4 BULLETIN 1141, U. S. DEPARTMENT OE AGRICULTURE. 



largest possible number of fruit growers. They are purposely so 

 planned as to be capable of enlargement or alteration to meet the 

 needs of a particular case. 



PRINCIPLES INVOLVED IN DRYING FRUITS. 



The purpose in view in drying any food material is to reduce its 

 moisture content to such a point that the growth of organisms therein 

 will no longer be possible, and to do this with a minimum of altera- 

 tion in the food value, appearance, and palatability of the product. 

 The necessity for avoiding changes in physical appearance and chem- 

 ical composition, other than actual loss of water, puts very definite 

 limitations upon the means which may be employed to bring about 

 drying and makes an understanding of certain principles a pre- 

 requisite to successful work. 



It is obviously of advantage that drying be brought about as rap- 

 idly as possible, since rapid drying minimizes opportunity for the 

 growth of organisms and for the spontaneous chemical changes which 

 set in as soon as the interior of the fruit is exposed to the air and 

 since a short drying period increases the working capacity of the 

 drying apparatus. There are three possible ways in which drying 

 may be accelerated, namely, by passing currents of air over the 

 material, thus giving a large volume of air for absorbing and carry- 

 ing away moisture, by raising the temperature and consequently the 

 moisture-absorbing capacity of the air, and by employing air which 

 has previously had all moisture removed from it by passing through 

 an air-drying apparatus. A theoretically perfect method of drying 

 would, of course, combine all three means of hastening the process, 

 and while such methods are in use in certain industries, they are im- 

 practicable for drying fruit because of their high cost. 



Consequently, practical drying methods rely upon the use of heated 

 air, with some means of maintaining the air in circulation over and 

 through the product. - 



The moisture-carrying capacity of the air is a function of its tem- 

 perature, and is practically doubled by every increase of 27 degrees 

 in temperature. Consequently, the application of a relatively mod- 

 erate degree of heat brings about a very great increase in its capacity 

 to absorb moisture. This is evident from a consideration of the fact 

 that raising the temperature of a quantity of air 108 degrees, from 60° 

 to 168° F., for example, results in a sixteenfold increase in its mois- 

 ture-carrying capacity. The temperature employed is consequently 

 the most significant factor in determining the drying rate, and it is 

 advisable to employ the highest temperature which can be used with- 

 out injury to the material. But the use of extreme temperatures in 

 drying fruits is impossible for three reasons. The various fruits 

 contain 65 to 88 per cent of water when prepared for drying. If 

 such water-filled material were suddenly exposed to dry air having 

 a temperature approaching that of boiling water, the rapid expansion 

 of the fluids of the tissues would burst the cell membranes, thus per- 

 mitting the loss of many of the soluble constituents of the fruit by 

 dripping. Some decomposition of the sugars of the fruit would also 

 occur at such temperatures, and the caramel formed would injure 

 both the flavor and the appearance of the dry product. Furthermore, 

 the very rapid drying of a thin layer at the surface of the material 



