66 



MISC. PUBLICATION 5 40, U. S. DEPT. OF AGRICULTURE 



materials that are initially very high in moisture have the higher 

 drying rates. The length of time required to reach, say, 10 percent 

 moisture bears no apparent relation to initial moisture content; it is 

 more likely that a relation could be traced with the amount and kind 

 of sugars and other soluble substances present in solution. 



20 

 10 



2 



I 

 0.5f 



£0.2 

 z 

 o 

 ^ O.I 



.05 



.02 



















































































V 



X 



Ss 























v Sweet Potal 



Prune 



Q 



5 











\ Cabbage 

 \ Shreds 



X^ Slices 















V 





































12 



14 



16 



2 4 6 8 10 



TIME (HOURS) 

 Figure 33. — Typical drying rates for three types of product. 



18 



20 



Estimation of Drying Time 



It should be apparent that the question, "How long does it take 

 to dry carrots?" can be answered only when a considerable number 

 of factors are specified. A reasonably good estimate of drying time 

 is a necessary element in dehydrator design, and it is now possible 

 to supply such estimates for many important cases as a result of cor- 

 relation of hundreds of experimental drying runs. 6 



The operator will not usually have any need for this information, 

 except in a qualitative way. The foregoing discussions indicate that 

 in general the following factors favor rapid drying: Thin pieces, 

 open metallic support, light open piling, circulation of air through 

 entire layer, high velocity of air past the moist material, high tem- 

 perature level, high wet-bulb depression. 



Drying Conditions Within a Dehydrator 



Drying conditions within a dehydrator operating with commercial 

 loads are never so simple as those of the experiments that have just 

 been described. In particular, the air temperature and humidity al- 



6 Information on the drying characteristics of a number of vegetables may be obtained 

 from the Western Regional' Research Laboratory, Albany, Calif. 



