COMMERCIAL CANNING OF FOODS. 9 



hard exhaust for three minutes is usually employed, but a better result ia obtained 

 ■ff-hen 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 aii". 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 i^er 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 arc 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- 

 p*irature, 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 turncnl in to cover tlie cans and the steam ia 

 admitted until the t<;mpcraliire has been attained. In processing fruits it is custom- 

 ary to use long vats conl^ining iKjiliiig wat<.'r and equipped with automatic conveyers, 

 which carry the cans or crates tlirougli at a spctid sufficient to process them for the 

 necessary length of time. Tliis period varies with the product. Sterilization depends 

 on adminiHtering 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. 



Ah Hi/on as tlie processing is completed, tlu; (!aiis sliould be cooled with watt^r. 

 UnleHH this i.s done, tlie licaL will b.; held so long that tlie contents become over- 

 'rKjkfd -friiil-H arr; sofu-ned, and tomatoes l)ccom<; liquid, ev<;n blackciii, p<!aH break 



iiid make niuddy liquor, while corn acquires a brown color and a Hcorchcd taste. 



1 he crxWing may be done by turning cold water into the ndort, by removing the 



