FAVORABLE CONDITIONS FOR BACTERIAL GROWTH 



57 



range. Some few species will grow at as high a temperature as 

 140 F., and B. bulgaricus will grow very rapidly at 110 F. 



The growth of bacteria at these extreme temperatures 4s 

 usually very slight. Even at 50 F. the rate of growth is very 

 slow. According to experiments conducted by Dr. Conn, the 

 multiplication of bacteria at 50 F. was 5-fold, while at 70 F. 

 the multiplication was 750-fold. The following table shows the 

 number of bacteria per cubic centimeter in milk kept at different 

 temperatures. 1 



All bacteria do not have the same optimum growing tem- 

 perature. Some species develop most rapidly at one tempera- 

 ture, while other species prefer a different temperature for the 

 greatest development. It is on this account that certain tem- 

 peratures are employed in ripening starters and cream. Accord- 

 ing to researches by Conn, Bacterium aerogenes develops very 

 rapidly in milk at 95 F. This particular species, producing 

 much gas and an unpleasant flavor, sours milk very rapidly. As 

 a rule, milk which has been held at this high temperature con- 

 tains a preponderance of this undesirable species of bacteria. 

 At 77 F. results are more uncertain; the species of bacteria 

 which will predominate in milk at this temperature depends in 

 large measure upon the number of each kind originally present. 

 According to Conn Streptococcus lacticus has the highest relative 

 growth at about 70 F. This particular species produces no gas, 

 and its presence is desirable in cream for butter-making. Milk 



1 Bull, 26 Storr's Sta., Conn. 



