210 BACTERIA 



The somewhat difficult problem of drawing off the pasteur- 

 ised milk from the vat without reinfecting it by contact with 

 the air is solved by placing a valve inside the chamber, and 

 by means of a pipe leading the pasteurised milk directly and 

 rapidly into the coolers. These are of two kinds, which may 

 be used separately or conjointly. In one set of cylinders 

 there is cold circulating water, in the other finely crushed ice. 



Domestic pasteurisation can be accomplished readily by 

 heating the milk in vessels in a water-bath raised to the 

 required temperature for half an hour. 



Without entering into a long discussion upon the various 

 methods adopted, we may summarise some of the chief 

 essential conditions. It need scarcely be said that the 

 operation must be efficiently conducted, and in such a way 

 as to maintain absolute control over the time and temper- 

 ature. The apparatus should be simple enough to be easily 

 cleansed, sterilised, and economical in use. Arrangements 

 must always be made to protect the milk from reinfection 

 during and after the process. The entire preparation of the 

 milk for market may be summed up in four items : 



1. Pasteurisation in heat reservoir. 



2. Rapid cooling in water- or ice-coolers. 



3. All cans, pails, bottles, and other utensils to be thoroughly 

 sterilised in steam. 



4. The prepared milk must be placed in sterilised bottles and 

 sealed up. 



The quality of the milk to be pasteurised is an important 

 point. All milks are not equally suited for this purpose, 

 and those containing a large quantity of contamination, 

 especially of spores, are distinctly unsuitable. Such milks, 

 to be purified, must be sterilised. Dr. Russell has laid 

 down a standard test for the degree of contamination which 

 may be corrected by pasteurisation by estimating the degree 

 of acidity, a low acidity (e. g., 0.2 per cent.) usually in- 



