VEGETABLE AND FRUIT DEHYDRATION 



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Air Temperature, Dry- Bulb (°F.) 



Figure 23. — Humidity chart — absolute humidity. 



Characteristics of Evaporation from a Moist Solid 



^ Moisture content. — The moisture content of a fruit or vegetable, 

 either fresh or dried, is usually expressed as a percentage by weight. 

 For example, the analysis of a sample of blanched potato strips may 

 be given as 80 percent moisture — that is, in 100 grams of sample there 

 are 80 grams of water and 20 grams of "bone-dry" matter. Calcula- 

 tions of processes in a drier are much simpler, however, if the moisture 

 content is expressed on the "dry basis" ; that is, as the ratio of water 

 content to dry matter. In the foregoing example this ratio is 4.0. 

 The convenience of this method arises from the fact that the weight 

 of dry matter remains constant as material progresses through the 

 drier, and provides a constant base for calculation of evaporation. 

 For example, the following important, but somewhat paradoxical, 

 statement can be verified by mental calculation : Most of the water 

 is evaporated while the material is still very wet. 



Suppose the material consists of carrot strips, in which the original 

 moisture content is 8 pounds of water per pound of dry matter. Then 

 8 of every 9 pounds loaded on a tray is water, and 1 pound is dry mat- 

 ter. When 7 pounds, or seven-eighths, of the water has been evapo- 

 rated, 1 pound of water is left with the same 1 pound of dry matter; 

 the moisture content has been reduced only to 50 percent. About 99.3 

 percent of the total water must be evaporated before the moisture 

 content of the product falls to 5 percent. 



If the moisture contents both of the fresh material entering the 

 drier and of the product leaving the drier are known, the drying ratio, 

 or its reciprocal, the drying yield, can be calculated. The formulas 

 may contain either "percent moisture" (wet basis) or moisture content 

 (dry basis) : 



Drvmp- mtin- Weigllt entering drier _ 100-.M / _ T o +l 

 trying ratio- Weight leaving drier -i00-M o ~"2/+l 



