54 MISC. PUBLICATION 54 0, TJ. S. DEPT. OF AGRICULTURE 



where M is moisture content (percent) in the fresh product, Mf is 

 the moisture content (percent) in the product leaving the drier, T 

 is the moisture content (pounds water per pound bone-dry) in the 

 fresh product, Tf is the moisture content (pounds water per pound 

 bone-dry) in the product leaving the drier. 



The drying yield is the reciprocal of the drying ratio. Blanched 

 carrot strips, for example, enter a drier containing 89 percent mois- 

 ture, and leave it containing 5 percent moisture. Then 



on c 



ikf =89, M f =5, T o =j|=8.09, 7>=^=0.053. 



-p. . ,. 100-5 8.09 + 1 Qao 



Drying ratio -^-^^-^-^ 8.63. 



~ . . , , 100-89 0.053 + 1 1 ni1 . 11A 

 Drymg ^6^=^^^=^^^=^3=0.116=11.6 percent. 



The drying ratio and drying yield do not express the total shrink- 

 age in weight of product in passing through the plant. The over-all 

 plant ratio, or its reciprocal, the over-all yield, must also take into 

 account the losses of product during preparation and inspection. 



The weight of water to be evaporated can be calculated from the fol- 

 lowing formula : 



T —T 



Evaporation = J, . -/ =pounds water per pound material en- 

 tering drier. 



From the preceding example, ' nQ , -. — = 0.884 pound water 

 evaporated per pound prepared material entering the drier. 



Evaporation from the Surface of a Wet Material 



When first exposed to a stream of air, cut and blanched pieces of 

 vegetable or fruit act essentially like a fine-grained sponge full of 

 water. The surface moisture evaporates very rapidly. 



Evaporation from a water surface has been studied extensively. The 

 observed relations are to a large extent explainable on the assumption 

 that a stagnant film of nearly saturated air persists at the surface. 

 Air flowing past the surface tears away vapor from the film, which is 

 replenished by evaporation from beneath. The more rapid the flow 

 of air, the thinner the film becomes, and the more rapid the transfer of 

 water vapor into the air stream. 



During this initial period (and, indeed, during the time that the 

 first half to three-quarters or more of all the moisture in the piece is 

 evaporating) the wet piece acts essentially like the wick of a wet- 

 bulb thermometer, and, if air velocity is reasonably high, the temper- 

 ature inside the piece stays at or near the wet-bulb temperature of the 

 air. 



Removal of Water From Deeper Layers 



Evaporation from the surface cannot proceed far before the surface 

 layers of the piece will become drier than those beneath them. The 



