6 BULLETIN 1141, U. S. DEPARTMENT OF AGRICULTURE. 



required for drying a given quantity of material, but are more ex- 

 pensive to construct and operate, are dependent for successful opera- 

 tion upon a source of power for driving the fans, and require a 

 higher class of labor because of their more complicated character. 

 In practice their efficiency is often lowered by reason of the fact that 

 the rate of air movement is excessive, causing rapid drying of the sur- 

 face of the material and thus retarding the escape of moisture from 

 the interior of the product. 



TVhatever the type of drier employed may be. it is essential to 

 success that drying be continuous and that the temperature be main- 

 tained fairly uniform throughout the process. The flesh of the 

 various fruits is an excellent medium for the growth of a great variety 

 of microorganisms, including both bacteria and fungi. As soon as 

 the material is opened up to the air it becomes subject to attack by 

 these organisms, which are present in great numbers in the form of 

 spores upon the skins of fruit and in the dust of the workroom, with 

 the result that considerable numbers become scattered over the sur- 

 faces of the prepared fruits. As long as the temperature of the 

 material is maintained fairly high and the escape of moisture from 

 the surface goes on uninterruptedly, the conditions are unfavorable 

 for the growth and multiplication of these organisms, but if the 

 temperature is permitted to drop and moisture accumulates upon the 

 surface of the material, the spores germinate, the organisms mul- 

 tiply rapidly, and fermentation or souring of the material begins. 

 For this reason material should never be prepared and allowed to 

 stand before being placed in the drier, and the drying should be 

 completed without interruption. 



Other conditions prerequisite to success in drying have to do with 

 the selection and preparatory treatment of the material to be dried, 

 and these determine the quality of the product to an even greater 

 degree than does the type and manner of operation of the drier. 

 Drying will not make an inferior article better, nor does it ever pro- 

 duce products indistinguishable from fresh. When conducted with 

 the greatest care and the employment of the best methods known, 

 dried products undergo perceptible modifications which do not affect 

 their food value, but, nevertheless, make it easy to distinguish them. 

 In order that these alterations, which affect texture and flavor in 

 some degree, may be at a minimum, the materials used should be in 

 prime condition, fully ripe, free from decay, and should be handled 

 throughout their preparation with scrupulous regard for cleanliness. 

 "With most of the fruits usually dried, chemical changes which modify 

 the color and appearance of the fruit set in as soon as the flesh is 

 exposed by peeling and slicing. The application of such a degree 

 of heat as can be used without injury to the material does not arrest 

 these changes, but, on the contrary, it causes them to go on at a much 

 more rapid rate than would be the case at ordinary air temperature 

 until most of the water of the fruit has been driven off. This is 

 particularly true of the oxidation of pigments and the changes in 

 tannin occurring in such fruits as the apple, pear, apricot, and peach, 

 which are responsible for the appearance of brownish discoloration 

 of the flesh of these fruits when peeled. While such discoloration 

 does not injuriously affect the palatability or the food value of the 

 fruits in which it occurs, the purchasing public has learned to dis- 

 criminate against such fruit, and the existing grades for commercial 



