VEGETABLE AND FRUIT DEHYDRATION 57 



but if it is expressed as the loss per unit of surface exposed, per unit 

 of time, it is substantially independent of thickness, at least while the 

 piece is very wet. This indicates that in the early phases of drying 

 the principal resistance to be overcome is that of the vapor film at the 

 surface. As drying proceeds further, the principal resistance shifts 

 to the internal resistance to diffusion of water. It may be regarded 

 as quite probable that the comparative slowness with which heavy 

 fruits can be dried (prunes, peaches, apricots) is traceable at least as 

 much to their thickness as to their high sugar content. 



."Case Hardening" 



The term "case hardening" was probably borrowed from the older 

 art of kiln-drying lumber. In drying lumber it is essential that the 

 inevitable shrinkage in volume shall take place uniformly and without 

 causing surface checks or cracks. Experience showed that if the dry- 

 ing were hastened too rapidly at first, such cracks invariably resulted. 

 The outer layers, drying more rapidly than the center, shrank down 

 upon a wet and nearly incompressible core. If the difference was 

 excessive, the resulting tension in the outer shell or "case" opened up 

 cracks. 



Dehydrators of heavy fruits similarly learned many years ago that 

 they must avoid exposing the moist fruit to too drastic drying con- 

 ditions. Such exposure, for example, would cause the skins of prunes 

 to check and the fruit would "bleed." It appeared, too, that if the 

 drying were started too fast, the final stage of finishing was likely 

 to be much slower. The fruit acted as though the initial rapid dry- 

 ing formed an impermeable "case" at the surface, through which the 

 remainder of the moisture could hardly diffuse. The reality of the 

 effect is indisputable. Whether it is due to a change in the permeabil- 

 ity of surface-cell walls as they become dry, or to the transport of 

 sugars to the surface and their deposit there as a layer of heavy, im- 

 permeable "taffy," has not been determined. At any rate, it was ob- 

 served that this trouble was not experienced when the initial rate of 

 drying was kept moderately low and when internal diffusion of mois- 

 ture was facilitated by keeping the temperature of the fruit moder- 

 ately high. Both conditions were obtained at once by keeping the 

 wet-bulb temperature of the air circulating in the fruit dehydrator 

 relatively high. 



In dehydrating cut vegetables there is little possibility of the oc- 

 currence of surface cracking and bleeding. "Case hardening" would 

 be recognized only by the persistence of a wet center, and drying to 

 the desired low final moisture content would require much longer 

 than would be deemed normal for that vegetable. The term "case 

 hardening" therefore lacks precision of meaning. In fact, it is very 

 difficult to produce a condition that can be clearly recognized as typi- 

 cal "case hardening" in cut pieces of vegetable. Ordinarily, cut vege- 

 tables can be dried to completion in the shortest total time when the 

 rate of drying is kept as high as possible at all times. 



The maintenance of high wet-bulb temperature does not have the 

 same significance, therefore, in drying vegetables as it has in drying 

 heavy fruits. It does have a function in the control of a dehydrator 

 and in economizing heat, as will be discussed in a later section. 



