J. RUSSELL ESTY 



295 



slightly toxic. The highest protection occurred from i to 2^ per cent. He also ob- 

 served that the resistance of the total number of spores is increased in a i| per cent 

 salt solution, and that the spores were destroyed according to a logarithmic law. Un- 

 published data on the heat resistance of thermophilic organisms and non-spore- 

 formers show that different concentrations of sugar, either alone or in combination 

 with salt, also influence the thermal death-time. 



The studies of Dickson and his associates' definitely indicate a protective in- 

 fluence when spores are heated in a medium stratified with oil. Their data leave no 

 doubt that CI. botuliniim spores heated and incubated in broth, covered with a thin 

 layer of mineral oil, exhibited thermal death-times which were very much greater 

 than those ordinarily recorded in identical broth without the covering of ofl. Table 

 III iflustrates their results, the data having been selected by the writer from Table 



III, page 479, of their article.' 



TABLE III 



The increased resistance is much more apparent when the spores are heated at 

 higher temperatures (115° and 121° C). According to Dickson, the spores immersed 

 in oil are exposed to the maximum heat for a shorter time than those suspended in 

 broth and, therefore, may not receive the same "sterilizing effect." Although the 

 exact mechanism of this protective influence of oil on the resistance of botulinum 

 spores is not understood, yet it is now appreciated that these observations have a di- 

 rect bearing on the processing time of all foods which contain oil, as was first pointed 

 out by Dickson. 



EFFECTS OF DORMANCY 



Another feature which deserves consideration in the interpretation of heat resist- 

 ance data is the dormancy factor characteristic for certain organisms. The germina- 

 tion of heated spores of many bacteria when transferred to a favorable environment 

 occurs promptly, whereas with some other organisms, notably CI. botulinum, the ger- 

 mination of the surviving cells may be markedly delayed even under ideal conditions 

 of subcultivation. Dickson and his associates' report the following maximum dor- 

 mancies for heated spores of CI. bctulinum: 



37 months for spores which were heated and incubated in oil-stratified broth' 

 22 months for spores which were heated and incubated in broth without oil 

 II months for spores which were heated and incubated in agar 



' Dickson, E. C, Burke, G. S., Beck, D., and Johnston, J.: loc. cit. 



■ ^ According to Dickson, since the publication of the foregoing data, observations have been made 

 at frequent intervals on the germination time of spores in the oil-stratified bath series. These show 

 to date a dormancy of sixty-six months for cultures incubated one month at 37.5° C. and then held 

 at room temperature. 



