EVAPORATIOX OF FRUITS. 41 



floor. By reason of the rapid escape of moisture, the actual tempera- 

 ture of the layer of fruit will be much lower, the difference vary- 

 ing with the efficiency of the ventilators, but usually being 20 to 30 

 degrees. After five or six hours the fruit is turned and opened up 

 so that the air passes through it more readily, and the fires are so 

 regulated that the temperature, measured in a bare spot on the floor 

 as before, .rises to 165° to 175° F., but is not allowed to exceed the 

 last-named figure. It is maintained as nearly constant as possible 

 until the fruit has lost approximately two-thirds of its water, when 

 the firing is somewhat slackened and the drying completed at a floor 

 temperature of 150° to 160° F. 



The practice just described can be commended in the light of ex- 

 perimental results obtained in the laboratories of the Bureau of 

 Plant Industry. The reduction in temperature as the fruit ap- 

 proaches dryness is especially to be advised, for the reason that the 

 temperature of the fruit, which is at first far below that of the sur- 

 rounding air, gradually rises as drying proceeds and approaches that 

 of the air when nearly dry. Hence caramelization of the contained 

 sugar and breaking down or volatilization of flavoring substances, 

 with resulting injury to the color and flavor of the product, is more 

 likely to occur as the process nears completion. In work in which 

 the temperature is under accurate control and can be quickly altered 

 at will, temperatures considerably higher than those named have been 

 employed in the earlier part of the drying without injury, but with 

 the imperfect control of temperature and ventilation found in actual 

 practice, the upper limits mentioned should not be exceeded. Some 

 operators aim at maintaining a uniform temperature of 150° to 

 155° F. throughout the entire process, and a few operate at still lower 

 temperatures. Such practice materially lengthens the time required 

 for drying and cuts down the working capacity of the kiln without 

 producing a corresponding improvement in the quality and appear- 

 ance of the product. 



In drying apples in the tunnel evaporator, most operators maintain 

 an air temperature of 165° to 175° F. directly over the air inlet at 

 the lower end of the tunnel, while the temperature at the base of the 

 ventilating shaft will be 20 to 25 degrees lower. In consequence the 

 freshly introduced fruit is subjected to a temperature of 145° to 

 150° F. at the outset, and this is progressively increased as the drying 

 proceeds. 



Turning the fruit. — To prevent the fruit from burning and from 

 sticking to the floor by remaining in contact with it too long and 

 to insure the most uniform drying that is possible, the fruit, in 

 the case of the kiln driers, is turned occasionally. The interval be- 

 tween turnings varies with different operators, with the condition of 

 the fruit, and with the degree of heat which is maintained. Some 

 operators do not turn the fruit until five hours have elapsed after 

 the furnace has been started, and this is to be commended as causing 

 less breaking of rings, although it is a more common practice to 

 make the first turning within two to three hours after the drying is 

 begun, or even sooner. After the first five or six hours it is generally 

 turned every two hours, and more frequently as the fruit becomes 

 drier, until perhaps it may require turning every half hour when 

 nearly dry. 



