ISE’s oxygen monitoring and control solution for controlled atmosphere storage (CA cold rooms) extends the storage life of fruit and vegetables by keeping O₂, CO₂, temperature and humidity at the optimum for each product. The system uses durable zirconia oxygen sensors and a PLC/SCADA controller that automatically purges N₂, runs the CO₂ scrubber and opens fresh-air valves – for fixed CA stores, CA reefer containers and small CA chambers.
Key benefits:
- Apples, pears, kiwifruit and cabbage stored under CA for up to about 10 months.
- CA containers extend banana and avocado shipping time from about 20 to 45 days – opening distant export markets.
- Preserves firmness, colour and flavour; reduces losses and chemical treatments.
- Lets exporters time sales to the market, increasing profit.
Why does oxygen control keep produce fresh longer?
After harvest, fruit and vegetables keep respiring: consuming O₂ and producing heat, CO₂, water vapour and ethylene, which drive ripening and then deterioration. Storage life is inversely proportional to respiration rate, so the goal is to slow respiration without causing physiological injury.
| Factor | Effect | Technical note |
|---|---|---|
| Initial quality | Only good produce at loading stores well | Establish CA within 5–7 days of harvest |
| Temperature | A 10 °C drop roughly halves metabolic rate (Q₁₀ ≈ 2) | Actual Q₁₀ 2–3, higher for some products |
| Humidity | Losing about 2 % of water weight noticeably affects quality | RH 90–95 %, modern CA up to 98 % |
| Gas composition | Reducing O₂ from 21 % to 2 % cuts metabolic rate by about 30 % | Combined with controlled CO₂ and ethylene removal |
- Low O₂: slows respiration, ethylene production and ripening without affecting flavour when applied correctly.
- Controlled high CO₂: inhibits ethylene action and limits some diseases and storage disorders.
- Ethylene: the ripening hormone – remove with scrubbers for sensitive products.
- Limits: too little O₂ causes anaerobic respiration and off-flavours; too much CO₂ damages flesh. Safe thresholds depend on variety and temperature – hence accurate sensors and automatic control.
CA storage technology levels
| Technology | O₂ (%) | CO₂ (%) | Control principle | Application |
|---|---|---|---|---|
| RA – regular cold storage | ≈ 21 | ≈ 0.03 | Temperature and humidity control only | Short-term storage |
| Standard CA | 1–5 | 1–5 | Automatic O₂/CO₂ measurement and correction to setpoint | Apples, pears, kiwifruit, cabbage |
| ULO (ultra-low oxygen) | < 2 (≈ 1) | 1–3 | As CA, with very gas-tight rooms and accurate sensors | Long-term apples, pears |
| DCA (dynamic CA) | 0.3–0.7 | 0.5–1 | O₂ lowered close to fruit tolerance, adjusted by physiological feedback | Apples, organic produce |
| CA container | 2–5 | 2–7 | Membrane separation, VPSA or automatic fresh-air valve | Bananas, avocados, mangoes – sea freight |
Reference CA storage conditions by product
Tropical fruit – relevant to Vietnamese exports
| Product | Temp. (°C) | O₂ (%) | CO₂ (%) | Published result |
|---|---|---|---|---|
| Dragon fruit | 6 | 2 | 5 | Optimum conditions; less bract yellowing, acidity retained |
| Dragon fruit | – | 2 | 0 | Storage extended to about 50 days |
| Durian | 14 | 3–5 | 5–15 | About 8 weeks vs 5 weeks in air |
| Durian | 14 | 3 | 14–17 | With periodic SO₂ fumigation: up to 60 days |
| Banana (CA tent) | Chilled | 2 | 5 | Combined with ethylene absorption |
| Banana, avocado (CA container) | Chilled | 2–5 | 2–7 | Shipping extended from about 20 to 45 days |
Apples and temperate fruit
| Product | Temp. (°C) | O₂ (%) | CO₂ (%) | CA storage time |
|---|---|---|---|---|
| Fuji apple | 0.3 | 1.4 | 1.0 | 7–11 months |
| Braeburn apple | 0.7 | 1.8 | 1.0 | 6–9 months |
| Gala apple | 1.3 | 1.7 | 1.6 | 2–9 months |
| Kiwifruit | 0–5 | 2 | 5 | Long term |
| Apricot | 0–5 | 2–3 | 2–3 | > 2 weeks |
| Strawberry | 0–5 | 10 | 15–20 | Short-term transport |
Recommended humidity 90–95 %. Values are indicative and must be adjusted for variety, growing region, maturity and import-market requirements. Chilling injury warning: tropical fruit is sensitive to low temperatures (e.g. durian is injured at 5 °C within a week); avocados are injured at O₂ < 1 % or CO₂ ≥ 25 %. CA does not replace correct temperature selection.
CA cold storage control system architecture
| Item | Function | Technology / requirement |
|---|---|---|
| Gas-tight room | Prevents gas exchange with outside | Panels, gas-tight doors, pressure-relief valve, breather bag; periodic tightness tests |
| N₂ generator | Rapid O₂ pull-down by N₂ purging | Hollow-fibre membrane (95–99 % N₂) or PSA (99.5–99.995 %) |
| CO₂ scrubber | Holds CO₂ below setpoint | Regenerated activated carbon, membrane or lime |
| Ethylene scrubber | Removes C₂H₄ | KMnO₄ on carrier, catalytic oxidation, adsorption |
| Refrigeration and humidification | Maintains temperature and humidity | Evaporators, defrost, humidification to 95–98 % RH |
| Sensors | Measure O₂, CO₂, temperature, humidity | O₂: zirconia; CO₂: NDIR infrared |
| Controller | Control loops, alarms, data logging | PLC/SCADA, Modbus RTU/RS485, 4–20 mA; traceability data |
| Fresh-air valve / sampling pump | Adds O₂ when below setpoint | Solenoid valves; multi-room sequential sampling |
Control principle:
- Pull-down: after loading and cooling, purge N₂ to lower O₂ from 21 % to setpoint.
