Abstract
1. This paper deals with the influence of hydrogen ion concentration of the media, in which the bacteria were cultured, on their thermal death times.
2. The thermal death times were determined by the following method:-The bacteria under experiment were cultured in bouillon nutrient broth, the reaction of which had been adjusted to pH 5.0, 7.0, or 8.6 with either N/1 HCl or NaOH, for 24 hours at 28°C. Test tubes containing 10 cc of the bouillon nutrient broth (reaction of pH 7.0) in which the bacteria were suspended, were placed at different times in a water bath, the temperature of which was electrically controlled at 50±0.5°C, after which the tubes were left to stand for two or three days at 28°C.
Nine strains of Bacillus A, four of Bacillus B, and two of Bacillus C were examined.
3. The thermal death time of Bacillus A, which was cultured in medium having a reaction of pH 5.0, seems to fall between 5 and 10 minutes, excepting that Str. 22, which was isolated from the seed of Ogami, a variety of rice, was killed between 15 and 20 minutes. The thermal death time of Bacillus B, grown in medium showing a reaction of pH 5.0, seems to fall between 10 and 15 minutes, while that of Bacillus C, grown in medium showing the same reaction, seems to lie between 15 and 25 minutes.
4. The thermal death times of the bacteria that were cultured in medium showing a reaction of pH 7.0, seem to fall between 15 and 60 minutes for Bacillus A, between 20 and 25 minutes for Bacillus B, and in more than 60 minutes for Bacillus C.
5. The thermal death times of Bacillus A and B, grown in medium showing a reaction of pH 8.6, seem to fall between 5 and 10 minutes, with an exception in the case of Str. 22 of Bacillus A, which was killed between 10 and 15 minutes, while that of Bacillus C, which was cultured in medium having the same reaction, seems to lie between 10 and 20 minutes.
6. Regardless of species, the thermal death time of bacteria grown in medium with a reaction of pH 7.0 was greater than that grown in medium having a reaction of pH 5.0 or 8.6, and that grown in medium showing a reaction of pH 5.0 was somewhat greater than that grown in medium having a reaction of pH 8.6. Judging from these results, the thermal resistance of bacteria seems to decrease with increase in the hydrogen or hydroxyl ions in the media in which the bacteria are cultured, and the rate of this reduction due to the hydroxyl ion seems to be somewhat larger than that due to the hydrogen ion.
