Controlled Atmosphere Cold Room: 12 Critical Systems for Longer Freshness

A Controlled Atmosphere Cold Room is an advanced storage environment designed to control not only temperature and relative humidity but also oxygen, carbon dioxide and, when required, ethylene levels. By slowing respiration and ripening, the system can help preserve the firmness, color, texture and commercial quality of selected fruits and vegetables for significantly longer periods.

Unlike a conventional cold room, a controlled atmosphere system requires a gas-tight structure, precise gas measurement, automatic control equipment and crop-specific storage programming. It is therefore not simply a refrigerated room with additional sensors. It is an integrated post-harvest storage system in which insulation, refrigeration, airflow, gas control, safety and automation must work together.

The Food and Agriculture Organization of the United Nations emphasizes that controlled atmosphere storage supplements correct temperature and humidity management; it does not replace them. Successful long-term storage begins with healthy produce, correct harvest maturity, rapid cooling and an uninterrupted cold chain.

What Is a Controlled Atmosphere Cold Room?

A Controlled Atmosphere Cold Room, frequently abbreviated as a CA cold room, is a gas-tight refrigerated chamber in which the composition of the internal atmosphere is measured and adjusted.

Normal outdoor air contains approximately 21% oxygen, a very small proportion of carbon dioxide and predominantly nitrogen. Inside a controlled atmosphere room, oxygen is generally reduced while carbon dioxide is maintained within a product-specific range. Temperature and relative humidity are simultaneously controlled.

The objective is to reduce the metabolic activity of the stored produce without causing physiological damage.

Fresh fruits and vegetables remain biologically active after harvest. They continue to:

• Consume oxygen
• Produce carbon dioxide
• Release moisture and heat
• Respond to ethylene
• Ripen and age
• Lose firmness and commercial quality

By carefully changing the storage atmosphere, the respiration and ripening processes can be slowed. The FAO’s controlled atmosphere storage guidance explains that altered oxygen and carbon dioxide conditions are used to delay ripening and senescence in suitable fruits, vegetables and cut flowers.

However, the optimum atmosphere is different for every product and may even vary between cultivars of the same fruit. Gas values must therefore be determined according to the product, variety, harvest maturity, planned storage period and applicable post-harvest recommendations.

Controlled Atmosphere Cold Room vs. Conventional Cold Room

The principal difference is the number of storage variables being actively controlled.

A conventional fruit cold room can maintain the temperature and humidity required for short- or medium-term preservation. A controlled atmosphere room adds active atmospheric management when the crop, storage duration and commercial strategy justify the additional investment.

How Does a Controlled Atmosphere Cold Room Work?

After the room is loaded, the produce is cooled to its required storage temperature. The door is then closed and the room is prepared for controlled atmosphere operation.

The main process can be summarized in five stages:

1. The product is harvested at the correct maturity.
2. Field heat is removed through appropriate precooling or pull-down cooling.
3. The product is loaded with sufficient clearance for uniform airflow.
4. The room is sealed and the oxygen level is gradually reduced.
5. Oxygen, carbon dioxide, humidity and temperature are continuously measured and corrected.

Oxygen can fall partly through the natural respiration of the stored produce. In modern installations, nitrogen generators or gas-flushing systems may be used to reach the operating condition more quickly.

Carbon dioxide produced by product respiration can be removed using a scrubber. Oxygen can be admitted in controlled quantities when it falls below the permitted limit. Depending on the product, ethylene management and humidity control can also be incorporated.

Air is continuously circulated so that temperature and gas composition remain reasonably uniform throughout the storage volume. Sensors send measurements to a controller or PLC, which operates the relevant equipment and records alarms, trends and deviations.

12 Critical Components of a Controlled Atmosphere Cold Room

1. Gas-Tight Insulated Envelope

A controlled atmosphere room must have both high thermal insulation performance and reliable gas tightness.

The walls, ceiling and floor are generally constructed with insulated sandwich panels. Nevertheless, panel insulation alone cannot create a successful CA room. Panel joints, corners, floor connections, pipe penetrations, cable entries, drains and structural interfaces must also be sealed.

High-performance cold room wall panels help limit heat transfer, but all construction details must be engineered to prevent uncontrolled gas exchange.

