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



easily from deep layers to the surface. If the surface layer tends to 

 become dry, diffusion into it from below will speed up because of the 

 increased difference in concentration. So long as the surface remains 

 moist, the temperature of the piece will remain at or near the wet-bulb 

 temperature of the air. The piece shrinks in volume, but since water 

 diffusing from the center must still traverse the same number of cell 

 walls, the resistance to its flow will not decrease ; indeed, it may well 

 increase because of a fall in the permeability of the cell walls as they 

 dry out. At the same time the diffusion pressure of the liquid will 

 fall as it becomes more concentrated. As more and more water is 

 lost, the possible difference in moisture concentration between succes- 

 sive layers of cells decreases. The result of all these changes is that 

 the rate of evaporation from the piece falls off continuously as drying 

 proceeds. At some point the evaporative cooling becomes insufficient 

 to hold the temperature of the piece down to the wet-bulb tempera- 

 ture of the air, and the temperature gradually rises approximately to 

 the dry-bulb air temperature. 



Experimental measurement of the temperature at the center of 

 carrot slices during dehydration under different conditions indicated 

 that the transition from wet-bulb temperature to dry-bulb tempera- 

 ture occurred generally within the range of moisture contents of about 

 65 percent (T=Q) to 33 percent (rt=0.50), i. e., after about three- 

 fourths of all the water had been evaporated. Data from a typical 

 experimental run are shown in figure 24. 



180 



160 



y 140 



3 

 < 

 UJ 



I 120 



100 



80 









55*2^- 







Dry^ 









\ 

 \ 



\ 

 \ 





\ 



\ Temf 

 I of K 



>erature 

 laterial / 



/A 





tfe 





v. 



/ 





















10 



2 4 6 8 



MOISTURE CONTENT (LBS. / LB. BONE-DRY) 



Figure 24. — Relation of temperature of material to dry-bulb and wet-bulb air 

 temperatures. Carrot slices three-sixteenths inch thick. 



Effect of Thickness of Piece 



Increasing the thickness of cut pieces of vegetable slows down the 

 completion of drying very materially. If the rate of drying is ex- 

 pressed as the loss in average moisture content per unit of time, this 

 rate is lower from the very start for thick pieces than for thin ones, 



