64 



MISC. PUBLICATION 54 0, U. S. DEPT. OF AGRICULTURE 



put of the dehydrator will actually fall off. Figure 31 presents imagi- 

 nary curves that show relative capacity of a particular dehydrator at 

 various loadings per square foot of tray. The curve for cabbage shreds 

 indicates that this dehydrator will reach its maximum capacity when 

 the trays are loaded about 1.1 pounds per square foot ; the curve for 

 potato strips appears to reach its maximum capacity at a load of 

 about 2 pounds per square foot. The drying times marked on the 

 curves indicate, however, a rapid increase in drying time at the heavier 

 loadings. 



Drying time can be considerably shortened by the use of a load some- 

 what lighter than the one that will give maximum output, and the 

 sacrifice of capacity will only be minor. In view of the well-estab- 

 lished fact that rapidly dried vegetables are superior in quality to 

 slowly dried ones, the wisdom of moderate loadings is apparent. 































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TRAY LOADING (LB. PER SQ.FT.) 



Figtjbe 31. — Relationship of loading of trays to daily capacity of a dehydrator. 

 (A numerical scale of tons input per day is not shown, because that will depend 

 on the kind of vegetable being dried, tray loading, total tray surface and the 

 temperature, humidity, and velocity of the circulating air.) 



Character of the Support 



In a cross-circulation drier, drying time is substantially shortened 

 by supporting the moist material on a wire-mesh surface such as hard- 

 ware cloth, which is high in heat conductivity and open to air circu- 

 lation. The support, whatever its construction, tends to reach the 

 dry-bulb temperature of air circulating past it; if its conductivity is 

 high, the product resting on it will warm up above wet-bulb tempera- 

 ture even while very moist, and will therefore dry faster. If the 

 conductivity is low, as in wood trays, the effect on product tempera- 

 ture will be much less. 



While the metal tray offers this advantage in drying rate, certain 

 other considerations favor wood-slat trays. Since corrosion-resistant 

 alloys and tinned screens are difficult to obtain under wartime condi- 

 tions, galvanized screen is the most common type of metal-tray bot- 

 tom. Corrosion of the screen and darkening of the product in con- 

 tact with the mesh may be experienced. Synthetic resin varnishes are 

 being used to some extent as protective coatings. Fruits generally 

 cannot be dried successfully on galvanized-screen trays. 



Manner of Preparation 



Fruits that are dried whole, for example prunes and cranberries, are 

 usually pretreated in a manner to rupture the tough, waxy skin so 



