Atmospheric pressure is like the thickness of frosting on a cake. Where
it is thickest it weighs more per square inch than where it is thin. At
sea level, where the atmosphere is the thickest, it is heavier than atop
a mountain.
As altitude increases, atmospheric pressure or its weight per square
inch decreases. Altitude affects the boiling point of water. Where
altitude is least, at sea level, water boils at 212° F. As altitude
increases the boiling point of water decreases.
The same is true in a pressure canner. Under 10 pounds pressure at sea
level, water boils at 240° F. As altitude increases, the temperature in
a pressure canner at 10 pounds of pressure is less than 240°. This
difference is enough to affect the safety of canned products at
altitudes above 2,000 feet.
[Illustration: Fred Farout
Processing times and pressures should be adjusted for altitude
change.]
Processing time for a particular vegetable is the time it takes to heat
the coldest part of the jar to a temperature of 240° F, and maintain it
long enough to kill any _C. botulinum_ spores present. At an altitude of
2,000 feet, it takes 11 pounds of gage pressure for water to boil at
240°. For each additional 2,000 feet increase in altitude, 1 pound of
pressure should be added.
For pressure canners that have the pressure control, the 15 pounds
pressure weight should be used at altitudes above 2,000 feet for canning
low-acid food.
At the end of processing, pressure inside the jars as well as inside the
canner is 10 pounds. The pressure inside the canner should be allowed to
drop slowly. If pressure inside the canner is released too rapidly,
pressure inside the jars will be great enough to force the contents,
especially liquid, out of the jars. This may prevent a jar from sealing
if a piece of food lodges on the top of the sealing rim. It may even
break the jar.
Remove the lid from the canner as soon as the pressure drops. Jars
should then be taken out and allowed to cool to room temperature
quickly. A type of non-toxic spoilage called flat sour can occur if the
jars are allowed to stand in the canner for long periods.
When using the steam-pressure canner, the pressure given is for
altitudes less than 2,000 feet above sea level. If you live in an area
with a higher altitude, it is necessary to make an adjustment in
pressure. See the next chapter for details.