300 DETERMINATIONS OF THERMAL DEATH-TIME 



phosphate solutions by constructing a curve through the percentage survival points 

 that have been experimentally determined. 



In general, the percentage survival greatly decreases as the heating progresses, 

 and the results are uniform in proportion to the number of tubes used. The larger the 

 number, the closer the range obtained in duplicate sets. However, for all practical 

 purposes it is believed that twenty-five to thirty tubes of the same suspension heated 

 for at least four different times will give reliable and fairly consistent information. 

 Single-tube results are valuable in preliminary tests to locate the range within which 

 all but the occasional spore is destroyed. A few tubes, all heated alike, determine the 

 approximate percentage survival and serve to establish the range of maximum re- 

 sistance. The actual death-time of the more resistant spores can be determined only 

 by heating a large number of tubes. This method has a direct bearing on all steriliza- 

 tion processes and permits of the accurate determination of thermal death-times.' 



' For a discussion of other aspects of this subject, cf. chaps, vi, xxii, and xxxii in this volume. 



