Keeping CO₂ at bay: avoiding CO₂ injury in apples
Emily Johnson, M.S. Student in Plant Sciences and Macarena Farcuh, Ph.D. Assistant Professor and Extension Specialist University of Maryland, College Park
What is CO₂ injury in apples?
Physiological disorders are abnormalities in fruit tissue not caused by mechanical damage or pathogens. Damage to fruit tissue from disorders can impair marketability, profitability and ultimately increase food waste due to unsuitability for sale. One important physiological disorder that can affect apple fruits, externally and internally, corresponds to carbon dioxide (CO₂) injury.
External CO₂ injury (Figure 1) is observed as light bronze lesions that are rough in texture that can become partially sunken and irregular in shape as the injury advances. Internal CO₂ injury (Figure 2) is characterized by brown cortex tissue, with healthy tissue surrounding the discolored areas. The inner necrotic tissue is initially firm and moist but desiccates and becomes a cavity as the injury progresses. The presence of a strong fermented aroma might occur. High CO₂ concentrations within cells cause oxidative stress, which can increase cell membrane damage. Membrane damage is followed by enzymatic oxidation of phenolic compounds resulting in brown flesh and a change in flavor. CO₂ injury is induced within the first four months of postharvest controlled atmosphere storage. Both external and internal injuries will progress until removal to normal atmosphere but can then develop fungal infections.
What factors cause or contribute CO₂ injury development in apples?
Genetic background: Some cultivars, such as ‘Honeycrisp,’ ‘Braeburn,’ ‘Fuji,’ ‘McIntosh,’ ‘Cortland,’ ‘Golden Delicious’ and ‘Empire,’ are more genetically susceptible to CO₂ injury. The manner of injury amongst these sensitive cultivars also varies in prevalence. For example, ‘Cortland’ apples have been associated with greater incidence of external CO₂ injury.
Maturity at harvest: Larger, overmature fruit harvested later in the season are usually associated with higher CO₂ concentrations in storage, yet this can vary by external or internal injury. Fruit with progressed maturity are at a greater risk of internal browning, while immature fruit tend to develop external skin lesions.
Preharvest environmental and orchard management factors: Incidence and severity of the disorder can vary by season, location, and orchard. There is a high association of development of CO₂ injury in fruit grown under cool and wet climatic conditions. This may be related to fruits that are denser, with a higher number of cells, which can, in turn, lead to altered gas diffusion and cellular metabolism. Furthermore, unbalanced fertilization practices may also increase CO₂ injury susceptibility. High Nitrogen (N) fertilization may increase competition with Calcium due to vigorous tree growth, thus affecting cell membrane stability.
Postharvest storage: CO₂ injury develops early in storage (within the first 4 months) and results from high carbon dioxide levels, more than 0.5%, and low oxygen levels, less than 1.5%, during controlled atmosphere storage. Rapid establishment of controlled atmosphere with high CO₂ levels before apples have cooled to reach the storage temperature led to increased risk of internal and external CO₂ injury. Additionally, the use of ethylene inhibitors, such as 1- methylcyclopropene (1-MCP, Smartfresh, Agrofresh) have shown to aggravate the disorder in ‘Honeycrisp’ apples.
What practices can be implemented to reduce CO₂ injury development?
Harvest strategies taking cultivar susceptibility into account, such as matching fruit maturity to storage potential and ensuring harvest at the proper ripening stage, may aid in injury reduction. The optimal fruit maturity for harvest should be targeted, especially for fruit that will be subjected to controlled atmosphere storage. Maintain low levels of carbon dioxide (< 1%) in storage atmospheres during early controlled atmosphere and in air storage. Implementation of controlled atmosphere storage should be delayed by around 4 weeks, as this aids in avoiding injury both internally and externally. This has been demonstrated in cultivars such as Fuji and Pink Lady. Maintain good air circulation/ventilation to avoid pockets with high carbon dioxide accumulation and sustain proper temperatures throughout storage. Rapidly cool fruit after waxing and packaging. Limit the use of 1-MCP in storage. Dipping or drenching apples in diphenylamine (DPA), an antioxidant traditionally used for inhibition of superficial scald, has been shown to help decrease the incidence of the disorder by reducing the potential for oxidative stress and damage.
This article appears in Volume 15, Issue 7 of the Vegetable and Fruit News.