76 



MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 



evident in the exhaust air. The general rule for minimizing this loss 

 of quality is to keep the temperature of the material relatively low, 

 but to keep the rate of drying as high as possible. Since the first 

 of these two measures slows down the rate of drying, any choice of 

 conditions will be a compromise based on experience. 



Drying conditions that expose the wet product for several hours to 

 warm air high in humidity are very unfavorable to product quality. 

 Under such conditions the growth of bacteria and molds may be ex- 

 ceedingly rapid, and the product may become sour or moldy before 

 the drying has progressed far enough to check further growth of the 

 micro-organisms. Drying should start promptly and proceed rapidly 

 to prevent this kind of damage. Trouble from this source may be 

 experienced in the drying of vegetables in a counterflow tunnel clehy- 

 drator if the tunnel is so long or the flow of air so slow that the air 

 temperature drops to only 5° or 10° F. above wet-bulb temperature 

 at the cool end. 



CONVEYOR-BELT DEHYDRATORS u 



Figure 36 illustrates diagrammatically a commercial form of con- 

 veyor dehydrator. The wet material is loaded evenly, and relatively 

 deeply, on a traveling conveyor. Hot air is blown through the layer 



W^ 



Figure 36. — Structural features of the conveyor dehydrator. 



of material and through the conveyor belt itself. The direction of 

 flow of the air may be reversed once or oftener in successive sections 

 of the drier, and the thickness of the layer of material may be main- 

 tained, or increased, in spite of shrinkage, by transfer of the material 

 to a conveyor that moves more slowly. 



The conveyor drier is one of the important types of equipment in 

 the current vegetable-dehydration program. It is fundamentally auto- 

 matic in operation, and may therefore be preferred in a highly mech- 

 anized plant. Unless a relatively thick layer of material is carried 

 on such a conveyor, the size of drier required becomes excessive. For 

 example, if 30 tons of moist vegetable are to be dried per 24 hours, 

 and the drying time cannot safely be reduced below 3 hours, then 6,250 



n While patent files and other publications describe hundreds of different designs for 

 food dehydrators, present commercial interest centers in less than a dozen general types. 

 In the following pages those types that are used most extensively for vegetable and fruit 

 dehydration are discussed in greater detail than are the others. Each type of dehydrator 

 has a preferred field of use. Most commercial production of dried vegetables and fruits is 

 obtained through the use of continuous hot-air types of driers. The noncontinuous, or 

 patch-type driers, are also used considerably and are valuable for experimental work. 



