VEGETABLE AND FRUIT DEHYDRATION 



121 



siderable extent. Table 10 indicates the rate at which dehydrated 

 potato strips cool when exposed to still air. 



Table 10. — Rate of decline in temperature of dehydrated potato strips exposed to 

 air — single layer, 04 pound per square foot 





Time of exposure in minutes— 





0.0 



0.5 



1.0 



2.0 



4.0 



Temperature of potatoes,.. . . . ..... 



°F. 



. 151 

 74 



°F. 

 123 

 74 



<*F. 



117 

 74 



°F. 

 106 

 74 



°F. 



94 





74 







Difference. _ .. ._ . 



77 



49 



43 



32 



20 







As an example let us assume that the inspection belt for dehydrated 

 potatoes is 12 feet long and moves 3 feet per minute. The tempera- 

 ture of the potatoes is 160° F. when they drop to the belt ; that of the 

 air is 70°. Time on the belt will then be 4 minutes. From table 10 

 we obtain an estimate of the temperature of the dehydrated potato 

 leaving the belt. In this case the temperature would be 90° to 100°. 

 With air at 85°, potatoes dropped on the belt at 150° would leave the 

 belt after 4 minutes at 105° to 115°. 



Table 11.- 



-Estimated cooling times for dehydrated julienne potatoes in 5-gallon 

 square cans, standing alone — packed 13 pounds to the can 



Initial difference in temperature (° F.) between 

 air outside and the center of the can 



Time required to reduce differences in 

 temperature to (° F.) — 



40° 



30° 



20° 



10° 



5° 



50 



Minutes 

 70 



Minutes 

 160 

 90 



Minutes 

 290 

 220 

 130 



Minutes 

 510 

 440 

 350 

 220 



Minutes 

 730 



40 . 



660 



30. 





570 



20 







440 



10 









220 















Measurements of the temperature of vegetables should be made after 

 they have cooled on the belt. Such measurements can be made by 

 thrusting a thermometer into the center of a filled carton or can and 

 by allowing 10 minutes before the reading is made. Measurements 

 on rate of cooling of potato strips in cans have demonstrated that 

 the center of a full 5-gallon can when filled at 105° F. and placed 

 in air at 85° will cool to 90° in 7 hours. Estimated cooling times for 

 dehydrated potato strips in 5-gallon square cans, standing alone, are 

 shown in table 11. 



The estimated time required to cool the same potato strips in 5-gal- 

 lon-size lead-foil packages placed in outer cartons approximately 

 13% x 7y 2 inches in size is the same as for the metal cans {38) . If the 

 cartons are stacked back to back on their sides, in a stack two cartons 

 thick, the estimated time required to cool the contents from 105° to 

 90° F. at the central plane of the stack with air at 75° is 7 to 8 days. 

 Similarly, for a stack that is four cartons thick the estimated time is 

 4 to 5 weeks (41). 



Cartons and cans should be kept apart or unstacked, as far as pos- 



