EXPERIMENT STATION BULLETINS. 



551 



COMTARISON OF METHODS FOR DETERMINING THE THERMAL DEATH POINT. 



The tliermal death points of four bacteria were determiued in sep- 

 arated milli and in cream by each of the three methods described above. 

 The results (Table II), show the harmony of these methods. 



TABLE 2.— COMPARISON OF THE THERMAL DEATH POINT AS DETERMINED BY 



DIFFERENT METHODS. 



THERMAL DEATH POINT OF LACTIC ACID BACTERIA FOUND IN PASTEURIZED 



MILK AND CREAM. 



A number of different species and strains of microorganisms were 

 found in samples of pasteurized milk and cream obtained from various 

 sources, but only those of the acid producing types are considered to 

 any extent in this work. The thermal death point of twelve strains of 

 Bact. lactis acidi and of four strains of B. coll were determined and are 

 tabulated in Table III. A variation of 22° C. (from 56° to 78° C) in 

 the thermal death point of these strains was found. Seven of the twelve 

 Bact. lactis acidi and two of the four B. coli have a thermal death point 

 high enough to survive pasteurization (145° F. for 20 minutes). When 

 bacteria with this inherent property are present in milk or cream we 

 can not hop© to exterminate them by the usual temperature of pasteuri- 

 zation. But when bacteria whose thermal death point is below the tem- 



TABLE 3.— THERMAL DEATH POINT DETERMINATIONS. 

 (Average of 5 determinations in bouillon, open tube method). 