Even a small leakage path may cause:

• Unstable oxygen concentration
• Excessive nitrogen consumption
• Longer atmosphere pull-down time
• Higher operating costs
• Frequent system corrections
• Inconsistent product quality
• Inability to maintain the storage program

A gas-tightness or pressure-decay test should therefore be performed during commissioning.

2. Atmosphere Control Cold Room Door

The door is one of the most sensitive points in a controlled atmosphere installation. A conventional insulated door may provide thermal separation but may not deliver the airtightness required for long-term CA operation.

A purpose-designed atmosphere control cold room door uses sealing systems engineered to reduce air and gas leakage. Depending on the project, the door assembly may include:

• Reinforced insulated door leaf
• Continuous perimeter gaskets
• Adjustable compression locking
• Gas-tight frame connections
• Inspection window
• Sampling hatch
• Safety signage
• Oxygen warning equipment
• Emergency operating provisions

The room should not be opened unnecessarily during storage. Every opening allows outside air to enter and changes the internal atmosphere.

3. Refrigeration System

Controlled atmosphere storage still depends on accurate refrigeration. Changing the gas composition cannot compensate for an incorrectly selected refrigeration system or inadequate precooling.

The refrigeration capacity must be calculated from the actual heat loads, including:

• Heat transfer through panels
• Product field heat
• Daily product intake
• Product respiration heat
• Air infiltration
• Fan motors and lighting
• Workers and handling equipment
• Outdoor design temperature
• Required pull-down time
• Available compressor operating hours

TunelGroup’s refrigeration units can be configured according to project temperature, capacity, ambient conditions and application requirements.

Equipment should never be selected only according to room volume or compressor horsepower. Two rooms with the same dimensions can require very different refrigeration capacities when their products, daily loading rates, outdoor temperatures and cooling times are different.

4. Evaporator and Airflow Management

The evaporator removes heat from the room and circulates conditioned air through the stored product. Its selection affects temperature uniformity, product dehydration, defrost frequency and energy use.

An evaporator with an excessively high temperature difference may remove too much moisture from the air and increase product weight loss. Insufficient airflow, on the other hand, can produce warm zones and uneven gas distribution.

Appropriate cold room evaporators should be selected according to:

• Required room temperature
• Refrigerant
• Cooling capacity
• Coil temperature difference
• Product sensitivity
• Desired relative humidity
• Air throw
• Storage layout
• Defrost method
• Fan energy consumption

Pallets and containers must be arranged to maintain designed air passages. A high-quality evaporator cannot correct a storage layout that blocks the return or supply airflow.

5. Oxygen Reduction System

The oxygen reduction system establishes the required low-oxygen environment.

Nitrogen is commonly used because it can replace part of the oxygen-rich air without introducing an unwanted reactive gas. Depending on the facility, nitrogen may be supplied from:

• An on-site nitrogen generator
• A membrane separation system
• A pressure swing adsorption system
• Bulk nitrogen storage
• Portable or project-specific gas supplies

The correct option depends on the number and size of rooms, required pull-down time, operating schedule and local gas availability.

Oxygen must not be reduced faster or further than permitted by the product protocol. Extremely low oxygen can trigger anaerobic respiration, fermentation, off-flavors and physiological injury.

6. Carbon Dioxide Scrubber

Fruits and vegetables release carbon dioxide as they respire. In a gas-tight room, carbon dioxide can accumulate and exceed the tolerance of the stored product.

A carbon dioxide scrubber removes excess CO₂ from the room air and returns the treated air to the storage environment. Scrubber capacity should be based on:

• Product type
• Product mass
• Respiration rate
• Storage temperature
• Storage maturity
• Room volume
• Target atmosphere
• Number of connected rooms

An undersized scrubber may not control peak respiration, particularly during the first storage period. An oversized or poorly controlled system can also create unnecessary cycling and energy consumption.

7. Ethylene Management

Ethylene is a natural plant hormone that influences ripening and ageing. Its importance varies considerably between commodities.

For ethylene-sensitive or ethylene-producing products, the storage design may include:

• Ethylene sensors
• Catalytic ethylene converters
• Adsorption filters
• Potassium permanganate media
• Controlled ventilation
• Product-separation strategies

Ethylene management should not be added automatically to every room. Its necessity and capacity must be determined from the stored product and operating strategy.

