Apparatus for Pasteurization 123 
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 soon have much more perfect apparatuses for 
this purpose than are at present available. The per- 
fect pasteurizing machine should cover the following 
points: Quick, perfect and uniform heating of the 
milk; perfect control of the temperature; quick and 
uniform cooling; 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 
cent of lactic acid is too sour for satisfactory re- 
sults. Inasmuch as this amount of acid cannot 
readily be detected by the senses of smell or taste, 
*Wisconsin Agr. Exp. Station, Bulls. 44 and 52. 
