Apparatus for Pasteurization. 79 
remain sweet thirty-six to forty-eight hours longer 
at ordinary temperatures than milk not pasteurized, 
from which germs have been excluded with ordinary 
care. 
The problem of successful pasteurization, then, 
depends upon the means of raising the milk in a 
short time to the required temperature, holding it 
there uniformly for ten or twenty minutes, and then 
cooling it rapidly to 50° or below. Several forms 
of apparatus have been devised for this purpose. 
Some of them are fairly perfect, but most of them 
are lacking in some important point. With the pres- 
ent activity in regard to this subject we shall un- 
doubtedly have in the course of a year or two, much 
more perfect apparatuses for this purpose than are 
at present available. The perfect pasteurizing ma- 
ehine should cover the following points : Quick, 
perfect and uniform heating of the milk; _ perfect 
control of the temperature ; quick and uniform cool- 
ing; compact form; ease of cleansing; absence of 
pumping arrangements; security against re-infection 
during the process. 
Selection of milk for pasteurization.—For the best 
results in pasteurizing, it is also essential that the 
milk be as fresh and free from fermentations as 
possible. Russel and Farrington have found* that 
milk that has developed as much as .2 of 1 per 
eent of lactic acid is too sour for satisfactory  re- 
sults. Inasmuch as this amount of acid cannot 
* Wisconsin Agr. Exp. Station, Bulls, 44 and 52, 
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