8. Gas Analyzers and Sensors

A controlled atmosphere system can only perform as well as its measurements.

The room normally requires accurate oxygen and carbon dioxide measurement. Depending on the application, additional sensors may monitor:

• Temperature
• Relative humidity
• Ethylene
• Refrigerant leakage
• Room pressure
• Door status
• Equipment operation
• Outdoor conditions

Sensors must be correctly positioned, periodically calibrated and protected from condensation or mechanical damage.

A sensor that displays a value is not automatically a reliable sensor. Calibration records, verification procedures and alarm tests should form part of the maintenance program.

9. Humidity Control

Many fruits and vegetables require high relative humidity to limit water and weight loss. However, uncontrolled condensation can promote decay, damage packaging and interfere with sensors.

Humidity performance is affected by:

• Evaporator coil temperature
• Air velocity
• Door openings
• Product moisture release
• Defrost operation
• Vapor barrier quality
• Room temperature stability
• Humidifier design
• Drainage and hygiene

The objective is not simply to create the highest possible humidity. It is to maintain the product-specific condition without persistent condensation or microbial risk.

10. Pressure Equalization System

Temperature changes, defrost cycles, nitrogen introduction, gas scrubbing and product respiration can cause pressure differences between the room and the surrounding building.

Because the room envelope is gas-tight, excessive positive or negative pressure can place significant stress on panels, joints, ceilings and doors.

A correctly selected pressure equalization device protects the room structure while limiting unnecessary gas exchange. Its capacity and operating characteristics must be coordinated with:

• Room volume
• Maximum temperature change
• Gas-control equipment
• Defrost cycles
• Structural limits
• Expected pressure fluctuations

Pressure protection is a technical necessity, not an optional accessory.

11. Automation, Data Logging and Remote Monitoring

Modern CA facilities use PLC, SCADA or digital control systems to coordinate refrigeration and atmosphere equipment.

A well-designed control system can monitor:

• Room temperature
• Relative humidity
• Oxygen concentration
• Carbon dioxide concentration
• Room pressure
• Door position
• Refrigeration status
• Scrubber operation
• Nitrogen system operation
• Active alarms

Trend recording is particularly valuable because product quality problems are rarely explained by a single measurement. Historical data can reveal gradual leakage, sensor drift, unstable refrigeration or excessive equipment cycling.

A digital cold room control panel can also support remote monitoring, alarm notification and maintenance planning when specified for the project.

12. Personnel Safety Systems

A controlled atmosphere room is intentionally operated with an oxygen concentration that may be dangerous or fatal to people. The room must therefore be treated as a restricted low-oxygen environment.

The UK Health and Safety Executive classifies reduced-oxygen enclosures and controlled atmosphere produce stores as confined-space hazards. Its guidance highlights the need for risk assessment, restricted access, warning systems, atmosphere testing, ventilation, trained personnel and emergency arrangements. See the official HSE guidance on hypoxic environments.

A project-specific safety system may include:

• External oxygen monitors
• Audible and visual alarms
• Door interlocks
• Warning signs
• Restricted access
• Lockout procedures
• Mechanical ventilation
• Atmosphere testing
• Staff training
• Emergency communication
• Rescue procedures
• Appropriate personal protective equipment

No person should enter a CA room until the atmosphere has been fully ventilated, tested and formally declared safe under the site’s approved procedure.

Workers must never rely on smell or physical sensation to identify oxygen deficiency. Untrained personnel must also never enter a hazardous room to attempt a rescue.

Applicable safety regulations vary by country and facility. A competent occupational safety professional should review the final design and operating procedure.

Which Products Are Suitable for Controlled Atmosphere Storage?

Controlled atmosphere storage is used most frequently for selected fruits and vegetables intended for extended storage.

Common applications include:

• Apples
• Pears
• Kiwifruit
• Selected citrus products
• Cabbage
• Certain berries
• Selected vegetables
• Cut flowers
• Research and seed applications

Apples are among the most established CA applications. However, storage conditions still vary between cultivars. The UC Davis Postharvest Technology Center publishes product-specific post-harvest information demonstrating why storage recommendations must be adapted to each commodity.

