84 U. S. BUREAU OF FISHERIES 
air, and later, when a hot day comes, the air expands and the cans 
swell. Before sealing, the cans and contents should be heated to 
drive out some of the air, for, if sealed hot, a partial vacuum will 
exist in the cans when they cool. Addition of hot sauce, followed 
by immediate sealing, is one way of warming the cans and contents. 
Another method, more widely used, is to exhaust the filled cans by 
heating them with “live” steam for about five minutes. This 
heating usually is accomplished by conveying the cans through a 
narrow chamber, into which the steam is turned. 
Sealing.—The cans are conveyed from the steamer (exhaust box) 
to an automatic sealing machine, which places a cover on the can 
and then seals it on. “The can carries a flange, which fits into a 
groove in the lid, having an extended edge bent downward. In 

Fic. 15.—Sealing the cans. Exhausted cans are leaving the exhaust box at A and are passing through 
the sealing machines B 
sealing, the flange of the can and edge of the lid are interlocked so 
as to form a double seam. The groove in the lid carries a very thin 
layer of rubber compound, which serves as a gasket between the 
seams. In some plants the can is sealed in two operations—one 
machine just crimps the cover to the can and another seals the can 
after it comes out of the exhaust box. 
Processing.—Sterilization of the contents of the cans is necessary 
to insure their keeping qualities; in addition, more cooking is required 
to soften the bones. Proper heat treatment accomplishes both of 
these steps at one time. 
Trucks containing cans from the sealing machines are wheeled into 
large steel retorts, where the cans are heated with steam under 
pressure. Various temperatures are used in this step. Usually, 
