PRESERVATION OF FRUITS AND VEGETABLES 



13 



any temperature is important, 

 since injury can be caused by 

 prolonged exposure at compara- 

 tively moderate temperatures. 



The rate of evaporation from 

 a free water surface increases 

 with the temperature and de- 

 creases with the increase of 

 relative humidity of the air. 

 The complex cellular structure 

 and chemical nature of fruit and 

 vegetable tissues retard evapo- 

 ration so that under no condi- 

 tions of temperature and humid- 

 ity does the rate of evaporation 

 from them equal that from a 

 free water surface. When con- 

 ditions are such that surface 

 evaporation from the tissues ex- 

 ceeds the rate of moisture diffu- 

 sion to the surface, the surface 

 becomes dry and hard and seals 

 in the moisture. This condi- 

 tion, known as case-hardening, 

 is overcome by reducing the 

 temperature of the air or by 

 increasing the humidity. The 

 maximum rate of drying, then, 

 is attained by using the highest 

 temperature which will not in- 

 jure the product, the humidity 

 being sufficient to prevent case- 

 hardening. In general prac- 

 tice the temperature of the air 

 entering the drying chamber 

 should not exceed 160° to 170° 

 F. The humidity at the air- 

 outlet end of the drier should 

 not greatly exceed 65 percent. 

 In driers employing recircula- 

 tion the conditions of tempera- 

 ture and humidity may be 

 largely controlled by varying 

 the recirculation. 



The velocities of air flow 

 which produce the most effi- 

 cient results in the drying cham- 

 ber depend upon several con- 

 ditions. In general the rate of 

 drying increases with the veloc- 

 ity of air movement. Low air 

 velocities tend to bring about 

 slow and uneven drying. Ex- 

 ceedingly high velocities may 



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