_by Dale E. Kirk and Carolyn A. Raab_[16]
Tasty ready-to-eat snacks and confections are some of the versatile
products you can create by drying fruits and vegetables at home. After
soaking in water, the rehydrated food can be used in favorite recipes
for casseroles, soups, stews and salads. Rehydrated fruits and berries
can also make excellent compotes or sauces.
Drying is appealing because the procedure is relatively simple and
requires little equipment. Only minimal storage space is needed. Food
can be dried in the sun, in the oven, or in a dehydrator.
Drying requires a method of heating the food to evaporate the moisture
present, and some means of removing the water vapor formed.
Sun drying utilizes both radiant heat energy and heat transferred to the
product from warm air. Natural air currents are usually adequate to
carry away the water vapor.
Trays of wood slats, plastic mesh, or aluminum screen may be placed in
the sun on support blocks or strips to allow air movement around and
through the trays. Galvanized wire is not recommended as a tray material
because high-acid foods will react with the zinc coating on the steel
wire.
If insects or birds are a problem, a wooden frame can be constructed
over the trays to support a plastic mesh or cheesecloth cover. Further
protection can be provided by using a totally enclosed frame and a
transparent panel to form a solar drying oven.
To dry in the kitchen oven, the thermostat should be set to its lowest
temperature (generally about 150° F). Since oven vents provided for
removing moisture from roasting and baking are adequate for drying only
small quantities of food at one time, the oven door should be left
partially opened. For larger loads, the air circulation rate can be
increased by placing a household fan outside the oven, directed at one
edge of the partially opened oven door.
Dehydrator cabinets may be purchased in many sizes and types. Or they
may be built using plans available from State universities or U.S.
Department of Agriculture plan services (ask your county Extension
office about plans). All cabinets are provided with a heat source and
vents for carrying off moist air.
Simpler units may rely on natural convection to carry moist air away,
and the heating unit may be limited in output so that the cabinet never
exceeds safe drying temperatures near the end of the drying period. This
type will be slow in achieving drying temperature if sizable amounts of
food are processed at one time.
Trays must be rotated during the processing period to insure even
drying. Trays nearest the bottom, exposed to the hottest, driest air,
will dry most rapidly.
If the natural convection type cabinet is equipped with a thermostat, it
may be fitted with a larger heater. This will provide higher drying
temperatures during the early stages but will not give even drying
across all trays.
By using a fan to force air across the trays more rapidly, even drying
can be obtained across each tray as well as between trays. The forced
air system may be used with or without a thermostat.
Most food products release moisture rapidly during early stages of
drying. This means they can absorb large amounts of heat and give off
large quantities of water vapor while remaining at a temperature well
below that of the drying air. Maximum drying rates can be achieved by
providing a larger, thermostatically-controlled heat source and a fan
for circulating air.
[Illustration: Enclosed frame solar drying oven with provision for
air movement.]
air in (screened)
trays
glass or transparent plastic
air out (screened)
[Illustration: Dehydrator with built-in heater relies on air
movement to carry off moist air.]
air in
heating element
heat shield
trays
air out
[Illustration: This forced draft dryer can recirculate much of the
drying air to conserve energy.]
air in
fan
heaters
heat shield
thermostat
trays
air out
To conserve energy and still obtain rapid, even drying across all trays,
much of the drying air may be reheated and recirculated. This is
particularly effective during the last 70% to 90% of the drying period,
when relatively small amounts of water are absorbed by the air as it
passes over the partially dried food.
The recirculating system requires either a thermostat or separate switch
controls on part of the heating unit to adjust heat output to match the
drying load. The amount of air recirculated is determined by the size of
the permanent inlet and outlet openings in the box. It can be further
controlled by adjusting the door to a partially opened position.
Detailed plans for constructing the recirculation-type drier can be
obtained by sending 25¢ to the Western Regional Agricultural Engineering
Service (WRAES), Oregon State University, Corvallis, Oreg. 97331 and
requesting WRAES Fact Sheet No. 18.