MECHANISM OF DEATH 317 



it took one year to kill 99.9% of all typhoid bacteria in 

 ice cream when held at approximately — 19°C. 



The slowness makes possible the explanation that the 

 fundamental process of death, aside from physical 

 destruction, might be an oxidation process, or, perhaps, a 

 starvation process. No data are available to decide 

 between these two and possibly other explanations. 



SUMMARY 



Death by freezing involves a number of different 

 possible causes. While the low temperature, as such, 

 is one of the factors, physical destruction by the forma- 

 tion of ice crystals has been shown to be important, 

 so that repeated thawing and freezing kills more rapidly 

 than long continued cold. The medium in which bac- 

 teria are frozen is also of significance for the rate of 

 death. 



Death by freezing is slow. The extremes recorded 

 in literature for a reduction of 99.9% are with Bad. 

 coli, eight weeks, B. suhtilis, more than eighty weeks, 

 Sacch. ellipsoideus five weeks, Sacch, cerevisiae, more 

 than three years. 



VI. DEATH BY HEAT 



Under ^^heat" in this chapter is understood ^' moist 

 heat," i.e., the application of high temperatures to 

 bacteria in their natural state of living. Death by dry 

 heat has been treated in the chapter on Drying (p. 309). 



(a) METHODS OF MEASURING 



While all that has been said about the technique of 

 disinfection experiments in general (p. 270 to 274) applies 

 here, some special features enter into the killing of 

 bacteria by moist heat that deserve special attention. 



