43. For canning foods by steam pressure, special equipment is necessary.
In one of the steam-pressure methods, what is known as a _water-seal
outfit_ is required, and in the other a device called a _pressure
cooker_ is employed. The work of getting the containers ready, preparing
the food for canning, packing it into the jars, and sealing and testing
the jars is practically the same in the steam pressure methods as in the
cold-pack methods. The difference lies in the cooking and sterilization
of the foods after they are in the jars and partly sealed and in the
rapidity with which it may be done.
44. CANNING WITH A WATER-SEAL OUTFIT.--A water-seal outfit, which may be
purchased in stores that sell canning supplies, consists of a large
metal vessel into which fits a perforated metal basket designed to hold
jars of food. This vessel is also provided with a tight-fitting cover
having an edge that passes down through the water, which is placed in
the bottom of the vessel. When heat is applied to the bottom of the
vessel, the water inside of it is changed into steam. The cover prevents
the steam from passing out, and it collects in and around the metal
basket supporting the jars of food. Enough steam is generated in this
outfit to raise the temperature about 4 to 6 degrees above the boiling
point. Thus, the water-seal outfit will cook the food in the cans in
about one-fourth less time than will the water bath of the one-period
cold-pack canning method.
[Illustration: FIG. 20]
45. CANNING WITH A PRESSURE COOKER.--For canning by steam pressure, a
number of different kinds of pressure cookers are to be had, but in
principle they are all alike and they are always made of heavy material,
so as to withstand the severe steam pressure generated in them. In Fig.
20 is shown one type of pressure cooker. It is provided with a bail, or
handle, for carrying it and with clamps that hold the cover firmly in
place. Attached to the cover is a steam gauge, which indicates the steam
pressure inside the cooker, and a pet-cock, which is used to regulate
the pressure. On some cookers, a thermometer is also attached to the
cover. Also, inside of some, resting on the bottom, is an elevated rack
for supporting the jars of food that are to be sterilized and cooked. In
operating a pressure cooker, water for generating steam is poured in
until it reaches the top of this rack, but it should not be allowed to
cover any part of the jars of food. Steam is generated by applying heat
to the bottom of the cooker, and the longer the heat is applied the
higher the steam pressure will go.
It is possible to secure a steam pressure of 5 to 25 pounds per square
inch in a cooker of this kind. This means that the temperature reached
will vary from a few degrees above boiling to about 275 degrees
Fahrenheit. At a pressure of 20 pounds, the temperature will be about
260 degrees. The heavier the material used for a cooker and the more
solid the construction, the higher may go the steam pressure, and, of
course, the temperature. Some cookers of light construction will not
permit of a pressure greater than 5 pounds, but even such cookers are
very satisfactory. It is the high temperature that may be developed in a
pressure cooker that greatly shortens the time required for cooking jars
of food and making them sterile.