PRESERVATION OF FRUITS AND VEGETABLES 



23 



Drying by Recirculation of Air 



After several years of research by the United States Department 

 of Agriculture, in which practically all types of driers and the problems 

 connected with them were studied, a compartment-type duplex 

 dehydrator (fig. 5) of 2 fresh tons capacity was built at Los Angeles, 

 California, and operated on a semicommercial scale on fruits and 

 vegetables (12). This apparatus and the fundamental principles 

 involved in its design and operation (18) suggest improvements in 

 drying practices with many other materials. This type of drier is 

 especially adapted for drying vegetables and is also well suited for 

 drying fruits. 



In designing the dehydrator, cognizance was taken of the cooling 

 effect of evaporation on a drying medium, as demonstrated by the 



Figure 5. — A compartment-type duplex dehydrator. 



wet bulb of the hygrometer. The development of a type of drier 

 was undertaken in which it would be possible not only to use maximum 

 temperatures at the time of maximum evaporation from the product 

 being treated, but also at the same time to control temperature and 

 humidity of the drying medium as well as its distribution. 



Heat is not an expensive item in the total cost of dehydration. 

 Uniformity of drying, however, is a troublesome problem and one 

 that costs the manufacturers dear. 



Proper distribution of conditioned air is perhaps the most important 

 fundamental in drying. The flow of air over the trays should be 

 uniform, so that drying will proceed at the same rate over the entire 

 spreading area of the tray surface. If this condition has been met in 



