VEGETABLE AND FRUIT DEHYDRATION 55 



process of moisture diffusion then starts, and this process must con- 

 tinue during all of the remainder of the drying. It is a complex 

 phenomenon that is still imperfectly understood. 



The moisture in fruits and vegetables exists as a dilute solution of 

 sugars, salts, proteins, and other organic compounds, held in the closed 

 cells that constitute living tissue. In order to travel from the inside to 

 the surface of a piece, water must be transferred through all of the 

 cell walls which lie between. That kind of transfer takes place quite 

 freely while the cells are still full of liquid. The direction of flow is 

 always from a cell with dilute content to an adjoining one in which 

 the solution is more concentrated. Evaporation of water at the surface 

 concentrates the cell solution there. Water from the deeper layers 

 then diffuses into it. Before very long this process will have caused 

 loss of moisture from all levels of the piece, all the way to the center. 



The cell walls tend, in general, to hold back the dissolved substances, 

 while permitting the water to pass freely ; the separation is not perfect, 

 however, so that there is some tendency to move a part of the salts and 

 sugars to the surface and deposit them there when the water evapo- 

 rates. Even though the cell solution is dilute in the fresh vegetable, 

 so large a proportion of the water is removed that the solution must 

 become exceedingly concentrated long before drying is complete. For 

 example, press juice from fresh carrots may contain about 6 percent 

 sugar; when evaporation has reduced the moisture content of a piece of 

 carrot to 10 percent, there is five times as much sugar present as water. 

 Such a solution is a taffy or glass — not a liquid. 



As water is lost from a cell an internal tension is set up which pulls 

 the cell walls inward. The cell partially collapses. This process, 

 occurring throughout the piece, is responsible for the shrinkage that 

 takes place during drying. Since the cell walls are relatively thin, the 

 decrease in volume of the piece is approximately equal to the volume of 

 water lost. Some of the tissues, however, may be more rigid than 

 others, or cell collapse may occur in some directions more freely than 

 in others, so that shrinkage is frequently very nonuniform. The piece 

 loses most of its resemblance to the original shape. 



At some point during the dehydration the rigidity of cell walls 

 resists further collapse with enough force so that cavities open up 

 within the cell contents, and the shrinkage stresses may loosen the 

 bonds between cells to some extent or even partly separate them. 

 During the final stages of drying it is probable that much of the 

 diffusion of water within the piece is a diffusion of water vapor through 

 open spaces, rather than transfer of liquid water. The concentration 

 of salts and sugars as drying proceeds slows down this process by 

 reducing the vapor pressure of the solution. 



Whether the transfer of moisture takes place by liquid or by vapor 

 diffusion, the rate at which water will move from one layer of cells 

 to the next will depend primarily on two factors — first, the difference 

 in concentration of water in the two layers, and second, the tempera- 

 ture. The difference in concentration provides the driving force for 

 diffusion; the greater the difference, the greater the force. Raising 

 the temperature decreases the viscosity of the liquid and increases its 

 diffusion pressure, whether it is liquid or vapor. 



The rate at which water can be removed from a thick piece of vege- 

 table after surface moisture has been evaporated will evidently change 

 radically as the drying progresses. At the start, water will diffuse 



