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



the dried product is continuously removed. Drying is facilitated by 

 a current of air. either coimterflow or parallel flow, induced by suit- 

 ably situated fans, and heat may be simultaneously supplied on the 

 outside of the drier. Construction is of either the single- or double- 

 shell type. The latter affords greater protection of the product from 

 damage by burning. Double-shell driers are sometimes provided with 

 louvers in the inner shell through which air is admitted in direct con- 

 tact with the material. 



Rotary driers are efficient and extremely useful in drying many 

 substances, but they have found little application in the field of fruit 

 and vegetable dehydration. The constant rolling or tumbling of the 

 wet product destroys the characteristic form of the pieces (slices, 

 strips, or cubes) and may result in production of a large proportion 

 of powder. Rotary driers have been used successfully for the drying 

 of precooked ground meat. 



Apple and Prune Kiln Driers 



A distinctive type of kiln drier finds extensive use in dehydra- 

 tion of apples and primes in the Pacific Northwest. The drier con- 

 sists of a two-story building in which the lower story houses the cen- 

 tral heating system, from which heated air is circulated through 

 wooden ducts to the kilns above. The kiln proper consists of a series 

 of stacked, shallow, wooden bins having slotted floors on which the 

 prepared fruit is placed in layers from 6 inches to 24 inches in depth. 

 Warmed air from the heaters below passes upward through the fruit 

 and is removed through a duct at the apex of the V-shaped roof. In 

 some modern kilns the air circulation is speeded up by an induced- 

 draft fan at the roof, and in some cases provision is made for re- 

 circulation of part of the exhaust air. When apples are being dried, 

 it is common practice to turn the material with shovels or forks after 

 4 to 6 hours of drying. 



Initial temperatures of 150° to 165° F. are usual. Final drying- 

 should be carried on below 160° F. Driers of this type are built in ca- 

 pacities ranging from a few tons to 120 tons of raw material per day. 

 Their initial cost is relatively low but difficulty of control is not con- 

 ducive to the highest quality of product and drying costs are likely 

 to be high as a result of large heat losses. Kilns are not well 

 adapted to the drying of cut vegetables because vegetables do not stand 

 the necessary rehandling so well as fruits, and because the relatively 

 low air velocity makes rapid drying impossible. 



MULTISTAGE DEHYDRATION 



It should be apparent that each of the types of dehydrators de- 

 scribed in foregoing pages has its peculiar advantages and disadvan- 

 tages. No one of them is "best" under all conditions. No one of 

 them is necessarily the best for all of the stages of dehydration of a 

 single lot of moist fruit or vegetable. The advantages of a com- 

 bination of parallel-flow and coimterflow tunnels over either of them 

 separately have been pointed out. Such a combination is one exam- 

 ple of a multistage dehydration process. 



While a complete discussion of the factors that determine the ad- 

 visability of combining different kinds of dehydrators belongs in the 



