24 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



the design of the drier, the entire charge of the product will be dried 

 to the same degree when removed from the drier. If the product 

 does not dry uniformly, it is in all probability because of a varying 

 degree of maturity or size of the fruit. This indicates lack of care 

 in grading. Uniform material should be put onto the trays of each 

 truck, in order to secure a uniformly dried product. 



Speed in drying is essential. On it depend not only the quality 

 of the product but, of equal importance, the capacity of the drier 

 and the day's or season's output. As the rate of evaporation is in 

 direct proportion to the temperature, it is advisable to start the drying 

 with a high temperature and suitable humidity, gradually reducing 

 the temperature and humidity to the end of the drying period when 

 the products have but little moisture left to protect them from 

 scorching. 



Tiemann (17) states that in drying lumber the actual temperature 

 of the wood, while it is moist, is that of the wet bulb, provided there 

 is sufficient circulation. In our investigations we did not find that 

 the vegetables or fruits being dried were at the same temperature 

 as the wet bulb. While considerable evaporation was taking place, 

 the fruit showed a temperature that ranged about midway between 

 the temperature of the wet bulb and the temperature of the dry 

 bulb of the hygrometer. After evaporation had slowed down per- 

 ceptibly, the temperature of the fruit began to rise, and by the end 

 of the drying period it had closely approached the dry-bulb temper- 

 ature. 



The optimum drying temperature for apricots is considered to be 

 150° F. After !}{ hours' drying in air at 175°, it was found that the 

 fruit had become heated to 150°. The temperature of the drying air 

 was then cut down to 150°, and the drying was finished. An excellent 

 dried product was obtained. The total drying time of 12 hours could 

 no doubt have been shortened by using a greater blast of air. Similar 

 adjustment of temperature during drying should be made for each 

 material dried. 



When blanching is necessary, as with vegetables, this operation 

 may be done in the drying compartment. This makes use of the 

 heat and moisture in the steam used for blanching, and the dehydrator 

 thus serves a double purpose. In addition to economies in heat, 

 there is a saving in labor for handling trays and trucks over that 

 required in a factory where it is necessary to transfer them from the 

 blancher to the drier. 



The dehydrator is 60 feet long, 14 feet high, and 4 feet wide. There 

 are six compartments, each of which accomodates one truck hoi ing 

 30 trays (fig. 6). Each tray is 30 inches square and 3 inches c^ep. 

 This gives a total spreading area of 1,125 square feet, providing a 

 loading capacity of 2 tons of such fruits as prunes, or about % ton of 

 ground oranges for marmalade, or }{ ton of trimmed spinach. Each 

 compartment is a complete drying chamber independent of the 

 others and may be used alone with all the others shut off, or it may 

 be used in conjunction with any or all of the others. A seventh 

 compartment houses the radiating units and the fan. Steam is used 

 as a heating medium to warm the air. A large blower fan, motor- 

 driven, is used to circulate the air through the drying compartments. 

 The air leaves the fan at the rate of 18,000 cubic feet per minute and 

 is distributed by means of a short sheet-iron duct, vanes, and sluices 



