PASTEURIZATION. 175 
time and temperature are under control. Various pasteuriz- 
ing machines are in use, and it is not within the province of 
this work to recommend any machine. A few words in regard 
to the principles which affect proper pasteurization will serve 
a better purpose. 
SELECTION OF PASTEURIZERS. 
Improper pasteurization is worse than none at all. If 
pasteurization is done at all, it is essential that it be properly 
accomplished. 
There are two things to be sought when purchasing a pas- 
teurizer; viz., (1) durability and capacity, and (2) economic 
efficiency. 
1, Durability and Capacity—The structure of most pas- 
teurizing machines is simple and substantial. They do not 
wear out like a complex piece of machinery. It is essential 
that the machine be strongly made. Heating-walls and other 
portions should be made heavy enough to be consistent with 
its use, and of a substance which will conduct heat rapidly. 
Nearly all the types of pasteurizers are made in different sizes 
to suit the demands. 
2. Economic Efficiency.—Obtaining efficient pasteurization 
economically is the most important question. It is important 
in this connection because it depends upon so many conditions 
which the operator has under control. The degree of con- 
ductivity of heating-surface during operation, may in one sense 
include most of the essential factors which affect pasteurization. 
This in turn depends upon a number of conditions which are as 
follows: 
(1) Kind of material from which the heating-surface is 
manufactured. 
(2) Degree of adhesiveness of milk or cream on heating- 
surface. 
(3) Thickness of layer of condensed steam, on steam side of 
heating-surface. 
(4) Difference in temperature on each side of heating-surface. 
