COMMERCIAL CANNING OF FOODS. 8 
ing apparatus. Thusa time and temperature curve is obtained directly. One of the 
important points learned from the latter apparatus was the effect of stirring or agitat- 
ing the contents of cans which ordinarily required long cooking. A can of corn in a 
retort requiring 65 minutes to reach 245° F. requires only 30 minutes when rolled back 
and forth. The effect of the agitation was a shorter cooking, a brighter color of the 
corn, and a bright can on the inside. The principle is good, but some mechanical 
difficulties in successful operation have yet to be-overcome. 
The penetration of heat in the can is dependent almost wholly upon the ease with 
which convection currents are set up, occurring most rapidly in products which per- 
mit the free circulation of water, weak brine, or sirup between the solids, as in peas, 
and least rapidly in the absence of free liquid, asin dry-packed sweet potatoes. Prod- 
ucts having a heavy though uniform consistency, like pumpkin and squash, require 
a long time for heat to penetrate to the center of the can. Heavy tomato pulp takes 
a much longer time to reach the boiling point than canned tomatoes, and soft ripe 
fruits, as apricots and peaches, need more time to become sterilized than green fruit, 
not because the germs are more resistant, but because the heat can not penetrate as 
readily as when the liquid circulates freely between the solid pieces. Failure to 
recognize this principle of the movement of heat in liquid, semi-liquid, and solid 
substances has caused the loss of thousands of cases of foods. Mechanical agitation 
shortens the period of cooking, especially in foods of heavy body; at the same time 
it places a greater strain upon the can, with a tendency to increase the number of 
leakers. : 
The varying temperatures and methods used in canning cause strains upon the 
containers, which may be comparatively light or so severe as to cause leakage. The 
contents expand as the temperature rises above that at which the sealing was done 
and contract as it goes below that point. The internal pressure, therefore, reaches 
the maximum in foods packed cold and processed at the boiling point or above in the 
open, and, conversely, the vacuum is highest in those filled near the boiling point and 
stored very near that of freezing. If the processing be done in a retort, the internal 
pressure will be the same as that induced by heating to 212° F., plus the number of 
pounds used in the retort, but a strain is produced only by such part of the pressure 
as is developed in bringing the contents to the boiling point as long as the retort is 
closed. For example, if a can of peas be sealed at 160° F., placed in a retort, and 
processed at 240° F., the can will first be subjected to a steam pressure of about 10 
pounds from without; the internal pressure will rise until the temperature has reached 
212 °F., at which point the pressure will show about 5 pounds, and as the temperature 
approaches that of the retort the pressure will show about 15 pounds, only 5 pounds 
- of which will exert any strain until the retort is opened, when the whole becomes 
effective, gradually decreasing as the cooling takes place. 
If, however, when the process is completed, the retort remains closed and a stream 
of cold water is admitted to chill the cans, the first effect is to condense the steam 
in the retort suddenly, thus causing a vacuum. This has the effect of removing 15 
pounds of atmospheric pressure from the outside of the can or virtually adding the 
equivalent of that much pressure within, giving a total of about 30 pounds internal 
strain. Thisis sufficient to break some cans, particularly No. 3 or above, and accounts 
for many slow leaks. The more suddenly the strains occur, the greater the percentage 
of leaks. Processing in retorts is accomplished with little loss, if on admitting the 
steam the pressure be gradually applied for a few minutes and in turning off a vent 
be opened and a little time be given to cool. 
In order to work the retorts rapidly and avoid the severe strains, air may be intro- 
duced under a pressure equal to that of the steam as the latter is being turned off. 
Water for cooling may then be introduced without danger. By this method square 
cans, gallon and even 5-gallon cans may be processed without the losses formerly 
experienced. In the open calcium chlorid or other bath the cans are subjected to 
the maximum internal pressure throughout the entire operation. 