- Maintenance: produce consumes O₂ and generates CO₂ – open the fresh-air valve when O₂ is below setpoint; run the scrubber when CO₂ is above setpoint; add N₂ if O₂ rises through leakage.
- DCA: lower O₂ step by step until fruit stress is detected, then raise O₂ 0.1–0.3 % above the threshold.
- Alarms: O₂/CO₂ out of limits, sensor fault, loss of gas tightness, power failure – time-stamped event log.
Zirconia oxygen sensors for CA storage
| Technology | Characteristics | Suitability |
|---|---|---|
| Zirconia (ZrO₂) | Non-consumable, long life, 0.1–100 % O₂, one-point air calibration, no reference gas | Excellent – CA, ULO, containers |
| Electrochemical | Low cost but consumable, needs periodic replacement | Safety monitoring, simple CA |
| Paramagnetic | High accuracy but expensive, vibration sensitive | Central multi-room analysis, ULO/DCA |
| Optical | Non-consumable, no heating; range model dependent | Monitoring, containers |
ISE recommends SST Sensing (UK) zirconia oxygen sensors, widely used in CA containers for their long life and no need for reference gas:
| Parameter | Value |
|---|---|
| Principle | Zirconium dioxide (ZrO₂), non-consumable element |
| Range | 0.1–100 % O₂ |
| Resolution / accuracy | 0.01 % O₂ / 0.5 % O₂ (with interface electronics) |
| Output | 4–20 mA or RS485 Modbus RTU |
| Electronics ambient temperature | −40…+60 °C |
| Power supply | 20–28 VDC |
| Calibration | One point in fresh air; no reference gas |
| Compliance | RoHS |
The exact model is confirmed from the current datasheet at quotation. See also the SST OXY-IE oxygen sensor.
Installation recommendations: at least one O₂ and one CO₂ sensor per CA room; redundant measurement for ULO/DCA. Locate in a representative air stream, away from direct N₂ injection and condensation. Check calibration in fresh air before each season.
Worker safety in CA stores
DANGER: the atmosphere in CA/ULO/DCA rooms is very low in oxygen and can cause asphyxiation and almost instant death without warning. Never enter a CA room before it has been ventilated and safe O₂ has been confirmed.
- Install personal O₂-deficiency alarms in corridors, the N₂ plant room and at CA room doors – safety detectors should comply with EN 50104. CA control sensors do not replace safety detectors.
- Lock CA room doors, post “Oxygen-depleted atmosphere – No entry” signs and use lock-out during maintenance.
- Confined-space entry requires a permit, forced ventilation, continuous gas monitoring, an attendant outside and breathing apparatus where required.
Reference standards
| Document | Scope |
|---|---|
| ISO 6949 | Principles and techniques of controlled atmosphere storage of fruit and vegetables |
| EN 50104 | Oxygen detection and measurement equipment – personal O₂-deficiency alarms |
| 2014/30/EU (EMC), 2014/35/EU (LVD), RoHS | Electrical equipment, sensors, control panels |
| EN 378 | Safety of refrigerating systems |
| (EC) 852/2004 | Food hygiene in storage |
| (EU) 2023/2429 | Marketing standards for fresh fruit and vegetables exported to the EU |
Operational risks and controls
| Risk | Control measure |
|---|---|
| Low-O₂ injury | Accurate sensors, regular calibration, low-limit alarms |
| CO₂ injury | Continuous CO₂ measurement, alarms, scrubber redundancy |
| Chilling injury | Product-specific temperature setpoints, temperature logging |
| Loss of gas tightness | Pre-season tightness test, monitoring N₂ consumption |
| Slow CA establishment | Establish within 5–7 days of harvest; correctly sized N₂ generator |
| Asphyxiation accidents | Safety detectors, interlocks, work permits |
ISE CA storage implementation process
| Step | Content |
|---|---|
| 1. Survey | Products, volumes, target storage time, existing store |
| 2. Design | Number of rooms, volume, N₂/scrubber capacity, measurement and control scheme |
| 3. Equipment supply | O₂/CO₂/temperature/humidity sensors, controller, valves, panels – with CO/CQ |
| 4. Installation | Wiring, PLC/SCADA programming |
| 5. Commissioning & acceptance | Calibration, tightness test, acceptance report |
| 6. Training & maintenance | Operation, safety, periodic maintenance |
FAQ
How does a CA store differ from a regular cold store?
A regular cold store controls only temperature and humidity (O₂ ≈ 21 %). A CA store also lowers O₂ and holds CO₂ at the optimum, slowing respiration so produce stays fresh many times longer.
Can an existing cold store be upgraded to CA?
Possibly, if the room can be made gas-tight by upgrading doors, seals and wall penetrations. ISE surveys and tests tightness before proposing.
Can dragon fruit and durian be CA stored?
Yes. Published research shows dragon fruit stored under CA for about 50 days and durian for about 8 weeks versus 5 weeks in air – with the correct temperature to avoid chilling injury.
Reference: FAO – CA and MA storage requirements for fruit and vegetables. Values in this article are technical references; actual storage results depend on site survey and trials.
Where to buy a controlled atmosphere storage solution?
EQUIPMENT INDUSTRIAL SOLUTION CO., LTD (ISE) – Ho Chi Minh City, Vietnam – supplies genuine a controlled atmosphere storage solution with technical consulting, fast quotation and on-site commissioning support across Vietnam and Southeast Asia.
Contact us: Sales +84 909 617 185 – Technical +84 979 117 185 – Email: [email protected]
