Forced Air Cooling

1. Why Cool Fruit?

1.1 Control of the Ripening Process

• Climacteric fruit (e.g., stone and pome fruit) continue to ripen after harvest.
• Uncontrolled ripening reduces marketability.
• Refrigeration is the most effective way to control respiration and slow ripening.
• Lowering pulp temperature also limits ethylene production.
• Storage period can be extended with:
  • Controlled atmosphere cold storage (regulates O₂ and CO₂).
  • Use of 1-MCP (inhibits ethylene action).
  • Adjusting cold chain for certain plums to enhance eating quality by varying shipping temperature.

1.2 Reduce Moisture Loss

• Any storage below 100% relative humidity causes moisture loss.
• Moisture loss is minimized by:
  • Reducing product temperature.
  • Keeping relative humidity high (preferably above 85%).
• Excessive moisture loss can cause defects (e.g., dry stems in grapes, shrivel in plums).

1.3 Control of Internal and External Defects/Diseases

• Fungal diseases (e.g., Botrytis) develop rapidly if fruit is not cooled soon after picking.
• Disease risk increases above 0°C.
• Different fruits have different cold sensitivities:
  • Grapes: -0.5°C ideal.
  • Citrus: Skin disorders if stored at -0.5°C for long periods.
  • Some plums (e.g., Songold): Internal browning if stored >10–14 days at -0.5°C.
  • Laetitia plums: After heat waves, cool gradually (e.g., to +10°C in 24h before forced air cooling to -0.5°C).

1.4 Control of Pests

• All fruit are exposed to orchard pests.
• Low temperatures can control pest development and damage.
• Some pests (e.g., Mediterranean Fruit Fly) can be killed by extended low temperature (basis for cold sterilisation required by some importers).

2. Principles of Forced Air Cooling

• Cooling rate depends on:
  • Heat transfer (conduction and convection).
  • Temperature differential between product and cooling medium (greater differential = faster cooling).
• Conduction: Slow process; heat moves from one fruit to another.
• Convection: Faster; cold air must reach fruit directly for efficient heat transfer.
• Without air movement, fruit temperature will eventually match the cold room, but cooling is slow.
• Fruit inside cartons and pallets cools slowly unless air is forced through the load.
• Adequate ventilation in pallets/cartons is essential for convective cooling.
• Even with ventilation, air must be forced through cartons—this is the role of forced air cooling.

3. Methodology

• Most common method: Suck cold air through pallet loads and cartons.
• Stack two rows of pallets, leaving a “tunnel” (plenum) between them.
• Cover the tunnel and pallet tops with a tarpaulin; place a suction fan at the end to create a vacuum.
• Cold air is drawn through the pallet loads, inducing convective cooling and speeding up the process.
• In fixed pre-cooling tunnels, exhaust fans may be above the tunnel in the false ceiling; both ends of the tunnel are sealed.
• Use vertical custom-made curtains (tarpaulin strips with netting) to close gaps between pallet loads and bases, allowing air to move freely through the loads.

4. Factors Influencing Forced Air Cooling Effectivity

4.1 Delivery Air Temperature (DAT)

• Target temperature
• Refrigeration capacity
• Defrosting installation design and cycle time
• Coil surface area
• Air volume and bypass
• Heat from fan motors
• Tunnel insulation and other surfaces
• Ambient air infiltration (e.g., open doors)
• Fruit temperature (difference between DAT and fruit temperature)
• Return Air Temperature (RAT)

4.2 Air Flow / Vacuum Level

• Extraction fans (number, size, speed, blades, pitch, position, cyclone design)
• Tunnel (length, width, distance between legs, design of curtains, sealing of gaps, insulation, height/number of pallets high)

4.3 Packaging

• Carton type and ventilation design
• Internal packaging (wrappers, trays, bags, punnets, other)
• Palletisation (neat and square)
• Orientation of pallet (1M or 1.2M face)
• Uniformity of packaging

4.4 Fruit Kind / Variety

• Temperature sensitivity
• Moisture loss risks
• Shape and size

5. Forced Air Cooling Guidelines

1. Calibrate thermometers before the season.
2. Regularly compare thermometers.
3. Check thermometers weekly in 50:50 flaked ice/pure water slurry (should read 0.0°C).
4. Ensure correct placement of thermocouples when palletising (optional).
5. Measure fruit temperature at receipt.
6. Apply sound forced air cooling principles (see Section 4). Use a refrigeration engineer for tunnel design.
7. Avoid different packs in one tunnel where possible:
   • Punnets cool slower.
   • Perforated bags cool quicker than non-perforated.
   • Different pallet heights cause problems.
   • Place slow-cooling packs closer to fans.
8. Check DAT regularly.
9. Seal tunnels properly.
10. Check sealing with a manometer (see Annexure 1).
11. Be careful with low-sugar fruit (may freeze just below zero).
12. Regularly measure and record:
    • Thermocouple readings
    • Air temperature
    • Probe readings (if any)
13. Know the cold room—identify hot spots and coldest positions.
14. Cooling rate slows as fruit approaches target temperature (e.g., 6 hours from 22°C to 5°C, but another 10 hours to reach 1°C).
15. Stop forced air cooling as soon as the target temperature is reached (to prevent moisture loss). This may require night shifts.
16. Measure fruit temperature at despatch.

Manometer Use (Annexure 1)

• A manometer measures pressure difference between spaces.
• One end is sealed in the tunnel, the other exposed to the cold room.
• The liquid in the tube moves toward the vacuum end (lower pressure).
• The height difference indicates vacuum level.
• A reading above 10mm (preferably 14mm) indicates a well-sealed tunnel.
• Use manometer readings to check tunnel sealing and save pre-cooling time by identifying leaks early.

Troubleshooting Cooling Problems (Annexure 2)

1. Room temperature rises as products are added:
   • Likely insufficient refrigeration capacity or under-designed cooler.
   • Solution: Divide room into separate cooling bays with curtains or uninsulated walls.
2. Product temperature in outer cartons decreases slowly, despite DAT being on spec:
   • Cause: Insufficient contact between cooling air and product (inadequate ventilation, blocked vents, insufficient airflow, or air bypassing cartons).
3. Product temperature in some outer cartons decreases rapidly, others cool slowly:
   • Cause: Non-uniform airflow due to poorly designed supply/return air channels.
   • Solution: Measure pulp temperatures and pressure drops at different heights/distances from the fan.

Reference

Thompson, J, et al., 1998. Commercial cooling of fruits, vegetables and flowers. University of California, DANR Publication 21567.