MECHANISM OF DEATH 333 



The thermal death-point is based on the same wrong conception 

 as the coagulation temperature of the proteins. In reality, we 

 have a reaction of measurable speed, but the very high temperature 

 coefficient obscured this fact for a long time. 



On the other hand, the high temperature coefficient prevents 

 the probable error in thermal death-points from being very large. 

 If, at a certain temperature, it takes ten minutes to reduce 1,000,000 

 ceUs to 1 cell, it will take 11.67 minutes to kill 10,000,000 bacteria, 

 and 13.33 minutes to kill 100,000,000, and 15 minutes to kill 1,000,- 

 000,000 cells. Figuring with a temperature coefficient of 20, it would 

 require an increase in temperature of 0.51°C., 0.96°C., or 1.35°C. 

 respectively if the larger numbers of cells were to be killed in exactly 

 ten minutes. These increases are quite within the limits of error of 

 the ordinary technique of determining thermal death points. 



The determination of thermal death-points becomes 

 rather uncertain by the observation of Gage and Stough- 

 ton (1906) that in old cultures of Bad. coli, the last 

 survivors are very hard to kill; they behaved very much 

 like spores. Figure 38 gives an illustration of the result. 

 While the great majority of cells (more than 99.99%) 

 is killed at 60°C. in five minutes, a few will remain alive 

 even after five minutes' heating to 85°C. The younger 

 cultures are more uniform in their reaction with heat, 

 and the smoother curve of their survivors indicates a 

 more constant temperature coefficient. 



This has led to the distinction of a ^'majority thermal 

 death-point.'' Ayers and Johnson (1914) found strepto- 

 cocci with a low '' majority thermal death-point," among 

 which a few cells are able to survive the pasteurization 

 temperature. ^'This ability . . . may be due to cer- 

 tain resistant characteristics peculiar to a few cells, 

 or may be due to some protective influence of the milk." 



The limitation of the thermal death-point to one 

 experimental time of ten minutes is not very satisfactory 

 for applied bacteriology. For the needs of food bac- 



