This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
All air must be exhausted from jars and cans or the unit may fail
to develop a good vacuum seal. The absence of air is critical also for
simple food safety as free oxygen is eliminated and the plethora of possible
bacteria which need oxygen do not have the where-with-all for life. The deadly
c.Botulinum which is anerobic ( does not require oxygen to sustain its life
cycle ) is handled in its own class.
"Hot-pack" is used for a good reason other than utility. When heated,
food expands and expels air. When we put hot food into a jar, spatula the air
bubbles out and lid it up, we have created an environment where "a" vacuum will
develop. As the food cools it will contract and create a partial vacuum. We
"process" even with hot-pack as we strive for a strong vacuum and sufficient
heat to kill toxins. It behooves a canner to use sterile vessels for food no
matter what will be the processing method because initial sterility reduces the
bacteria count. It is a well established fact of food science that the number
of bacteria at the origin has a huge bearing on the outcome.
Headspace was not the answer here. Headspace ( a valuable tool) is
required so that the food may expand and thus drive the air out. Getting
headspace correct is necessary so that the food expansion is just right so
that the food then occupies the entire vessel. If the headspace is too large
the air will compress on top of the food with a weak vacuum. Canners new to
wide mouth jars see this problem. eg. They are used to 1/2" - vertical height
in a standard mouth jar. When usng a wide mouth jar of dsame size and same 1/2"
height of headspace there about 30% more volume in the wide mouth 1/2"
headspace than in the standard mouth jar. The solution here is adjust the
headspace properly - reduce the headspace by 30%.
Last word on getting the air out has to do with storage. Processed jars
which develop a good vacuum may still contain (trapped bubbles) residual air.
This air may rise to the surface and release the seal. If the product is viscous
enough it will remain internal and simply cause premature oxygenation -
Canned foods held for several years may be seriously oxydized and be relatively
unrecognizable. Do process jars properly coming OUT of the pantry as well as
prior to going in. - ED.