TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 5 
the subsequent development of the organisms causing spoilage must 
be provided for first, but for the sake of the quality it should not be 
prolonged beyond the time essential to insure the keeping of the 
product and the safety of the food for human consumption; in other 
words, the temperature factor must be under careful control. 
Another factor greatly affecting the quality of the finished product 
is the amount of air present in the can. The presence of air may 
result in enormous strains on cans, causing leakage and loss, in the 
discoloration of the product, and in increased activity of the food 
material upon the metal of the container. In other ways also it has 
‘a direct bearing upon the quality of the product. It is evident, there- 
fore, that this factor likewise must be kept under control. This may 
be accomplished by some method of exhaust. 
Inasmuch as under present conditions the exhaust is effected pri- 
marily by the use of heat and inasmuch as the initial temperature of 
the material in the can affects the time-temperature curves, it is ap- 
parent that in determining the proper temperature and duration of 
the processing period the exhaust must be carefully considered. In 
other words, the exhausting procedure must be standardized before 
the processing temperatures can be prescribed and the processing 
periods determined. 
As is well known, exhausting may be accomplished in three ways: 
First, by filling the cans with the material, after it has been heated to 
the desired temperature, as with corn, sweet potatoes, etc.; second, 
by subjecting the cans of material to a preliminary heating before 
sealing; and, third, by mechanically removing the air, or sealing in 
a vacuum chamber. 
Whichever method is used, it is especially important that so far as 
possible uniform temperatures shall exist throughout the entire mass 
in the can and that processing shall begin at the earliest possible 
moment, preferably at once. As has been shown graphically by 
Bigelow and his collaborators (3, p. 63), unless this is the case the 
time-temperature curve at the center of the can may actually fall dur- 
ing the first part of the processing period and thus make impossible 
an accurate knowledge of the temperature changes taking place dur- 
ing the processing period. Under such conditions careful standard- 
ization is out of the question. 
With these things in mind and with a view to gaining a definite 
knowledge of the temperature changes taking place under carefully 
controlled conditions as regards the time-temperature relation in 
food materials sealed at different temperatures and the relation of 
these matters to pressures developed and vacuum obtained, the pres- 
ent work was undertaken. 
