- u- 



This oversight can lead to serious consequences in either of 2 ways: if 

 humidity is too high, decay is accelerated; and if humidity is too low, 

 weight loss is accelerated. As we are now entering into the storage sea- 

 son, a review of these problems seems appropriate. 



Fungal spores are always present on the surface of fruits. For the 

 spores to germinate and the fungi to attack the fruit, moisture is needed. 

 If the humidity around the fruit is at or near 100% relative humidity 

 (RH) , spores will rapidly germinate, and if free moisture is on the fruit, 

 germination is even more intense. Once spores are germinated, high hu- 

 midity must continue as the young fungi develop, for if they dry out 

 they will die. However, growth is very rapid, and high humidity is nec- 

 essary only for a period of hours before infections become established 

 and unaffected by humidity changes. To reduce the disease problem, stor- 

 age recommendations usually call for 90-95% RH, never 100%. 



However, at anything less than 100% RH, moisture loss occurs and 

 the fruits consequently lose weight and may shrivel. To understand and 

 appreciate the seriousness of the moisture-loss problem, we must under- 

 stand the concept of "vapor pressure." Relative humidity tells us the 

 percent of saturation with water vapor of the atmosphere at a given tem - 

 perature. But as temperature changes, the saturation value changes, and 

 therefore RH changes, for it is a relative rather than absolute value. 

 Vapor pressure (VP) , on the other hand, is an absolute measure of the 

 moisture in the atmosphere. Vapor pressure is the pressure exerted by 

 the water vapor in the atmosphere. If the atmosphere is kept saturated 

 while temperature is increased, RH will continue to be 100%. However, 

 VP will rise markedly, simply because at a higher temperature the atmos- 

 phere can hold more moisture. Since we dare not maintain 100% RH within 



a storage, another term becomes important vapor pressure deficit (VPD) . 



This value is the difference between the VP at saturation (100% RH) and 

 that at whatever RH and temperature exist at a given time. VPD is ex- 

 tremely important, because rate of moisture loss is directly propor- 

 tional to it. 



VPD increases sharply as RH falls at a given temperature, or as tem- 

 perature rises and RH remains constant, and it rises extremely fast as 

 temperature rises and RH falls. This is illustrated in Table 1. At 32° F, 

 VPD increases from 0.229 to 2.290 as humidity drops from 95% to 50%; and 

 at a constant 95% RH, the VPD increases from 0.229 to 0.887 as temperature 

 rises from 32° to 68 F. The meaningfulness of these figures lies in the 

 fact that rate of moisture loss is directly proportional to VPD, and that 

 a fair estimate of the differences in effects of various conditions on 

 rate of moisture loss can be found by dividing the larger VPD by the 

 smaller VPD. Therefore, in our above examples, at 32° F moisture will 

 be lost 10 times as fast at 50% RH as at 95% RH (2.29 '- 0.229). Any 

 comparison can be made among the values in Table 1, and so it can be 

 seen that if fruit is changed from 32° and 95% RH, to 50° and 85% RH, 

 rate of moisture loss will be increased 6 times (1.38 - 0.229); or from 

 32° and 95% RH to 68° and 60% RH, moisture loss will be increased about 

 31 times (7.016 ^ 0.229). It should now be clear that both humidity and 

 temperature are important factors influencing moisture loss. And, as 

 temperature increases, humidity control becomes increasingly important 

 in reducing moisture loss. 



