PASTEURIZATION 199 



beat, that is, they are all rendered practically inert at a 

 temperature of 60° C. Therefore, the ordinary tempera- 

 tures used in pasteurization would also destroy the true 

 toxines. 



Are there any poisonous substances in milk that are not 

 destroyed by heat? There may be such substances, but we 

 do not know what they are. Experience teaches us that 

 they must be exceedingly rare. It must further be re- 

 membered that it heat-resisting chemical poisons (so-called 

 ptomaines) are present, they will be in the raw milk as 

 well as in the pasteurized milk. 



The effect of heat upon the harmful bacteria and their thermal 

 death-points 



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 diflBculties and pitfalls; diflferent investigators have 

 come to widely diflferent results. Some of these discre- 

 pancies 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 differences 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 surface layer may remain cooler than the body of 

 the Uquid. 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 the thermometer in the deeper layers. 



