COMMERCIAL DEHYDRATION 13 



Fruits are not usually blanched but may be treated with sulfur to 

 keep them from darkening and for other purposes. (See p. 19 for a 

 discussion of sulfuring.) 



Equipment for blanching may be a small tunnel through which 

 material is conveyed on a wire-screen belt. Live steam in the tunnel 

 partly cooks the food material as it passes through. Blanching can 

 also be done on trays stacked in a stationary steam cabinet. 



The temperature of live steam at atmospheric pressure (about 212° 

 F.) is usually maintained in the blancher. The vegetables themselves 

 should attain a temperature of at least 190° F. Some products require 

 longer blanching and some a higher temperature than others. The 

 plant operator can determine by certain chemical tests whether his 

 blanching operation is effective and can make adjustments accordingly. 

 Government specifications set up standards for blanching. 



The Drying Process 



A piece of vegetable or fruit is in many lX'spects like a small fine- 

 grained sponge full of water. It is far more complex in structure. 

 but for the first few minutes in the dehydrator it dries just as a wet 

 sponge would. Evaporation is at first entirely from the surface and 

 is very rapid. Later, water must move from the center to the surface. 

 and drying becomes slower. As water comes to the surface and evap- 

 orates, the concentration of salts and sugars in the juice increases. In 

 genera], the hotter and drier the air and the more rapidly it circulates 

 in the dehydrator, the faster the moisture will be drawn to the surface 

 and evaporate. Too rapid drying may result in the formation of an 

 impervious surface, with moisture trapped inside. This condition, 

 called case-hardening, is more likely to occur with fruits than with 

 vegetables. It may be prevented by controlling the rate of drying. 

 Too high a temperature may also cause scorching. Food should be 

 spread on the drying trays evenly and in a shallow layer to insure 

 uniform drying. The new dehydration methods are more successful 

 than previous ones because a lower moisture content is reached. 



TYPES OF DEHYDRATORS 



There are several types of dehydrators. For cut materials or whole 

 small fruits, the tunnel and cabinet driers are most commonly used. 

 The chief difference between these two types is that in the tunnel 

 drier (fig. T) the trays of food are loaded on trucks that move from 

 one end to the other, whereas in the cabinet the trays remain stat ionary. 

 The trays have wire-screen or wooden-slat bottoms, and currents of 

 heated air circulated bel ween and through them evaporate the moisture 

 from the food. 



The kind of raw material and the quantity available, the availability 

 and cost of building materials, and other factors a fleet the design and 

 construction of dehydrators. As a result of study and experience, 

 improvements are constantly being made. 



The tunnel drier is well suited to continuous operation and is eco 

 nomical in the use of heat. A single well-designed machine of this 

 type will dehydrate 15 to 25 tons of raw material during a 24-hour 

 run. For greater economy, tunnels are often constructed in groups of 

 two or more. 



