294 DETERMINATIONS OF THERMAL DEATH-TIME 



The hydrogen-ion concentration of food juices heated in soft-glass changed less 

 than in hard-glass tubes, due largely to the fact that the reaction of the food juices 

 generally decreases during heating. In the case of corn juice, with an initial pH of 

 6.0 inoculated with 22,000 spores of a thermophilic organism, the destruction times at 

 100° were 25 hours in soft-glass and 21 hours in hard-glass tubes. Although no general 

 statement can be made regarding the relative merits of these two types of glass in 

 heat-resistance tests, yet the influence should be determined for each solution. It is 

 just as important, however, to know the hydrogen-ion concentration of the solutions 

 during the entire heating period. 



INFLUENCE OF NUMBERS OF BACTERIA 



Harrison and Hood' observed that the thermal death-time depended on the 

 presence or absence of clumps. Spores from filtered material having a thermal death- 

 time ranging from i to 10 minutes in boiling water survived from 30 minutes to 3 

 hours if unfiltered. 



The decided influence of numbers on the heat resistance of bacteria has been 

 pointed out by several investigators.^ However, this is observed only when dilutions 

 of a given suspension are used in the tests. In fact, owing to the remarkable varia- 

 bility of the resistance of spores of the same strain produced under identical condi- 

 tions, cultures containing less than a million may be more resistant than others con- 

 taining several billion spores. 



Our observations have consistently shown that the larger the number of spores in a 

 given suspension the longer the time necessary to destroy them. This statement is 

 supported by experiments involving an extensive series of packs of canned foods,-' 

 both artificially and naturally contaminated with varying numbers of spoilage organ- 

 isms. It was shown that the greater the contamination, the heavier the spoilage losses 

 from understerilization. This appears to be contrary to the statement made by Hast- 

 ings, Fred and Carroll to the effect that "certainly one million spores should include 

 all grades of heat resistance." 



EFFECTS OF SALTS AND OTHER SUBSTANCES 



The heat resistance is also affected by the presence of varying amounts of sodium 

 chloride in the heating medium. The thermal death-time of CI. botulinmn was not 

 reduced until the salt concentration reached 8 per cent, and a very decided de- 

 crease was noted with 10 and 20 per cent solutions of sodium chloride. The addition 

 of 0.5 and I per cent sodium chloride made the destruction more difficult than in a 

 2 or 3 per cent concentration.^ Viljoen,^ studying the effect of different amounts of 

 sodium chloride on thermophilic spores, reports a protective influence up to a con- 

 centration of ^-Tyh per cent. A 4 per cent solution showed either no effect or was only 



'Harrison, F. C, and Hood, E.: Proc. Roy. Soc, Canada, 17. 1923. 



' Bigelow, W. D., and Esty, J. R.: loc. ciL; Esty, J. R., and Meyer, K. F.: loc. cit.; and Weiss, 

 H.: op. cit., 28, 70. 1921. 



3 Esty, J. R.: Canning; Age, 5, 179-81, 236. 1924; Cameron, E. J., Williams, C. C, and Thomp- 

 son, R. J.: article to appear in early number of J. Bad. 



"Hastings, E. G., Fred, E. B., and Carroll, W. R.: Centralbl. f. BakterioL, Abt. H, 67, 165. 

 1926. 



sEsty, J. R., and Meyer, K. F.: loc. cit. H^iljoen, J. A.: J. Infect. Dis., 39,286. 1926. 



