Michigan State University Extension
Preserving Food Safely - 01600501
10/13/97
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 3 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.