EVAPORATION OF FRUIT. 



407 



Fig. 76. 



Attention must especially be drawn to the fact that dextrin, 

 the formation of which is due to dry heat, is only found in the 

 second column, and must be considered 

 as an essential disadvantage of drying 

 in the oven. The absence of this sub- 

 stance in evaporated fruit, as well as 

 the presence of a larger quantity of 

 water (chemically fixed), is to be as- 

 cribed to the influence of moisture dur- 

 ing evaporation. 



A presentation of the process of evapo- 

 rating fruit must be preceded by a de- 

 scription of the apparatus used. Fig. 

 76 shows the Alden apparatus as re- 

 cently perfected. A is the air-furnace, 

 which is formed by the fire-box Z>, the 

 ash-box D v and the doubled horizontal 

 pipes G, of which, according to the size 

 of the apparatus, there are from 3 to 6, 

 each 4 inches in diameter, and running 

 parallel to each other ; the products of 

 combustion pass through them in the 

 direction of the arrows, and escape 

 through the smoke-pipe at the back 

 of the apparatus. The fire-box is sur- 

 rounded with an air-space provided at Mwith apertures. Similar 

 apertures to permit the entrance of cold air are provided on the 

 side near the foot of the brick casing. The cold air comes first 

 in contact with the lower, only moderately heated, pipes, then 

 rises to the second, and finally to the third and hottest series of 

 pipes. It is thus gradually heated, and the pipes lying close to- 

 gether, each atom of air comes in contact with them, which is con- 

 sidered a better mode of heating than by radiation, formerly used. 

 The pipes are of cast-iron, and an escape of smoke into the dry- 

 ing-tower is impossible. By always keeping the pipes clean, 

 which -can be conveniently done, the heat passes rapidly through 

 their walls, and ascends immediately into the drying-tower with- 

 out the possibility of super-heating. 



