Temperature Effects on Produce Degradation 609
is a balance in respect to moisture, and virtually no moisture is lost from the produce
to the air. If the RH of the room is low, water vapor is readily transferred from the
humid interiors of the produce to the relatively dry air. This moisture loss may
manifest itself as shriveling and wilting, and many products show visible effects of
moisture loss after losing 3 to 5% of their initial weight.^1
The amount of moisture the air can hold rises as the temperature increases. More
water is required to saturate air at 16°C than at 4°C.^10 Therefore, at 16°C and 90%
RH the air is drier than at 4°C and 90% RH, and there is more rapid dehydration
of produce at the higher temperatures. Because of the relationship of RH to tem-
perature, RH is always expressed with temperature.
Vapor pressure deficit (VPD) is a term used to indicate the combined influence
of temperature and relative humidity and is related directly to the rate of water loss
from produce.^1 The VPD may be expressed as VPD = VP (100 – RH/100), where
VPD = vapor pressure deficit (millimeters or inches of Hg), VP = vapor pressure of
water at a given temperature (millimeters or inches Hg), and RH = relative humidity
(percentage). It is apparent from the equation that the VPD increases as the VP
increases. An increase in VP in the produce would occur with increased temperature
since VP is directly related to the temperature of water and most produce is largely
water. In addition, the VPD will increase as the RH is lowered. Thus, it would be
expected that VPD would be especially high during the hot, dry weather that accom-
panies harvest in many growing regions.
In addition to a greater VPD at higher temperatures leading to more moisture
loss from produce, water evaporates from the produce more quickly at higher
temperatures.^11 The dual effect of temperature on RH and increased tendency for
evaporation at higher temperatures means that as storage temperature increases the
quality of produce tends to decrease more rapidly. Produce in the field at 25°C and
30% RH loses water 36 times faster than it does when cooled and stored at 0°C and
90% RH. Thus, maintaining low product temperature can be a mechanism for
reducing water loss and subsequent shriveling and wilting.
FIGURE 20.2Ethylene-induced floret yellowing of broccoli is enhanced by increasing tem-
peratures. (Photo courtesy of A.A. Kader, University of California, Davis.)