74 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



the humidity chart, a = 0.0005. Then r= 0.800, # = 2,460 B. t. u. per 

 pound, or 1,640,000 B. t. u. per hour. 



b. What will be the heat consumption if the outside air has a tem- 

 perature of 90° F., wet-bulb 65° (arid conditions, 24 percent relative 

 humidity) ? a o = 0.0075, r=0.748, 5 = 1,800 B. t. u. per pound or 1,200,- 

 000 B. t. u. per hour. 



c. What will be the heat consumption if the outside air has a tem- 

 perature of 90° F., wet -bulb 85° (tropical conditions, 82 percent rela- 

 tive humidity) ? a o = 0.0247, r=0.267, 5 = 2,830 B. t. u. per pound or 

 1,890,000 B. t. u. per hour. 



3. The greater the load of evaporation that is put on a given 

 dehydrator, the more efficient will be the utilization of heat, but 

 the greater will be the amount of heat required per hour. 



Example : Consider that in the simple counter flow tunnel the input 

 of moist carrots is increased from 18,000 pounds to 24,000 pounds dur- 

 ing 24 hours. The weight of water to be evaporated will increase 

 from 11.1 pounds per minute to 14.8 pounds per minute, and the tem- 

 perature will fall at the cool end to 128° instead of 137.5°. If the 

 wet-bulb temperature is 100°, a' = 0.0271 as before, but a" = 0.0360. 

 If the humidity of the fresh air, a o = 0.0080, and * o =60°, r=0.682, 

 5 = 1,980 B. t. u. per pound (a decrease of 8 percent), but the heat 

 usage per hour is 1,760,000 B. t. u. (an increase of 23 percent.) 



Balance Between Efficiency and Effectiveness 



It is evident that the operator has it within his power to change 

 to some extent the cost of heat used in the process, through his control 

 of the wet-bulb temperature. He may be confronted, however, with 

 the necessity of choosing between low heat cost and high output of 

 dry product. The matter of striking a proper balance between the 

 two is largely a question of relative costs, but it is also affected by the 

 influence of dehydrator conditions on the quality of the product. 

 That subject is discussed in the following section. 



EFFECT OF DRYING CONDITIONS ON QUALITY OF PRODUCT 



The dehydration of fruits and vegetables is primarily a technique 

 of preserving a perishable food product. The product must retain 

 the qualities of an acceptable food or the entire purpose of the process 

 is defeated. Consideration of the drying step solely from the stand- 

 point of efficient evaporation of moisture would therefore be fatally 

 one-sided. Certain hazards that are encountered during the drying 

 process are discussed here briefly; reconstitution and quality of fin- 

 ished products are treated in subsequent sections. 



Heat Damage — Scorching 



The temperature to which a piece of fruit or vegetable can be heated 

 is limited by the occurrence of scorching. The reconstituted and 

 cooked product must have the best possible appearance, flavor, and 

 texture. It must be free from the characteristic flavor and browning 

 produced by scorching. 



Scorching occurs in some materials at much lower temperatures than 

 in others. Sweetpotatoes, sweet corn, and spinach and other green 

 leafy vegetables will stand much higher temperatures without scorch- 



