176 AGRICULTURAL BACTERIOLOGY 



volved in artificial refrigeration and their application to 

 the cold-storage industry are largely the outcome of the 

 relation of cold to the preservation of food supplies. 



The temperature zone within which bacterial growth can 

 take place extends from a few degrees below the freezing 

 point of water to about 158 F. It is true that no one 

 organism possesses the ability of growing throughout this 

 entire range of temperature. Most kinds are able to de- 

 velop from temperatures in the neighborhood of 50 F. 

 to somewhat above blood heat. Also there are groups cap- 

 able of multiplying at or near the freezing-point, and 

 others at temperatures of 120-140 F. Those types habi- 

 tuated to low temperatures are of much practical signifi- 

 cance in the storage of foods. 



The greatest importance of low temperatures in the 

 preservation of foods is not found in the extreme temper- 

 atures that are employed in cold-storage warehouses, but 

 in the range that occurs in daily life under the conditions 

 obtaining in the ordinary household. The great majority 

 of bacteria grow most rapidly from 60 to 100 F. If the 

 temperature is reduced to 50, the rate of bacterial multi- 

 plication is much retarded, as is to be noted from the fol- 

 lowing table in which is given the time required for the 

 division of a single bacterial cell into two completely grown 

 daughter cells at different temperatures. 



The generation time of B. coli 



45 C. (113 F.) 20 minutes 



40 C. (104 F.) 17.2 minutes 



35 C. ( 95 F.) 22 minutes 



30 C. ( 86 F.) 29 minutes 



25 C. ( 77 F.) 40 minutes 



20 C. ( 68 F.) 95 minutes 



16 C. ( 60 F.) 120 minutes 



10 C. ( 50 F.) 14 hours, 25 min. 



As ordinary refrigerators maintain a temperature vary- 



