 

 considerably lower degrees of heat they retain life, and so^e 

 forms continue to multiply. Freezing does not kill them. Some 

 species can withstand a temperature of many degrees below 

 zero, and with the return of suitable conditions again commence 

 to grow. 



Up to a certain point the higher temperatures have the same 

 effect as cold, i. e., make the germs inactive. But when the 

 heat is raised to 125 degrees F. some are killed; others, not 

 harmed by this temperature, are destroyed by greater heat. A 

 sufficient temperature to kill almost all of the growing forms 

 found in milk is 165 F. Spores require still more heat; some 

 can withstand boiling temperature, 212 degrees F. 



If milk is heated high enough to kill all the living forms of 

 bacteria and then suddenly cooled to a low temperature, it will 

 keep sweet a long time, because it is free from growing germs. 

 It must be quickly cooled, however, or the spores will develop 

 while the temperature is ranging from iio down to about 60 

 degrees, and the bacteria thus formed may continue to increase 

 slowly after the cooling is completed, at the low temperature at 

 which the spores would not have germinated. When milk is 

 heated for the purpose of killing bacteria (the process is called 

 pasteurization or sterilization) it should be held at the highest 

 temperature at least ten minutes, as some forms 'are not killed 



