286 DETERMINATIONS OF THERMAL DEATH-TIME 



in a few minutes at temperatures below ioo° C, while others, notably certain soil or- 

 ganisms, B. cylindricus and B. tostus, studied by Blau,' are reported to have with- 

 stood heating in boiling water at ioo° C. for ig hours but were killed in 20 hours. 

 They^ have also determined the thermal death-point of the spores of several aerobic 

 thermophilic bacteria from water. 



Spores of certain obligate thermophiles were found by Bigelow and Esty^ to with- 

 stand boiling at 100° C. in corn juice, pH 6.1, for 21 hours. More recently the same 

 strains, when heated in a phosphate solution, pH 7.0, survived 45 hours' continuous 

 boiling at 100° C. but were killed in 46 hours. This group of organisms (group 100) 

 had been isolated by Cameron and Esty^ as the causative spoilage agent from certain 

 understerilized canned foods. As such, the foregoing findings are not only of academic 

 interest but are also significant when adequate sterilizing processes are considered. 



Numerous articles have been published on the subject of thermal death-points, 

 and in many of these a definite time and temperature have been reported for specific 

 organisms. However, this time- temperature relation has been frequently based upon 

 the study of a single strain or merely a few superficial tests without taking into con- 

 sideration certain fundamental biological principles involved. As a result, one is con- 

 fronted with many conflicting statements which make it extremely difficult, if not 

 impossible, to interpret discordant observations. In many cases it is necessary to 

 make additional tests before the results can be evaluated and safely applied to all 

 life-processes. 



EXPERIMENTS WITH B. TUBERCULOSIS 



Take, for example, the problem of establishing a safe standard for the pasteuriza- 

 tion of milk. This involves a study of the heat resistance of pathogenic organisms 

 which are present in milk, particularly B. tuberculosis, B. typhosus, and the patho- 

 genic streptococci. As regards the tubercle bacillus, at least twenty-five reports were 

 published from 1883 to 1906. 



It appears from the results reported^ that under certain conditions the heat re- 

 sistance of B, tuberculosis may vary from i4o°-i56° F. for 15 minutes, i4o°-i55° F. 

 for 20 minutes, and at 140° F, from 10 minutes to 6 hours, and at 212° F. from less 

 than 30 seconds to 3 hours. 



When we consider these variable results in the light of the apparently successful 

 time and temperature requirements for pasteurization that were in effect in 1923 in 

 one hundred of the larger American cities (142° F. for 30 minutes was required for 

 pasteurization in forty of these cities and the maximum in any case was 145° F. for 

 30 minutes),* it is evident that some of these observations on the heat resistance of the 

 tubercle bacillus are either in error or are not applicable to the commercial process. 

 In this connection it should be borne in mind that heat-resistance data obtained from 



' Blau, O.: Cenlralbl.f. Bakteriol., Abt. II, 15, 97. 1Q05. 



^ Morrison, L. E., and Tanner, F. W.: Bot. Gac, 77, 2, 171. 1924. 



3 Bigelow, W. D., and Esty, J. R.: /. Infect. Dis., 27, 602. 1920. 



•• Cameron, E. J., and Esty, J. R.: ibid., 39, 2, 89. 1926. 



5 U. S. Pub. Health Bull. 147, Part X, Table I, p. 129. 1925. Also cf. chap, xxxii in this volume. 



* Secured from data collected in an unpublished survey by the American Public Health Associa- 

 tion and the U.S. Public Health Service {Am. J. Pub. Health, p. 375. 1927). 



