crystals are changed to vapor, and the vapor passes from the food by diffusion. The
removed vapor is returned to atmospheric pressure and eliminated as water. Freeze-
dehydrated products require protection, since they tend to absorb moisture and oxygen
from the air, because there is a higher concentration of moisture in the atmosphere than
in the food product.
b. In a Processing Plant. The freeze dehydration process in a plant proceeds
as follows: a frozen product is received at the plant from the original processor and is
stored until ready for use. When the product is to be processed, it is spread thinly on
trays to provide the greatest surface area for evaporation, and the trays are placed on
carts. The loaded carts are placed in a freezer at 20F (-7C). To begin the sublimation
process, the frozen product on trays is placed in sublimator cabinets that are then
sealed and placed in a chamber where the internal pressure is reduced to 0.5 mm of
mercury per inch by drawing a vacuum. When the frozen product is at a sufficiently low
pressure, heat is applied through coils underneath the trays to change the ice within the
food to a vapor. The vapor is removed. When the process is considered complete, the
vacuum is broken by injecting nitrogen into the chamber. Nitrogen is used to prevent
oxygen from entering and combining with the product, which could cause oxidative
rancidity. If the moisture content is low enough, the trays are removed from the
cabinets. The moisture content should be about two percent.
c. Packaging Requirement. Freeze-dehydrated products are extremely
reactive to oxygen and moisture and must be protected from them to remain good.
Proper packaging is required. The primary requirement during packaging is to prevent
absorption of moisture and oxygen from the air. To accomplish this, the product is
transported through an airlock to the humidity-controlled packaging room. The product
is carefully weighed and placed in cans. A lid is placed on each and is crimped in place
but not yet sealed. The cans are placed in a vacuum chamber. A vacuum is drawn to
remove the oxygen. Nitrogen is introduced to replace the oxygen. The cans are then
removed from the chamber, inspected for vacuum by the sound of a tap on the lid, and
the can lids are sealed.
3-22. PRESERVATION BY RADIATION
a. Effect of Radiation on Foods. The preservation of foods by the use of
ionizing radiation is a relatively new means of preservation. The food is preserved
because the quick dose of radiation destroys all microorganisms, including bacteria,
destroys insects in all life cycle stages, inhibits sprouting on vegetables, and prevents
some chemical (enzymatic) deterioration. The dosage will not inactivate all enzymes,
and spoilage from enzymatic activity is a major problem in irradiated foods. After
irradiation, the products are normally placed in hermetically sealed containers to keep
out contaminating microorganisms.
b. Sources and Dosage of Radiation. The sources of ionizing radiation used
are gamma radiation, such as cobalt 60, beta radiation, electron beam, and x-ray beam.
The radiation source is held a few centimeters from the product. A dosage of 2.5 to 5
million rad is required to kill all bacteria.