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.
Drying is a relatively simple process, but there are a number of recommended techniques. You may need to use a “trial and error” approach to find the drying procedure which works best in a particular situation.
Fruits and vegetables can be dried in pieces or pureed and dried in a thin sheet as a “leather.”