VEGETABLE AND FRUIT DEHYDRATION' 59 



If the finishing stage is to proceed at a reasonably high rate, there- 

 fore, the humidity of the air must be decidedly lower than that 

 corresponding with equilibrium at the desired final moisture content. 



Typical Drying Curves, Constant Drying Conditions 



General physical laws and the known properties of air and water 

 vapor make it possible to calculate accurately some important char- 

 acteristics of a dehydrator. The methods used and some of the 

 results are discussed in a later section. These methods have the strict 

 limitation that they offer no information about the rate at which 

 evaporation will take place; in other words, the time required to 

 bring about a given degree of drying. The foregoing discussion 

 indicates qualitatively the effect of various factors on the rate of 

 drying, but quantitative information can be determined only by 

 experiment. 



At least nine separate factors have marked effects upon drying 

 rates (45)- These are: Variety of fruit or vegetable, shape and 

 size of piece, method of pretreatment (blanching, etc.), method of 

 support in the drier, thickness of layer of moist pieces on the sup- 

 port, manner of exposure to the air stream, air velocity, air tem- 

 perature, and air humidity. In a vacuum dehydrator still other 

 factors appear. The number of possible combinations of these vari- 

 ables is so enormous that the only practical way to investigate their 

 effects is to conduct a series of controlled experiments; in a group 

 of such experiments all conditions except one are kept constant, and 

 several values of that one will be tried in successive experiments; 

 then in other groups the effects of other variables will similarly be 

 tried one at a time. After careful analysis of a long series of ex- 

 periments of this kind it becomes possible to estimate with consider- 

 able accuracy what the rate of drying will be under any combination 

 of conditions. 



Detailed results of experimental work on drying rates are not in- 

 cluded here, because such information is principally valuable to de- 

 hydrator designers. The following general discussion is introduced 

 for the value it may have in promoting intelligent understanding of 

 what goes on in a dehydrator. 



Suppose that the following choice of conditions is made for a single 

 experimental run : Russet potato, "julienne" strips five thirty-seconds 

 inch square, steam-blanched 6 minutes, wood-slat trays, original load 

 1.5 pounds per square foot of tray surface, air flow across the trays, 

 air velocity 500 feet per minute, air temperature 150° F., wet-bulb 

 temperature 90°. If these conditions are maintained constant and 

 the progress of drying is determined from time to time by weighing 

 the tray, a drying curve such as that reproduced in figure 26 may 

 be drawn. 



This curve is typical of hundreds that have been observed. Two 

 characteristics are especially important: (1) The very rapid initial 

 fall in moisture content, and (2) the very slow final approach toward 

 equilibrium moisture content (in this case, from figure 26, equilibrium 

 ^=0.025, about 2.5 percent). Note that an entire hour is consumed 

 in drying from 7 percent to 6 percent moisture, whereas three-fourths 

 of the total original moisture is evaporated in only an hour and a 

 quarter. 



