Heat is a form of energy which flows from hot to cold substances. This
flow occurs by convection, conduction, and radiation. In a pressure
canner, convection and conduction are the primary methods of heat
transfer.
Convection heating occurs in thin liquids and in gases like air and
steam. As molecules are heated, they become lighter and rise to the top
of the jar, displacing cooler ones toward the bottom. This movement is
visible in water that is being heated in a clear glass container.
Convection heating occurs best in liquids like fruit and vegetable
juices or broths. These heat rapidly and thus have shorter processing
times.
Small quantities of starch either added or leached from vegetables slows
down the convection and increases processing time. For example, jars of
liquid containing pieces of green beans or peas would heat more slowly
than apple juice because the pieces would interfere with convection.
A tightly packed jar takes longer to heat than a loosely packed one
which allows some convection heating. Thus it is important not to
over-pack jars as this will decrease the rate of heat penetration.
Some types of food heat by a combination of conduction and convection.
One example is a peach half in thin sirup. The sirup heats by convection
while the peach heats by conduction. Another example is cream style
corn. Initially, the liquid is thin and heats by convection. As the
liquid thickens, it heats by conduction.
Conduction occurs when heat is transferred from one particle or
substance to another right next to it. This is the slowest type of heat
transfer. Foods that mat together, like spinach, or viscous material
like mashed pumpkin, heat by conduction. The larger the pieces of food
or the thicker the puree, the slower the heat penetration.