Abstract
1. This paper deals with the effects of the hydrogen ion concentration of the media of the bacteria (Bacillus A, B and C isolated from the interior of rice seeds) on their thermal death times.
2. The experiments were carried out by such method that the test tubes containing 10 cc of the bouillon nutrient broth, the reaction of which had been adjusted to pH 5.0, 7.0 or 7.2, and 8.6 or 8.8 (after steam sterilization) with N/1 HCl or NaOH, and in which the bacteria were suspended, were placed for various lengths of time in a water bath, the temperature of which was electrically controlled, after which the tubes allowed to stand for two or three days at 28°C. The bacteria tested were cultured in bouillon with reactions of pH 7.0 or 7.2 for 24 hours at 28°C. Nine strains of Bacillus A, four strains of Bacillus B, and two strains of Bacillus C were used.
3. The thermal death time of Bacillus A at 47.5±0.5°C. seems to fall between 35 and 60 minutes in the medium of pH 5.0, between 15 and 50 minutes in that of pH 8.6 or 8.8, and in more than 60 minutes in that of pH 7.0 or 7.2; that of Bacillus B at the same temperature in more than 60 minutes in that of pH 5.0 and 7.0 or 7.2, and between 25 and 35 minutes in that of pH 8.6 or 8.8; while that of Bacillus C at the same temperature seems to be more than 60 minutes, regardless of the hydrogen ion concentration of the medium.
4. The thermal death time of Bacillus A at 50±0.5°C seems to fall between 10 and 20 minutes in the medium of pH 5.0, between 15 and 60 minutes in that of pH 7.0 or 7.2, and in less than 5 minutes or between 5 and 10 minutes in that of pH 8.6 or 8.8; that of Bacillus B at the same temperature between 10 and 20 minutes in that of pH 5.0, between 20 and 25 minutes in that of pH 7.0 or 7.2, and in less than 5 minutes in that of pH 8.6 or 8.8; that of Bacillus C at the same temperature between 50 and 60 minutes in that of pH 5.0, in more than 60 minutes in that of pH 7.0 or 7.2, and between 15 and 30 minutes in that of pH 8.6 or 8.8.
5. The thermal death time of Bacillus A at 52.5±0.5°C seems to fall between 5 and 15 minutes in the medium of pH 5.0, between 10 and 20 minutes in that of pH 7.0 or 7.2, and in less than 5 minutes or between 5 and 10 minutes in that of pH 8 6 or 8.8; that of Bacillus B at the same temperature between 5 and 10 minutes in that of pH 5.0, between 15 and 20 minutes in that of pH 7.0 or 7.2, and in less than 5 minutes in that of pH 8.6 or 8.8; that of Bacillus C at the same temperature between 10 and 20 minutes in that of pH 5.0, between 20 and 25 minutes in that of pH 7.0 or 7.2, and between 5 and 15 minutes in that of pH 8.6 or 8.8.
6. The thermal death times of Bacillus A and B at 55±0.5°C. seem to fall in less than 5 minutes, irrespective of the hydrogen ion concentration of the medium, excepting that strain No.22 of Bacillus A, isolated from the seed of Ogami, a variety of rice, were killed in less than 10 minutes in the medium of pH 7.0 or 7.2; that of Bacillus C at the same temperature in less than 5 minutes or between 5 and 10 minutes in that of pH 5.0, between 10 and 15 minutes in that of pH 7.0 or 7.2, and in less than 5 minutes in that of pH 8.6 or 8.8.
7. Regardless of the hydrogen ion concentration of the medium and of the strains tested, the thermal death times of Bacillus A, B, and C seem to fall in more than 60 minutes at 45±0.5°C and in less than 5 minutes at 57.5±0.5°C. and 60±0.5°C.
