12 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 



sometimes the organisms were not all destroyed, and in this connec- 

 tion state : 



Nothing in our experiments throws any doubt upon the thermal death points 

 of the microorganisms tested. We are sure that if the milk reaches 140 F. 

 and is held there for 20 minutes it will kill tubercle, typhoid, and diphtheria 

 bacilli. Our experiments show that milk pasteurized at this temperature for the 

 specific time may not always, in practice, reach these minimum requirements. 

 It is therefore evident that a liberal factor of safety is necessary in the opera- 

 tion of this type of pasteurizer under commercial conditions. 



They state further: 



Perhaps the best temperature to meet practical conditions is 145 F. and the 

 milk should be held from 30 to 45 minutes. This should give sufficient leeway. 

 If the pasteurizer is set at 145 F. care will probably be taken that it does 

 not go above 148 F. on account of destroying the cream line, and it is not likely 

 that the mixed milk in the holding tank would drop below 140 F., which is 

 the minimum. 



Other experiments are reported by Pease and Heulings (in the 

 Report of the Committee on Milk Supply of American Public Health 

 Association, 1920), in which the destruction of pathogenic organisms 

 was tested under commercial conditions of pasteurization. Some of 

 the pathogenic types were found living after heating to from 140 

 to 141 F. and holding for 15 minutes, but none were found alive after 

 30 minutes' holding. Here again is evidence of the narrow margin 

 of safety when milk is pasteurized at 140 F. for 30 minutes, and 

 the committee expressed the following opinion : 



The committee feels that while enough has been done to indicate clearly that 

 a proper application of heat to a temperature of 140 F. for a minimum period 

 of 30 minutes will destroy substantially all the pathogenic bacteria in milk, 

 still they believe, as already expressed, that a margin of safety for biological 

 reasons calls for the use of higher temperatures of not lower than 145 F. 



The United States Department of Agriculture, since 1910, (2) has 

 advised the use of a temperature of 145 F. for a period of 30 minutes 

 for the pasteurization of milk. Besides insuring an ample margin of 

 safety, a temperature of 145 F. causes a considerably greater destruc- 

 tion of bacteria in milk than 140 F. when held for the same period 

 of 30 minutes. 



Extensive experiments (3) in the research laboratories of the 

 Dairy Division have shown that the thermal death point of a con- 

 siderable number of bacteria lies between 140 and 145 F.; there- 

 fore an increase of 5 above 140 F. produces a great increase in the 

 destruction of bacteria. 



There is a marked tendency in commercial work to pasteurize at 

 or near the minimum temperature requirement necessary to destroy 

 pathogenic organisms, namely, 140 F. Such seems to be the case 

 because of the fear of injuring the cream line. In fact, the opinion 

 is often expressed by milk-plant operators that a temperature of 