Under properly controlled conditions, some apple varieties may be stored for many months. The USDA reports that suitable cultivars can remain marketable for extended periods under controlled atmosphere storage. This must not be interpreted as a universal storage guarantee, because product quality, maturity, disease pressure, temperature history and cultivar all affect the result.

Not every product benefits from reduced oxygen or elevated carbon dioxide. Some commodities are highly sensitive to atmosphere-related injury. Before defining operating values, the designer and operator should determine:

• Exact product and cultivar
• Harvest date
• Harvest maturity
• Growing region
• Initial product quality
• Precooling method
• Storage duration
• Packaging type
• Final market destination
• Applicable scientific recommendations

Why Gas Tightness Is Essential

Gas tightness determines whether a controlled atmosphere can be established and maintained economically.

If outside air continuously leaks into the room, the system must repeatedly remove oxygen or introduce additional nitrogen. This increases operating costs and causes concentration fluctuations.

Typical leakage points include:

• Panel joints
• Wall-to-floor connections
• Ceiling suspensions
• Door gaskets
• Pipe penetrations
• Electrical conduits
• Drain lines
• Sensor fittings
• Inspection windows
• Pressure valves

Sealants must be compatible with cold-room materials, temperature changes, moisture and cleaning procedures. Construction details should allow for thermal movement without losing their sealing performance.

Gas-tightness testing should be carried out before the room is filled with product. Correcting leakage in an empty room is safer, faster and less expensive than finding it during the storage season.

CA, ULO and Dynamic Controlled Atmosphere

Controlled atmosphere systems can be configured at different levels of sophistication.

Standard Controlled Atmosphere

The system maintains predetermined oxygen and carbon dioxide limits based on an established storage program.

Ultra-Low Oxygen Storage

Ultra-low oxygen, commonly known as ULO, operates at lower oxygen conditions than conventional CA storage. It requires excellent gas tightness, accurate analyzers and carefully validated product limits.

Dynamic Controlled Atmosphere

Dynamic controlled atmosphere systems adjust operating conditions according to the measured response of the stored produce. Depending on the technology, the system may use respiration, fluorescence or other biological indicators.

These advanced methods can offer commercial advantages in suitable applications, but they also increase the importance of calibration, automation, technical supervision and cultivar-specific expertise.

Terms such as CA, ULO and DCA should not be used interchangeably. Each method has different technical and operational requirements.

Main Benefits of a Controlled Atmosphere Cold Room

Longer Commercial Storage Period

Reduced respiration and controlled ripening can extend the period in which suitable produce remains marketable.

Better Quality Retention

When correctly managed, CA storage may help preserve:

• Firmness
• Color
• Acidity
• Texture
• Moisture
• Appearance
• Market value

The actual result depends on the product and storage protocol.

Reduced Post-Harvest Losses

Longer storage and more stable quality can reduce spoilage, shrinkage and rejection. This can be particularly valuable for high-volume growers, packing houses and exporters.

Greater Marketing Flexibility

Producers may be able to avoid selling the entire harvest immediately after harvest. Storage can support more flexible market timing, longer export programs and improved supply continuity.

Improved Export Planning

Long-distance distribution requires a consistent cold chain. CA storage can help stabilize product quality before dispatch, but transportation and destination storage must maintain the required conditions.

Potential Financial Return

The commercial value of a CA room may come from:

• Lower product losses
• Longer selling periods
• Access to distant markets
• Better quality classification
• Reduced distress selling
• Improved supply consistency

Nevertheless, a controlled atmosphere room is not automatically profitable. The expected benefits must be compared with the additional investment, energy consumption, gas equipment, maintenance, calibration and operational expertise required.

How to Evaluate the Investment

A useful feasibility analysis should compare the CA project with a conventional cold-storage alternative.

A simplified annual evaluation can be expressed as:

Annual CA value = reduced product losses + improved selling value + logistics benefits − additional operating and maintenance costs

The calculation should include:

• Annual stored tonnage
• Product purchase or production value
• Historical storage losses
• Expected loss reduction
• Potential selling-price differences
• Additional storage duration
• Electrical energy
• Nitrogen production or gas supply
• Scrubber operation
• Sensor calibration
• Maintenance and spare parts
• Financing cost
• Product disposal risk
• Staff training

The most expensive system is not necessarily the best investment. The optimum system is the one designed around the actual product, market and operating strategy.

