292 



DETERMINATIONS OF THERMAL DEATH-TIME 



The phosphate mixture is easily duplicated and undergoes practica^y no change during heat- 

 ing. Experience has shown that the resistance of spore suspensions can be readily standard- 

 ized on this basis, and this method is highly recommended. 



INFLUENCE OF THE HYDROGEN-ION CONCENTRATION 



The effect of the hydrogen and hydroxyl-ion concentration on the heat resistance 

 of spores of CI. botulinum is shown in Chart L' In the tests illustrated by the chart, 

 carefully washed spores had been suspended in two different series of solutions of vary- 

 ing pH values: {a) phosphate mixture series f — Na2HP04 and KH^POJ varying 



N 

 from pH 3.5-8.8; (//) double-strength veal infusions to which varying amounts of 



hydrochloric acid, citric acid, and sodium hydroxide had been added. It will be seen 



TABLE II 

 Heat Resistance or CI. botulinum Spores in Various Solutions 



* The figures given in the -|- column are observed survival times and those in the — column destruction times. 



that the maximum resistance in the phosphate series is slightly on the acid side (pH 

 6.3-6.9), and in the veal-infusion series the influence of the hydrogen-ion concentra- 

 tion on the heat resistance is striking below pH 6.0 and above pH 9.0. 



Chart I also shows graphically the spore resistance of two dilTerent suspensions of 

 CI. bohdinum in juices of different varieties of canned food. The results differed funda- 

 mentally from the phosphate and veal-infusion series, indicating that other factors to- 

 gether with the hydrogen-ion concentration affect the thermal death-time. For ex- 

 ample, spores heated in ripe ohves with a pH 7.93; corn, pH 6.35; and spinach, 

 pH 5.05 exhibited approximately the same resistance, while in asparagus with a pH 

 5.25 and 5.55 sterilization was more readily accomplished. In food juices with a pH 

 value below 4.5 the hydrogen-ion concentration had a decided influence on the de- 

 struction of CI. botulinum. The results by Weiss^ are also included in Chart I by a 

 broken line, and although the spores appear to be less heat resistant, the .same rela- 

 tionship exists. Similar observations, many of which are unpublished, have been 

 made by the writer and his associates with thermophilic organisms both in phosphate 

 solutions and food juices. 



' Ibid. 



^ Weiss, H.: op. cil., 29, 362. 1921. 



