TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 3 
involved in work of this sort, use has been made of numerous experi- 
mental curves, tables, and formulas, which make the work of much 
value. 
Especially useful are the formulas given for calculating from a 
previously determined curve the time-temperature curves for cans 
of food having any initial temperature; for calculating curves for 
cans processed at any temperature from a curve for a can with a 
known processing temperature; and for calculating curves for a con- 
tainer of any size from an experimental curve determined for a can 
of known size. 
Few cooling curves are given. The statement is made, however, 
that the temperature will descend during the cooling operation by ex- 
actly the same curve that is followed during the processing. What 
has been said upon this matter in connection with the review of 
Thompson’s article above applies equally here. 
Since certain matters discussed in these papers bear directly upon 
the work herein reported, more detailed reference will be made to 
them from time to time in the body of this bulletin. 
The necessity of properly exhausting cans of food material either 
by short heating, by filling in the material while hot, or by mechani- 
cally removing the air in the can before the final sealing, has long 
been recognized. It was early believed that the partial vacuum thus 
formed was largely responsible for the keeping of the food. Later 
when it was found that food spoilage was due to the activities of 
microorganisms this idea was abandoned, though the practice of ex- 
hausting was continued for other reasons. More recently the work 
of Weinzirl (10) has focused attention upon the question whether 
after all the vacuum may not be largely responsible for the keeping 
of many of our canned foods. 
The relation of exhaust and vacuum to swells and springers in 
canned foods was clearly set forth by Bigelow (1) in 1914. The 
following year Bitting (4) called attention to the effect which the 
vacuum has upon the amount of dissolved tin in canned foods and 
pointed out how the action of the food material on the metal of the 
container may affect the vacuum. 
In 1916 the same author (5) entered into a more detailed consider- 
ation of the subject of exhaust and vacuum. Some of the reasons 
given for exhausting were as follows: 
(1) To draw in the ends of the cans, thus giving an index to the condition of 
the contents. 
(2) To minimize the action of the contents upon the container. 
(3) To prevent overfilling. 
(4) To prevent unnecessary strains on Gans. 
(5) To produce a desirable effect upon the product itself. 
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