Common Controlled Atmosphere Storage Mistakes

Copying a Standard Gas Value

Using a generic oxygen and carbon dioxide setting without considering the product and cultivar can cause physiological injury or flavor problems.

Ignoring Initial Product Quality

Controlled atmosphere storage slows deterioration; it cannot reverse bruising, decay, over-maturity or poor harvesting practices.

Starting CA Before Proper Cooling

Gas control should be coordinated with product cooling. A warm product can generate a large heat and respiration load.

Selecting Refrigeration by Room Volume Alone

Volume-based rules are useful only for early budgeting. Final selection requires a complete heat-load calculation.

Poor Panel and Door Sealing

A high-capacity nitrogen generator cannot permanently compensate for a badly sealed room.

Insufficient Sensor Calibration

Incorrect gas readings can cause the control system to create unsafe or product-damaging conditions.

Blocking the Airflow

Pallets placed against the evaporator, walls or air-return route can create temperature and gas-distribution problems.

Opening the Room Frequently

Every door opening disrupts the atmosphere. Sampling and inspection should be planned through suitable hatches or monitoring systems whenever possible.

Neglecting Safety Procedures

A CA room may appear normal from outside while containing a life-threatening atmosphere. Safety controls must be part of the original design.

Designing the Right Controlled Atmosphere Cold Room

Before equipment is selected, the project team should answer the following questions:

1. Which product and cultivar will be stored?
2. What is the harvest maturity?
3. How many tonnes will enter the room each day?
4. What is the product entry temperature?
5. How quickly must the product be cooled?
6. What is the required storage temperature?
7. What relative humidity is required?
8. What atmosphere limits are recommended?
9. How long will the product be stored?
10. How often must the room be accessed?
11. What are the room dimensions and pallet layout?
12. What is the maximum outdoor temperature?
13. How quickly must the atmosphere be established?
14. Is an on-site nitrogen generator economically justified?
15. Is ethylene control required?
16. What level of remote monitoring is needed?
17. Which alarms and safety systems are required?
18. Who will operate and maintain the system?
19. What are the local electrical and safety regulations?
20. Is future capacity expansion planned?

These answers form the basis of the room layout, insulation design, refrigeration capacity, evaporator selection, gas-control system and automation strategy.

Can an Existing Cold Room Be Converted?

Some conventional cold rooms can be converted to controlled atmosphere operation, but conversion is not always technically or financially suitable.

The assessment should examine:

• Panel condition
• Joint design
• Floor and ceiling sealing
• Door airtightness
• Structural pressure resistance
• Pipe and cable penetrations
• Refrigeration capacity
• Evaporator suitability
• Drainage
• Electrical infrastructure
• Available space for gas equipment
• Ventilation and personnel safety

A leakage test is normally required. In many cases, additional sealing, a dedicated CA door, pressure equalization equipment, gas analyzers, automation and safety systems must be installed.

If the existing structure cannot maintain gas tightness or safely withstand pressure variation, constructing a purpose-designed room may be more economical.

Maintenance and Commissioning Checklist

Before the first storage season, the following items should be verified:

• Panel joints and penetrations are completely sealed
• Door gaskets provide uniform compression
• Gas-tightness test is completed
• Pressure equalization equipment is operational
• Refrigeration capacity is verified
• Evaporator airflow is unobstructed
• Temperature sensors are calibrated
• Oxygen and carbon dioxide analyzers are calibrated
• Humidity sensors are checked
• Nitrogen equipment is tested
• CO₂ scrubber performance is confirmed
• Alarms are tested
• External oxygen monitors are operational
• Emergency ventilation is tested
• Door interlocks and warning lights function correctly
• Data logging is active
• Operating personnel are trained
• Entry and rescue procedures are documented

During operation, trend data should be reviewed regularly. A gradual increase in nitrogen use, for example, may indicate a door-gasket problem, structural movement or leakage around a penetration.

The Future of Controlled Atmosphere Storage

Controlled atmosphere technology is becoming more closely integrated with data analytics, remote monitoring and product-response measurement.

