12 CIRCULAR 65 9, U. S. DEPARTMENT OF AGRICULTURE 



Atmospheric Humidity 



Keeping the apples from wilting and shriveling is as important as 

 keeping them from getting overripe and can be done by controlling 

 the humidity in the storage space. A certain amount of moisture is 

 continuously lost by the apples through the process of transpiration 

 (evaporation). 



Although it can be cut down by reducing the temperature, it is par- 

 ticularly serious in apples which are not properly matured and on 

 which lenticels are not normally corked over and the wax coating is not 

 sufficiently developed. The natural barrier to loss through evapora- 

 tion is the waxy skin of the fruit. Any injury to the skin or the re- 

 moval of the wax, as in some washing processes, facilitates moisture 

 loss and increases the severity of wilting. Careful handling to avoid 

 bruising and injury of the skin and the use of proper washing processes 

 are important therefore in preventing excessive wilting or shrinkage 

 in storage as well as blue mold rot and other types of decay. The drier 

 the air the greater is the degree of wilting. The faster the air is moved 

 over the apples the more moisture will be removed from the fruit. The 

 extent to which this occurs in apples has not been determined exactly, 

 but in grapes Allen and Pentzer (7) found that doubling the air 

 movement increased moisture loss by about one-third and was equiva- 

 lent to about a 5-percent drop in relative humidity. If the relative 

 humidity of the storage is maintained at 85 percent or higher, which 

 is the approximate water content of apples, the "pull" on the fruit for 

 moisture or the evaporating power of the air will be slight and shrink- 

 age of the fruit will be correspondingly lessened. Observations in 

 commercial cold storage plants have shown that the relative humidity 

 in the apple storage rooms is f requently less than 85 percent, sometimes 

 as low as 70 percent. 



To maintain desirable humidity conditions and to reduce chances of 

 wilting, it is always desirable to fill the storage rooms, thus reducing 

 as much as possible the ratio of air to fruit. In such cases, however, it 

 is essential to have a gentle air movement through and around the 

 stacked fruit to insure proper cooling. This is not a serious problem 

 when apples are packed in bushel baskets (fig. 4), but it is when boxes 

 or crates are used. In the latter event, the stacks should be spaced an 

 inch or two apart and laths or other dunnage strips should be used to 

 separate the boxes or crates in each stack. When a storage room is 

 completely filled with apples, the humidity will be almost automati- 

 cally adjusted at the desired level. 



It is desirable to have sufficient coil surface in a cold-storage room 

 to provide the desired cooling without using excessively low tempera- 

 ture in the cooling medium. The greater the difference in temperature 

 between the cooling surface and the air of the storage room, the more 

 the air will be dried by the freezing out of moisture and the greater 

 will be the pull on the fruit with corresponding wilting effects. If 

 there is sufficient coil surface to permit use of the cooling medium at 

 not more than 7° or 8° F. below the desired temperature of the room, 

 the freezing out of the moisture from the air onto the coils will be 

 minimized. 



