COMMERCIAL CANNING OF FOODS. 9 
hard exhaust for three minutes is usually employed, but a better result is obtained 
when a lower temperature is maintained for five or six minutes. Meats and fish are 
held from 8 to 20 minutes in a vigorous exhaust. Milk is one of the few products 
packed cold. 
One of the difficulties connected with thorough exhausting has been that it causes 
- expansion of the contents, so that the solids float on the top, which makes capping 
difficult. This is now obviated with the open-top cans by attaching the covers and 
crimping them lightly before they enter the heater, thus holding the solids in, at the 
same time permitting the expulsion of air. It also makes possible the sealing at a 
higher temperature than formerly. The importance of thorough exhausting has 
_ been established in connection with studies upon the effect of the contents on the 
container. Some products which have shown a rather high content of dissolved 
salts of tin are found to have much less when well exhausted, and practically all 
show some diminution. 
A well-exhausted can shows 8 inches or more of vacuum when the can is cold. 
The average of a number of experiments gives the vacuum reading at room tempera- 
ture, where the tipping was 200° F., 16.5 inches; 190°, 15.4 inches; 180°, 13 inches; 
170°, 10 inches; 160°, 8.5 inches; 150°, 8 inches; 140°, 7 inches; 130°, 5 inches; 
and 70°, 1 inch. There are many cans which show 15 inches of vacuum shortly after 
being put up, but this gradually decreases as the pack stands, the rate depending 
principally upon the activity with which the product attacks the container. 
CAPPING AND TESTING FOR LEAKS. 
Open-top cans are sealed by a special machine known as a double seamer. The 
lid is pressed into place and steel rollers crimp it on without acid or solder. This 
action is automatic, a single can at a time, but at the rate of 30 per minute, or 1,800 
per hour. Cans with solder tops are sealed by automatic machinery, 12 at a time, 
85 per minute, or 5,000 per hour. The top is wiped, the cap placed on, acid applied, 
the hot soldering irons drop into place, and the vent is afterwards closed, all in one 
series of operations, without touching by hand. As the cans pass from the capping 
machine they may be submerged in a bath of boiling water to test for leaks. Any 
- imperfection in the can or defect in sealing will be shown by a series of air bubbles 
issuing from the opening, and the can is at once taken out by the inspector for repairs. 
PROCESSING AND COOLING. 
After capping, the cans are processed according to the nature of the contents. The 
cans are collected in large iron baskets, which usually hold 270 No. 2 or 180 No. 3 
cans, and three baskets fill a retort. If the processing is conducted at boiling tem- 
perature, the retort is not closed, but steam is turned into the water which covers 
the cans. If the temperature is to be above the boiling point, the retort is closed 
and either the steam is turned into the retort until the proper pressure and tempera- 
ture have been reached, or water is first turned in to cover the cans and the steam is 
admitted until the temperature has been attained. In processing fruits it is custom- 
ary to use long vats containing boiling water and equipped with automatic conveyers, 
which carry the cans or crates through at a speed sufficient to process them for the 
necessary length of time. This period varies with the product. Sterilization depends 
on administering the proper amount of heat, and heating above the required temper- 
ature or for longer than is necessary only cooks the material to no purpose. 
As soon as the processing is completed, the cans should be cooled with water. 
Unless this is done, the heat will be held so long that the contents become over- 
cooked—fruits are softened, and tomatoes become liquid, even blacken, peas break 
and make muddy liquor, while corn acquires a brown color and a scorched taste. 
The cooling may be done by turning cold water into the retort, by removing the 
