334 PHYSIOLOGY OF BACTERIA 



teriology, especially for milk pasteurization and for 

 canning of vegetables and meat, this conception has been 

 broadened to the ^'thermal death time," i.e., the time 

 required to kill all cells at a certain temperature, as 

 determined by the endpoint method. 



The thermal death time is dependent upon all the 

 factors mentioned above. It is not more accurate 

 than the thermal death-point, but it is more useful in 

 applied bacteriology. A detailed discussion has been 

 given by Esty (1928). 



(/) SUMMARY OF FACTS 



In the death of bacteria by heat, the logarithmic 

 order is well established. 



Dead cells give to living cells a certain protection 

 against heat which is too pronounced to be attributed to 

 a decreased rate of heat transmission. 



At higher temperatures, the bacteria die much more 

 rapidly. There is considerable disagreement in the 

 temperature coefficients of death of vegetative cells. 

 For bacterial spores, the temperature coefficient of 

 death is approximately 10, and is constant over a 

 temperature range of about 30°C. 



In concentrated solutions, death by heat is retarded. 



SUMMARY OF THEORIES 



Death by heat shows such close analogies to the heat 

 coagulation of proteins, in temperature range as well as 

 in the high temperature coefficients, that the general 

 explanation of death by heat being due to coagulation 

 of some parts of the protoplasm seems well founded. 

 The greater resistance of dry cells corresponds well with 

 the absence of coagulation of proteins when heated 



