Cox’s Orange Pippin is widely considered the most sensitive apple variety to CO2 injury in storage, along with other classic varieties such as Bramley’s Seedling and certain Jonagold strains. These varieties can show internal browning, skin discoloration, and off-flavors at CO2 concentrations that many modern cultivars tolerate without issue. The sections below unpack the causes, thresholds, variety-specific sensitivities, and practical steps growers can take to protect their crop. If you want to discuss storage compatibility for specific varieties, feel free to get in touch with us directly.
What causes CO2 injury in apple storage?
CO2 injury in apples occurs when fruit tissue is exposed to elevated carbon dioxide concentrations for a sustained period, disrupting normal cellular respiration and causing toxic byproducts to accumulate inside the flesh. The damage is primarily internal, presenting as browning of the cortex tissue, cavities, or skin breakdown depending on the variety and exposure level.
Under normal conditions, apples produce CO2 as a natural byproduct of respiration. In controlled atmosphere (CA) storage, operators intentionally lower oxygen and may allow CO2 to rise to slow ripening. However, if CO2 climbs too high or accumulates too quickly before the fruit has properly equilibrated, cellular membranes are disrupted. The result is fermentation-like chemistry inside the flesh, leading to the characteristic brown discoloration known as CO2 injury, sometimes called brown heart or internal CO2 injury.
Temperature, storage duration, and the speed at which CO2 is introduced all influence how much damage occurs. Fruit loaded into a cold store immediately after harvest and exposed to rising CO2 before it has fully cooled is at considerably higher risk than fruit that has been pre-cooled and equilibrated gradually.
Which apple varieties are most sensitive to CO2 injury?
Cox’s Orange Pippin is consistently identified as one of the most CO2-sensitive apple varieties, with Bramley’s Seedling, Jonagold, and certain older Delicious strains also showing high sensitivity. These varieties can develop internal browning at CO2 levels as low as 1 to 2 percent if maintained for extended periods, making careful atmosphere management essential for long-term storage.
The sensitivity of Cox’s Orange Pippin is particularly well documented among UK and European growers. Even brief exposure to CO2 levels above 2 percent during the early weeks of storage can trigger brown heart in a significant proportion of the crop. Bramley’s Seedling, widely grown in the UK for culinary use, shares a similarly low tolerance threshold.
Jonagold and its many sport mutations show variable sensitivity depending on the specific strain and growing region, but the variety as a group is generally regarded as moderately to highly sensitive. Older heritage varieties tend to be more vulnerable overall, partly because they were developed long before modern CA storage practices were standard and were never selected for postharvest atmosphere tolerance.
Which apple varieties are most tolerant of high CO2 levels?
Granny Smith is among the most CO2-tolerant commercial apple varieties, capable of withstanding concentrations of 3 to 5 percent CO2 in controlled atmosphere storage without significant internal damage. Fuji and Gala are also regarded as relatively tolerant, making them well-suited to long-haul storage and export supply chains that require extended CA conditions.
Modern club varieties developed through advanced breeding programs tend to perform better in high-CO2 environments than many older cultivars, partly because postharvest storability is now a deliberate selection criterion. At Better3Fruit, storability is one of the key traits we evaluate across our breeding pipeline, recognizing that a variety’s commercial value depends not only on flavor and appearance at harvest but also on how well it maintains quality through the supply chain.
Pink Lady (Cripps Pink) also shows reasonable CO2 tolerance, which has contributed to its success as a long-storage, export-oriented variety. Braeburn occupies a middle ground, tolerating moderate CO2 levels but becoming sensitive if concentrations rise above 3 percent.
What CO2 thresholds trigger damage in sensitive varieties?
For the most sensitive varieties such as Cox’s Orange Pippin, CO2 concentrations above 1 to 2 percent can trigger internal browning during long-term storage. For moderately sensitive varieties like Jonagold, the threshold is typically 2 to 3 percent. Tolerant varieties such as Granny Smith generally remain undamaged up to 5 percent, though exceeding this level poses a risk for almost all commercial cultivars.
These thresholds are not fixed values. Exposure duration matters enormously. A sensitive variety exposed to 2 percent CO2 for a short period during atmosphere establishment may escape injury, while the same variety held at that level for several months is at high risk. Temperature also modulates the threshold. Fruit stored at slightly higher temperatures tends to be more sensitive because metabolic activity is greater.
Storage operators managing sensitive varieties typically aim to keep CO2 below 1 percent throughout the storage period, sometimes using CO2 scrubbers to actively remove excess gas. For high-value crops where a significant proportion of the harvest is destined for premium markets, this level of atmosphere control is a worthwhile investment.
How does harvest maturity affect CO2 sensitivity?
Fruit harvested early, before it has reached optimal maturity, is generally more susceptible to CO2 injury than fruit picked at the correct maturity window. Immature fruit has higher metabolic activity and less stable cell membranes, making it more vulnerable to the toxic effects of elevated CO2 in controlled atmosphere storage.
This relationship between maturity and CO2 sensitivity is particularly relevant for Cox’s Orange Pippin and Bramley’s Seedling, where early picking to extend the storage season can inadvertently increase injury risk. Growers who use starch-iodine pattern tests, firmness measurements, and background color assessments to time harvest more accurately tend to see lower rates of CO2 injury across the season.
Overmature fruit presents a different problem. Fruit picked too late has softer flesh and reduced storability overall, but its CO2 sensitivity may be slightly lower than immature fruit. The practical takeaway is that harvesting within the optimal maturity window is one of the most effective ways to reduce postharvest losses from CO2 damage, independent of storage conditions.
How can growers reduce CO2 injury risk during storage?
Growers can reduce CO2 injury risk by pre-cooling fruit rapidly before loading into controlled atmosphere storage, establishing the target atmosphere gradually rather than abruptly, monitoring CO2 levels continuously, and using CO2 scrubbers for sensitive varieties. Harvesting at the correct maturity stage is equally important and reduces vulnerability before the fruit even enters the store.
The practical steps most effective in reducing risk include:
- Pre-cooling before CA establishment: Allowing fruit to reach target storage temperature before sealing and adjusting atmosphere reduces early CO2 accumulation stress.
- Gradual atmosphere transition: Stepping down oxygen and managing CO2 rise over several days rather than immediately sealing at target levels gives fruit time to adapt.
- Active CO2 scrubbing: For sensitive varieties, CO2 scrubbers that continuously remove excess gas are the most reliable way to hold concentrations below injury thresholds.
- Continuous monitoring: Real-time atmosphere sensors allow operators to detect and correct CO2 spikes before damage accumulates.
- Variety-specific storage protocols: Not all varieties can be stored under the same conditions. Operators managing mixed-variety stores should apply the most conservative CO2 limits required by the most sensitive variety present.
Choosing varieties with strong inherent storability also reduces exposure to these risks from the outset. Exploring our apple variety portfolio is a useful starting point for growers and packers looking for cultivars bred with postharvest performance in mind. If you want to discuss which varieties best fit your storage infrastructure and market requirements, contact us to plan a conversation with our team.
This content was generated with the help of AI and it may contain mistakes