THE THERMAL DEATH POINTS OF PATHOGENIC MICRO- 
ORGANISMS IN MILK." 
By Milton J. Rosenau, 
Surgeon and Director Hygienic Laboratory, U. S. Public Health and Marine- 
Hospital Service. 
The temperature at which milk should be pasteurized hinges on the 
thermal death points of the pathogenic micro-organisms which con- 
taminate it. The micro-organisms pathogenic for man most frequently 
found in market milk are those causing tuberculosis, typhoid fever, 
diphtheria, scarlet fever, dysentery, and Malta fever. Fortunately 
none of the organisms causing the above-mentioned acute diseases 
have resisting spores. Moderate degrees of heat are, therefore, suffi- 
cient to render milk safe so far as these dangers are concerned. The 
streptococci, staphylococci, and most of the bacteria associated with 
infantile diarrhea are also readily destroyed by heat. 
Although it would appear to be a comparatively simple matter 
to determine precisely the temperature at which micro-organisms die, 
such work is in fact surrounded by many difficulties and pitfalls; 
different investigators have come to widely different results. Some 
of these discrepancies are only apparent and may be explained by the 
relation of time to temperature. The longer the time of exposure, the 
lower the temperature necessary to kill any organism. Differences in 
methods are also responsible for difference in results. 
Among bacteria some strains or races are more resistant to heat 
than others. These differences, which correspond to similar known 
variations in all animal and vegetable species, must be taken into 
account. 
Evaporation takes place so rapidly from exposed fluids that the sur- 
face layer may remain cooler than the body of the liquid. This is 
especially a matter of concern with milk, which sometimes forms a 
scum above 60 ° C., owing to rapid evaporation of the surface layer. 
This scum consists of coagulated albumins in which is enmeshed much 
fat. The bacteria entangled in this surface pellicle may escape the 
heat indicated by thn thermometer in the deeper layers. 
a This article is a brief summary of Hygienic Laboratory Bulletin No. 42, 
entitled “ The thermal death points of pathogenic micro-organisms in milk.” 
For all the details of the work the reader is referred to that bulletin. 
(683) 
