VEGETABLE AND FRUIT DEHYDRATION 115 



realm of dehydrator design, rather than operation* the following 

 summary may assist the operator of such a system to understand what 

 can be done with it. 



Changes in Material as Drying Progresses 



The fundamental factor at the base of all combination systems is 

 the enormous change in properties of the material as it dries. A 

 freshly blanched piece of vegetable is soft and easily crushed, is in- 

 deed a mass of water held together by a tenuous cell structure; its 

 vapor pressure is nearly equal to that of pure water. In contrast, the 

 same piece of vegetable after drying to 5 percent moisture is greatly 

 shrunken in volume and distorted in shape, is hard and may be very 

 brittle, and its vapor pressure may not be more than 10 or 15 percent 

 of the vapor pressure of water. These properties all change as the 

 drying progresses, but not all at the same rate. 



Hence the conditions which would be "ideal" for drying change 

 as the piece dries. When the jjiece is very wet the water will evaporate 

 very rapidly if a great deal of air is supplied, and that air may be 

 very hot without burning the piece. At the same time the piece will 

 be too tender to stand more than a minimum amount of handling. 

 Essentially similar conditions prevail while at least the first half to 

 three-quarters of the weight is being lost through evaporation. As the 

 moisture is still further reduced the piece (which will have already 

 undergone most of the shrinkage in volume that it will experience) 

 becomes tough or leathery. The rate of evaporation may not be more 

 than a half or a third of what it was at first, even though drying 

 conditions are kept the same ; increasing the air velocity past the piece 

 has no perceptible effect on the rate ; the temperature of the piece rises 

 almost to the dry -bulb temperature of the air. "Ideal" conditions now 

 demand a lower temperature than at first, and the use of a high air 

 velocity would be a needless waste of power. However, the piece will 

 now withstand handling, tumbling, or deep piling without damage. 

 Conditions substantially similar to these prevail down to a moisture 

 content of about 10 to 15 percent. Below that point, and more and 

 more clearly as the moisture content falls further, conditions change 

 again. The piece becomes hard and brittle. The rate of further 

 evaporation is determined almost entirely by the thickness of the 

 piece and the dryness of the air. 12 The rate will perhaps become less 

 than 1 percent of what it was at the beginning of the drying. "Ideal" 

 conditions now call for a slow current of dry air, not too warm, and 

 very little mechanical disturbance of the product. 



The usefulness of different kinds of dehydrators at different stages 

 of drying will depend not only on these physical factors but also on 

 economic ones. It is an economic waste to use a large and expensive 

 piece of equipment to perform work that a small, cheap one will do 

 as well. Consideration of the labor or complications involved in 

 rehandling the product may favor one kind of transition over others. 



Examples of Multistage Systems 



Out of the very great number of possible combinations, the following 

 may be mentioned. Not all of these are now in commercial use. 



12 More exactly, by difference between the vapor pressure of the piece and the partial 

 pressure of moisture in the air. 



