Preserving Appeal

Preserving Appeal

Fresh Florida citrus is well known for its excellent taste and juiciness. However, peel disorders sporadically develop after harvest that make the fruit unmarketable, or even worse, show up upon arrival at destination markets. These can be difficult to mitigate because a wide range of peel disorders can produce similar symptoms, and we often do not fully understand the underlying cause(s). Resulting economic losses can run into the millions of dollars and reduce buyer confidence for future purchases. Some types of peel breakdown, such as oil spotting (oleocellosis), chilling injury (CI), and one type of postharvest pitting (PP) are easier to mitigate because they are more clearly related to specific harvest or postharvest handling practices. For example, PP has been related to low internal oxygen concentrations within warm (>50°F) fruit coated with waxes that restrict gas diffusion (e.g., some shellac-based waxes). Improved cooling practices and the use of coatings with better gas diffusion have reduced the occurrence of PP. CI is caused by holding fruit at low, but non-freezing temperatures during storage and transit. However, modern handling practices have made CI a rare occurrence on Florida citrus.

Other types of peel disorders, such as stem-end rind breakdown (SERB) and general peel pitting are influenced by both pre- and postharvest factors and their occurrence can vary considerably from one season to the next. While the cause(s) of these disorders are not completely understood, they appear to be most related to the water status of the fruit. They may first appear during the winter months, especially after cool and/or windy weather with low relative humidity (RH), and continue into the spring at the end of the dry season and as field temperatures increase. Tree water stress from the lack of rain or insufficient irrigation before harvest can significantly increase peel breakdown after harvest. Conversely, application of an antitranspirant (e.g., 1% or 2% Vapor Gard) to the trees decreases the permeability of the fruit cuticle to water loss and reduces postharvest peel breakdown.

Health Factors

Plant nutrition, especially low potassium (K), also has been associated with peel disorders in citrus fruit such as creasing and pineapple orange peel pitting. Potassium plays an important role in osmotic (water potential) regulation of cells and in regulating stomatal closing. Recently, we found a single preharvest foliar application of mono-potassium phosphate (0-52-34; 23.5 lb. MKP per acre; 8 lb. K2O per acre) can often significantly reduce postharvest peel breakdown. Thus, before harvest, it is important to maintain an adequate supply of water to the trees and consider a preharvest foliar K or an antitranspirant spray if the block has a history of developing peel disorders.
 
After harvest, drying conditions that promote peel breakdown arise from delays in packing, holding the fruit under low RH and high temperatures, and excessive air movement around the fruit. Studies by Dr. Jacqueline Burns’ group (UF/IFAS) and others worldwide have shown that postharvest fruit exposure to low RH conditions (i.e., 30%) for as little as a few hours, followed by high RH (e.g., 90%) conditions induces peel pitting. Even low RH conditions in the field at harvest can result in greater pitting of the fruit after harvest. Excessive brushing during packing also increases water loss and enhances peel breakdown.

Time And Care

After harvest, the best way to prevent peel breakdown is to minimize the time between harvest and waxing, especially during hot, dry, or windy weather, and to keep the fruit continuously under high RH conditions (>90%). Avoid warm temperatures because even with the same RH, warmer air dries fruit faster. During packing, avoid excessive brushing that increases water loss, keep brush speeds below 100 rpm, and use automatic wipeouts to prevent fruit from sitting idle on the brushes. Cool the fruit as soon as possible and maintain low (but non-injurious) temperatures throughout storage, transport, and marketing.

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