COMMEIM [AL DEHYDRATION 15 



affords better opportunity to regulate humidity and temperature 

 than the tunnel types. It is suited to smaller volume and noncon- 

 tinuous use and is less economical of heat than the tunnel. It is es- 

 pecially useful where shortage of labor makes 34-hour operation im- 

 possible or where the available volume of raw materia] is insufficient 

 to justify large-scale operation. 



Vacuum drying is used for various types of materials, especially 

 those that dry best at low temperature or with minimum exposure to 

 air. In the vacuum dehydrator moisture is removed from the food 

 in a chamber from which nearly all the air has been exhausted. 

 Vacuum driers require much metal and are more complicated than 

 other types. The method is used largely to "ultradry" certain foods 

 from which most of the water has already been evaporated by other 

 means. 



There are driers of other types for handling materials that have 

 been liquefied, pulped, pureed, or finely ground. In -pray drying, 

 liquid material is sprayed into a chamber through which a current 

 of hot aii - moves, taking away the moisture and reducing the solid 

 matter to a powder. Large quantities of spray-dried milk and eggs 

 are being sent to the armed forces and to lend-lease nations. Cer- 

 tain fruit and vegetable juices can also be spray-dried, and though 

 production is now small, research and experience may show the way 

 toward quantity production and better quality of Mich products. 



Pulpy matter and meat may be dried on a hot drum from which 

 the dried material is scraped in Hakes. Tomato flakes, cranberry 

 Hakes, and similar products are being made by the drum-diving 

 method. Like other methods id' dehydration, spray and drum dry- 

 ing are being studied and are developing rapidly. 



DRYING TEMPERATURES AND LENGTH OF DRYING PERIOD 



Drying follows immediately after blanching. In the tunnel and 

 cabinet methods the damp, partly cooked material is spread on trays 

 so that the warm dry air of the dehydrator can reach as much of the 

 surface of each piece as possible. This means that trays must not be 

 overloaded (fig. 8). 



Temperatures of air. rates of air movement, and length of time re- 

 quired to complete the drying vary widely with the method used and 

 the material. Some products take longer to dry than others, but the 

 usual range is between '■) and 8 hours, due initial temperatures may 

 be as high a- •_!■_'<) F. for certain foods, bill the temperature at the end 

 of the drying period is usually 40° to 80° lower. A.S high an air 

 velocity as can be attained economically and as high a temperature as 

 can be used without scorching or otherwise injuring the product are 

 recommended. Vitamin (' retention has been found to be better in hot, 

 rapid drying than in slower, cooler drying. 



The dried food is sometimes removed from the trays and placed in 

 finishing, or curing, bins, where a uniform moisture content of the en- 

 tire mass is attained. Food containing 7 to l.~> percent of moisture 

 may be dried in finishing bins to a 5-percent moisture content by a 

 slow movement of air at 110 to 120 F. From the bins the dehy- 

 drated food goes into the packages. 



