Tin cans are sealed before processing. The temperature of the food in
the cans must be 170° F. or higher when the cans are sealed. Food is
heated to this temperature to drive out air so that there will be a good
vacuum in the can after processing and cooling. Removal of air also
helps prevent discoloring of canned food and change in flavor.
Food packed raw must be heated in the cans (exhausted) before the cans
are sealed. Food packed hot may be sealed without further heating if you
are sure the temperature of the food has not dropped below 170° F. To
make sure, test with a thermometer, placing the bulb at the center of
the can. If the thermometer registers lower than 170°, or if you do not
make this test, exhaust the cans.
_To exhaust_, place open, filled cans on a rack in a kettle in which
there is enough boiling water to come to about 2 inches below the tops
of the cans. Cover the kettle. Bring water back to boiling. Boil until a
thermometer inserted at the center of the can registers 170° F.—or for
the length of time given in the directions for the fruit or vegetable
you are canning.
Remove cans from the water one at a time, and add boiling packing liquid
or water if necessary to bring head space back to the level specified
for each product. Place clean lid on filled can. Seal at once.