Future developments are expected to focus on:

• Dynamic atmosphere adjustment
• Improved optical and biological sensors
• Predictive maintenance
• Automatic leak detection
• Cloud-based storage records
• Artificial intelligence-supported alarm analysis
• Variable-speed compressors and fans
• Low-global-warming-potential refrigerants
• Renewable energy integration
• Product-level traceability

These technologies can make storage more precise, but they cannot replace correct engineering and disciplined operation. Reliable insulation, airflow, refrigeration, gas measurement and safety remain the foundation of every successful system.

TunelGroup Controlled Atmosphere Cold Room Solutions

TunelGroup supports controlled atmosphere projects with integrated cold-storage engineering and equipment solutions.

Depending on the application, a complete project may include:

• Insulated cold room panels
• Gas-tight panel connections
• Atmosphere control cold room doors
• Refrigeration units
• Evaporators
• Digital control panels
• Temperature and humidity monitoring
• Oxygen and carbon dioxide analyzers
• Pressure equalization equipment
• Remote monitoring options
• Project-specific accessories
• Installation and commissioning support

Rather than treating each component as an independent product, TunelGroup evaluates the cold room as a complete system. Room dimensions, storage capacity, crop requirements, outdoor climate, pull-down time, airflow, gas-control strategy and operational safety are considered during the engineering process.

For a project-specific evaluation, visit TunelGroup Cooling Systems and share your product type, storage capacity, entry temperature, required storage period and project location.

Frequently Asked Questions

What is a Controlled Atmosphere Cold Room?

A Controlled Atmosphere Cold Room is a gas-tight refrigerated storage room that controls temperature, humidity, oxygen and carbon dioxide levels to slow the respiration and ripening of suitable fresh produce.

Is controlled atmosphere storage the same as refrigeration?

No. Refrigeration controls product and room temperature. Controlled atmosphere storage adds oxygen and carbon dioxide management. CA operation cannot replace correct refrigeration or humidity control.

What is the difference between CA storage and modified atmosphere storage?

Controlled atmosphere storage actively measures and corrects gas concentrations. Modified atmosphere conditions may be created passively or inside packaging and can change over time without the same level of active control.

Which products can be stored in a CA cold room?

Apples, pears, kiwifruit and certain other fruits and vegetables are common applications. Suitability and operating conditions must be confirmed for the exact product and cultivar.

How long can fruit remain in controlled atmosphere storage?

Storage duration varies according to cultivar, harvest maturity, product quality, temperature, atmosphere settings and disease control. Some apple cultivars may remain marketable for several months, but no universal duration can be guaranteed.

Is a controlled atmosphere room dangerous?

Yes. Its low-oxygen atmosphere can be fatal to people. Access must be restricted, and the room must be ventilated, tested and declared safe before entry.

Can a normal cold room be converted into a CA room?

Conversion may be possible if the structure can be made gas-tight and safely equipped with atmosphere control, pressure protection, automation and personnel safety systems. A detailed technical inspection is required.

Does a CA cold room need a special door?

Yes. A controlled atmosphere door must provide dependable gas sealing in addition to thermal insulation.

How are oxygen and carbon dioxide controlled?

Oxygen can be reduced through product respiration, nitrogen flushing or a nitrogen generator. Excess carbon dioxide is normally removed by a scrubber. Sensors and automatic controls maintain the permitted operating range.

How much does a Controlled Atmosphere Cold Room cost?

Cost depends on room capacity, insulation, refrigeration load, door design, gas-tightness requirements, nitrogen equipment, scrubber capacity, automation, monitoring, safety systems and installation location. A technical project specification is required for an accurate quotation.

Conclusion

A Controlled Atmosphere Cold Room can create major post-harvest advantages when it is designed for the correct product and operated under a validated storage program.

Its performance depends on much more than reducing oxygen. Refrigeration, humidity, airflow, gas tightness, carbon dioxide removal, sensor accuracy, automation and personnel safety must operate as one coordinated system.

For growers, packing houses, exporters and food-storage operators, the technology can support longer storage, lower product losses and greater marketing flexibility. However, these benefits are only achieved through crop-specific engineering, correct commissioning and disciplined operation.

TunelGroup provides project-specific controlled atmosphere cold room solutions combining insulated construction, atmosphere control doors, refrigeration equipment, monitoring and technical support.