Many fresh foods contain from 10 percent to more than
30 percent air. How long canned food retains high
quality depends on how much air is removed from food
before jars are sealed.
Raw-packing is the practice of filling jars tightly
with freshly prepared, but unheated food. Such foods,
especially fruit, will float in the jars. The
entrapped air in and around the food may cause
discoloration within 2 to 3 months of storage.
Raw-packing is more suitable for vegetables processed
in a pressure canner.
Hot-packing is the practice of heating freshly
prepared food to boiling, simmering it 2 to 5 minutes,
and promptly filling jars loosely with the boiled food.
Whether food has been hot-packed or raw-packed, the
juice, syrup, or water to be added to the foods should
also be heated to boiling before adding it to the
jars. This practice helps to remove air from food
tissues, shrinks food, helps keep the food from
floating in the jars, increases vacuum in sealed jars,
and improves shelf life. Preshrinking food permits
filling more food into each jar.
Hot-packing is the best way to remove air and is the
preferred pack style for foods processed in a
boiling-water canner At first, the color of hot-packed
foods may appear no better than that of raw-packed
foods, but within a short storage period, both color
and flavor of hot-packed foods will be superior.
Controlling Headspace
The unfilled space above the food in a jar and below
its lid is termed headspace. Directions for canning
specify leaving 1/4-inch for jams and jellies,
1/2-inch for fruits and tomatoes to be processed in
boiling water and from 1- to 1-1/4-inches in low- acid
foods to be processed in a pressure canner This space
is needed for expansion of food as jars are processed,
and for forming vacuums in cooled jars. The extent of
expansion is determined by the air content in the food
and by the processing temperature. Air expands greatly
when heated to high temperatures; the higher the
temperature, the greater the expansion. Foods expand
less than air when heated.
* USDA Agriculture Information Bulletin No. 539 (rev. 1994)
* Meal-Master format courtesy of Karen Mintzias
