62 MILK AND THE PUBLIC HEALTH CHAP. 



Using milk artificially inoculated with tubercle bacilli and 

 then made into cheese, Galtier found the bacilli alive in cheese 

 2 months and 1 days old respectively. Harrison made cheese 

 from milk artificially inoculated with tubercle bacilli. In cheese 

 made by the Emmental method, they died between the 34th 

 and the 40th day; in Cheddar cheese, after 62 to 70 days. 



D. THERMAL DEATH-POINTS OF PATHOGENIC BACTERIA IN 



MILK 



In view of the extent to which the sterilisation and parti- 

 cularly the pasteurisation of milk is carried out commercially, 

 it is of fundamental importance that accurate information 

 should be available as to the temperatures at which the patho- 

 genic bacteria possibly to be found in milk are killed. For 

 practical purposes it is necessary to distinguish between 

 laboratory results and those obtained under the modifying 

 influences of practical working conditions. 



Thermal death-points cannot be stated as definite tempera- 

 tures since obviously there are two closely related facts the 

 temperature of exposure and the period of exposure. In 

 addition, factors not without influence are the vitality of 

 particular strains of bacilli and the physical conditions of 

 exposure. Among bacteria some strains are more resistant 

 to physical conditions than others ; this must be taken into 

 account, and any accepted figure must relate to the death- 

 point of the most resistant strains of individual bacilli likely 

 to be present. In experimental work it is common to find 

 that certain bacilli, even from a single culture, are more 

 resistant than the rest and survive the heat or chemical 

 employed. 



As regards the physical conditions of exposure, since 

 only a single fluid is under consideration, it might be 

 supposed that these would always be the same, but this is 

 not quite the case. The chemical reaction of milk varies, 

 and this may influence a little, while much more important 

 is the presence or absence of surface scum. If milk is 

 rapidly evaporated a surface pellicle forms consisting of 

 coagulated albumens with entangled fat and bacilli. Such 

 a pellicle is at a lower temperature than the fluid beneath, 



