Controlled Atmosphere Storage: How Oxygen and CO2 Control Keeps Fruit Firm for Months

An apple you buy in Bangladesh may have been picked eight or ten months earlier on the other side of the world. It is still firm because it spent most of that time not just cold, but in an atmosphere with the oxygen deliberately turned down and the carbon dioxide turned up. That is controlled atmosphere storage — CA storage — and it is the technology that lets apples, pears and a few other crops be held for the better part of a year without turning soft and mealy. For anyone importing or planning to store these fruits, it is worth understanding what CA does that a plain cold room cannot.

Fruit is alive in storage

A harvested apple is still a living tissue. It breathes — respiration — taking in oxygen and giving off carbon dioxide, and it keeps ripening. Cold slows this down: every ten degrees of cooling roughly halves the respiration rate, which is why a cold room extends shelf life. But cold alone has a floor. Below a certain temperature the fruit suffers chilling injury, and even at the ideal temperature respiration and ripening continue, just slowly. After a few months in a normal cold store an apple softens, loses acidity and eventually goes mealy. CA storage attacks the problem from a second direction: change the air the fruit is breathing.

What "controlled atmosphere" actually means

Normal air is about 21 percent oxygen and a trace of carbon dioxide. In a CA room the fruit itself, sealed in a gastight space, consumes oxygen and produces carbon dioxide until the balance shifts; equipment then holds that shifted balance steady. Typical targets for apples are oxygen pulled down to a low single-digit percentage and carbon dioxide raised to a low single-digit percentage, with the exact set-points varying by variety. The effects:

• Low oxygen starves respiration. With less oxygen to burn, the fruit’s metabolism slows far below what cold alone achieves, so ripening almost stops.
• Raised carbon dioxide suppresses ethylene action. Ethylene is the plant hormone that drives ripening; high CO2 blunts the fruit’s response to it.
• Firmness and acidity are retained for months longer, and disorders like scald are reduced.

Push the oxygen too low or the carbon dioxide too high, though, and the fruit switches to fermentation, producing alcohol and off-flavours and internal browning. CA storage is a narrow window that has to be monitored, not set and forgotten.

What a CA room needs that a cold room does not

• A gastight envelope. Sealed panel joints, gastight doors, sealed cable and pipe penetrations, and a pressure-relief and breather bag system to handle the expansion and contraction of the gas as temperature moves. A room that leaks cannot hold its atmosphere.
• Oxygen control — usually a nitrogen generator that floods the room to pull oxygen down quickly at the start rather than waiting days for the fruit to do it.
• Carbon dioxide control — a scrubber that removes excess CO2 as the fruit keeps producing it.
• Continuous gas monitoring with alarms, because a drift outside the window damages the whole room of fruit at once.
• Discipline on opening. Every time the room is opened the atmosphere is lost and has to be re-established, so CA rooms are filled, sealed and left shut until the batch is sold — they are not day-to-day working stores.

Which crops it suits

CA storage earns its cost on high-value crops that store for a long time and respond well to low oxygen — apples and pears above all, and to varying degrees kiwifruit, cabbage, and some stone fruit and berries for shorter periods. It does little for crops that are already short-lived or that suffer in low oxygen. For most vegetables in Bangladesh, getting plain refrigerated storage and the cold chain right is a far bigger win than jumping to CA.

Where it fits in Bangladesh

Most CA-stored fruit reaching Bangladesh was stored abroad and shipped in refrigerated containers; the importer’s job is to hold it in good refrigerated storage and move it before the clock that started at the overseas CA room runs out. Building CA storage locally makes sense mainly for a business handling large, steady volumes of apples or pears — a major importer or a distributor consolidating for a region — where extending the sell-through window by months changes the economics. It is a specialised build: the room sizing, the gastight construction, the nitrogen and scrubber capacity and the set-points all depend on the crop, the variety and the volume, so it is scoped per project.

Common mistakes

• Treating a CA room as a normal cold store and opening it repeatedly, so the atmosphere is never stable.
• Loading fruit that is already over-ripe or bruised — CA slows deterioration, it does not reverse it, and rots spread in a sealed room.
• Skimping on the gastight envelope, then being unable to hold the oxygen level.
• No independent gas monitoring, so a scrubber or generator fault is discovered only when the fruit comes out damaged.
• Using one set-point for every variety instead of the values each variety actually needs.

Related reading: ethylene ripening chambers, humidity control in cold storage, cold storage for fruit exporters and pre-cooling fresh produce. See our services, contact DE International, our sourcing service and shop.