VEGETABLE AND FRUIT DEHYDRATION 



65 



that drying will be accelerated. Vegetables, on the other hand, are 

 always dried in the form of cut or formed pieces. The pretreatment 

 they receive, particularly blanching, has a complex effect on the rate 

 of drying. 



Blanching has a marked effect on the physical structure of potato 

 pieces. Starch granules swell, a certain amount of soluble salts is 

 leached out, a layer of gelatinous starch may be formed on the surface. 

 There is little information on the effect of these changes on the speed 

 of drying, mainly because present practice invariably includes blanch- 

 ing. The same situation exists with regard to other vegetables. 

 There is experimental evidence, however, that there is no material 

 difference between the rate of drying of potato strips blanched 4 

 minutes and those blanched 10 minutes in atmospheric steam. It is 

 well known that vegetables made soft by blanching must be handled 

 with care to prevent the formation of "mush" on the trays, since mush 

 dries very slowly. The tendency of blanched leafy vegetables to "mat 

 down" also limits the depth to which they can be piled, especially 

 in a through-circulation dehydrator. 



Shape and Size of Pieces 



Internal resistance to diffusion of moisture is the controlling influ- 

 ence during most of the drying of vegetables and fruits in commercial 

 sizes. The shape and size of the pieces is therefore a very important 

 factor in determining rate of drying. Figure 32 compares drying 



S2 



O o 



3 -J 



0.5 



0.1 



0.05 









































XifeTh 



ickness 









W ? 



\3fix 



























i 







TIME ( 



2 



HOURS) 







30 



Figure 32. — Effect of thickness of potato strips on drying time. 



curves for potato strips of three different thicknesses. Cubes gen- 

 erally dry faster than strips of the same cross section, and slices of the 

 same thickness dry slower than either. 



Nature of the Fruit or Vegetable 



Figure 33 presents typical drying curves for two common vegetables 

 and one stone fruit. While these curves are not strictly comparable 

 with regard to density of loading, size of piece, and other factors, they 

 illustrate the general range of differences between products. 



The drying rates show the widest divergence at low moisture con- 

 tent, but there are substantial differences at all levels. In general, 



569074 — 44 5 



