MOIST HEAT 113 



lower water content, since there is evidence to show that desiccation can raise 

 the time-temperature limit necessary to cause coagulation of proteins (see Hewlett 

 1909, Cameron 1930). The resistance of both vegetative bacteria and of spores varies 

 considerably with the different species, some being killed much more rapidly than 

 others. The spores of moulds are intermediate in resistance between the vegeta- 

 tive and sporing bacteria ; they require a temperature of 110-115° C. for 1 J hours 

 for their destruction. 



As with desiccation, the higher the temperature, the shorter is the survival 

 time. Thus, if the temperature is raised from 140° to 160° C. spores are killed 

 in 1 to U hours. At 400" C. they are killed in 20-30 seconds (Oag 1940). 



Koch did not regard dry heat as an efficient method of disinfection. Though 

 satisfactory when dealing with naked bacteria, it is quite ineffectual within the 

 times usually employed when the bacteria are protected by textile or other rela- 

 tively non-conducting material ; this is due to the low power of penetration of 

 hot air. Thus, when a bundle of tow measuring 55 X 50 cm. was exposed to a 

 temperature of 140-150° C, the interior after 3 hours had only reached the 

 temperature of 74-5° C. — a temperature quite inadequate to kill the spores enclosed 

 in the bundle. Moreover a temperature of 140° C. is sufficient in a short time 

 to ruin most cloth fabrics. 



Flaming is a useful method of surface disinfection for non-inflammable sub- 

 stances ; its efficacy appears to depend on the temperature to which the exposed 

 surface is raised (Mayser 1925). 



Moist Heat. — Koch (Koch et al. 1881), in conjunction with Gaffky and Loeffler, 

 was the first to make a quantitative study of the germicidal action of moist heat. 

 He found that the temperature required for sterilization of spores was much lower 

 than with dry heat. Thus anthrax spores were killed in 10 minutes at 95° C, 

 and spores present in garden earth in less than 10 minutes at 105° C. He also 

 showed that steam under pressure is more efficient than steam at atmospheric 

 pressure. For the disinfection of clothes, too, he foimd moist heat to be preferable 

 to dry heat, as it has a greater penetrating power. Thus after 4 hours' dry 

 heat at 140-150° C. the temperature inside a roll of flannel was only 83° C, 

 and the contained spores germinated freely, whereas after 1| hours of moist heat 

 at 120° C, the temperature inside was 117° C, and all the spores were dead. Koch 

 was greatly impressed by the value of boiling water ; from numerous experiments 

 he concluded that even spores seldom survive its action for more than a few minutes. 

 We now know that Koch rather over-estimated its efficacy, for there are certain 

 bacteria the spores of which will resist the action of boiling water for hours. This 

 is especially marked with the thermophilic bacteria ; thus, Bigelow and Esty 

 (1920) exposed the spores of thermophilic organisms, suspended in a nutrient 

 medium of pH 6-1 in sealed glass tubes, to various temperatures in oil baths, with 

 the following results : 



Temperature. Killed in 



100° C 1,320 minutes. 



110° C 225 „ 



120° C 23 „ 



130° C 3-5 „ 



140° C 10 „ 



Thus at a temperature of 100° C. they remained viable for nearly a day. 



