Relative humidity (RH) is another important consideration in storing fruit and is defined as a ratio of the quantity of water vapor present to the potential maximum amount of vapor at a given temperature and pressure. The major constituent in a piece of fruit is water (Table S3.1). Intercellular space within healthy fruit is saturated at approximately 100 percent RH. To decrease the exposure time requirement in precooling, refrigeration coils have relatively low set points, which create a large difference between coil and fruit temperature. Actively growing and harvested fruit regulate temperature and give up heat by releasing water vapor. The cooling process results in an unavoidable loss in fruit weight. The relatively large amount of water given up by fruit during the precooling process is frozen on the coils and requires several defrost cycles each day. After fruit reach the targeted storage temperature, the objective in the cold-storage room changes to maintaining a stable storage temperature. At this point, coil temperature can be raised to roughly 0.5°C lower than the targeted fruit storage temperature.
A refrigerated storage room filled with fruit will come to equilibrium at around 85 to 90 percent RH. As air circulates around the cold coils, suspended water vapor freezes on the coils and the RH of the room is lowered. The difference between the internal fruit RH and the surrounding atmosphere creates a gradient that draws additional moisture out of fruit. Several techniques can be employed to reduce fruit water loss. Restricting fan speed to lower velocities of 15 to 23 meters per minute can reduce the rate of water loss. Covering individual fruit bins or containers with polyethylene increases the RH of the atmosphere surrounding fruit and slows the rate of water loss. Compared to conventional wooden bins that can absorb water equivalent to approximately 15 percent of their weight, plastic bins do not absorb water and thus reduce fruit shrinkage due to water loss. Adding standing water to the floor of a cold-storage room does little to raise the RH or prevent fruit weight loss, as water changes physical states from liquid to vapor at the interface of the liquid and the atmosphere. The best method to increase RH is to supply water in a fashion that increases the total available surface area of the liquid. This is best accomplished by adding water in the form of a fine fog. The combined surface area of a fog comprised of millions of small, micron-sized droplets is several orders of magnitude greater than the entire surface area of a floor covered with standing water.
With humidity, relative is an important term. In contrast to warm air, cold air has a very limited capacity to support water vapor. The atmosphere of a 100 cubic meter cold room at 0°C and 100 percent RH can support 480 grams of water. The difference between 85 and 95 percent RH at 0°C storage can have significant quality effects, even though it represents only 50 grams of suspended water. Keeping fruit at or close to 100 percent RH over long periods of time, however, can have some detrimental effects, such as fruit splitting, skin cracking, or increased decay. Extremely hydrated fruit also are more susceptible to bruising during handling and packing following storage.
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