K'ew York Agricultural Experiment Station. 143 



When plates that had been kept 48 hours at 30° C were left 

 3-5 days at 21° C (70° F.) there was a small average increase on 

 the second count. This indicates that some of the organisms 

 present in the milk did not thrive at the higher temperature. 



The above results obtained from tests upon 14 days at 70° C. 

 (158° F.) illustrate the uncertainty of the pasteurizing action at 

 that temperature and in this particular quite agTee with the re- 

 sults on Ip previous days when preliminary trials were being made. 

 They also show what misleading conclusions might be drawn when 

 generalizations are made after one or two observations. 



It should be remembered that in any heating of the milk the 

 most desirable class of acid formers will be among the first to be 

 killed and the residue is composed of germs not likely to improve 

 the flavor of the butter. Just how many of this class of bacteria 

 may be left in the milk without impairing the quality of the prod- 

 uct, like the problem of how many weeds can be left in the field 

 without detriment to the crop, is not clearly 'understood and a 

 conservative disposition would favor their reduction to the lowest 

 practical limits. 



A most important fact shown is that 70° C. (158° F.) lies near 

 the lower limit of the killing effect of heat applied in this way. 

 When operating a pasteurizer in a practical way, temporary re- 

 ductions of temperature are almost certain to occur, and if this 

 reduction goes much below 70° C. the killing effect upon bacteria 

 will be very slight. 



The results of pasteurizing at 80° C. (176° F.) show a surpris- 

 ing reduction in the germ life and this reduction was accomplished 

 with very slight variation on each of the 25 days tested. These 

 25 tests gave an average of only 117, with a maximum of 297 

 and a minimum of 20 living germs per c. c. in the pasteurized 

 milk. 



Comparing this average of 25 determinations made after con- 

 tinuous pasteurization with 6,140, the average number of germs 



