VEGETABLE AND FRUIT DEHYDRATION 113 



vacuum. Neither of these devices has as yet been extensively used in 

 commercial dehydration. 



The principal handicap to practical use of vacuum driers for dehy- 

 drating fruits and vegetables, namely, the difficulty of transmitting 

 heat to the product rapidly enough to evaporate a large quantity of 

 water effectively, can be largely overcome if the drier is used only 

 for the finishing step in a multistage dehydration process. Prelim- 

 inary steps may remove 95 percent or more of the moisture originally 

 present in the material ; from that point on the vacuum drier is able 

 to operate very effectively. A combination process of this nature is 

 being used commercially for the final dehydration of certain fruits, 

 which are first dried in hot air to the usual moisture content for dried 

 fruits — about 20 to 25 percent. 



Dehydration of fruits or vegetables to final moisture contents of 

 only 1 to 2 percent can be accomplished in one of two ways — by the 

 use of highly desiccated air as the drying medium or by the use of a 

 vacuum drier. Choice between these two methods may be determined 

 by availability of the equipment or by the sensitivity of the nearly dry 

 product to oxidation by warm air. 



Rotary-Drum Driers 



Drum drying provides means for rapid dehydration of solutions, 

 slurries, purees, and the like, spread in a thin layer on a heated sur- 

 face from which the dried product is removed continuously. The 

 equipment consists of an internally heated revolving drum, upon the 

 outer surface of which the substance being processed is spread at one 

 point while the dried product is removed by means of a blade at 

 another. The temperature and rate of revolution of the drum are 

 so chosen that drying is completed in the time required for the 

 material to travel from the point of spreading to the blade that 

 removes the product. 



The operation can be carried out in air or in a vacuum. In the 

 latter case heat-sensitive products are better protected since lower 

 operating temperatures can be maintained. The material being de- 

 hydrated can be spread on the surface of the drum by spraying it, by 

 a "doctor" blade, or by use of two drums placed parallel with a small 

 clearance between. In the latter case both drums can be used as 

 driers, in which event they will be similar in size, or one may be small 

 and used only for the purpose of providing a small clearance through 

 which even spreading on the large drum can be attained. 



Many products are removed from the drum in the form of con- 

 tinuous sheets which are conveyed to a mill operating in a suitably 

 conditioned atmosphere where the dry product is reduced to a powder 

 and then packaged. 



The drum-drying process is widely used for the production of dried 

 milk and to a lesser extent for certain fruit and vegetable purees. The 

 form of the product is confined to thin flakes and powders. 



Rotary Driers 



Eotary driers are rotating drums inclined at a slight angle from 

 the horizontal. The wet substance is fed in at the high end and 

 is conveyed with constant tumbling toward the low end, from which 



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