1. Why Cool Fruit?
1.1 Control of the Ripening Process
- The ripening process of climacteric fruit (such as stone and pome fruit) continues after picking.
- If not controlled, fruit ripens within days, reducing marketability.
- The most effective way to control respiration and ripening is refrigeration.
- Lowering fruit pulp temperature also limits ethylene production, further retarding ripening.
- The ripening process can be manipulated to extend storage using:
- Controlled atmosphere cold storage (adjusting O₂ and CO₂ levels)
- 1MCP (e.g., SmartFresh)
- Adjusted cold chain for certain plums (e.g., raising and reducing shipping temperature en route)
1.2 Reduce Moisture Loss
- Storing fruit at relative humidity below 100% causes moisture loss.
- Loss is controlled by reducing product temperature and keeping RH high (preferably above 85%).
- Excessive moisture loss can cause defects like dry stems in grapes and 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.
- Different fruits react differently to cold:
- -0.5°C is ideal for grapes, but citrus may show skin disorders at this temperature.
- Some plums (e.g., Songold) develop internal browning if stored >10–14 days at -0.5°C.
- For Laetitia plums, especially after a heat wave, avoid rapid cooling; cool to +10°C in 24 hours before forced air cooling to -0.5°C.
1.4 Control of Pests
- All fruit are exposed to orchard pests.
- Pests may still be present during packing.
- Low temperatures can control pest development and, for some (e.g., Mediterranean Fruit Fly), kill them after extended exposure—basis for cold sterilisation required by some importing countries.
2. Principles of Forced Air Cooling
- The cooling rate of warm fruit depends on:
- Heat transfer by conduction and convection
- Temperature differential between product and cooling medium (greater differential = faster cooling)
- Conduction in fruit is slow, transferring heat from one fruit to another as the ‘cold front’ moves.
- Convection speeds up cooling by transferring heat directly from warm fruit to cold air.
If fruit is left in a cold store with little air movement, its temperature will eventually match the cold room.
However, to cool fruit quickly, cold air must move over the fruit.
Because fruit is inside cartons and pallet loads, air takes the path of least resistance and may not reach the inside fruit.
Sufficient ventilation in pallet loads/cartons is required for convective cooling.
Even with good ventilation, cold air will not move through cartons unless forced—this is where forced air cooling is vital.
3. Methodology
- The most common method is to suck cold air through pallet loads and cartons.
- Stack two rows of pallets, leaving a ‘tunnel’ (plenum) between them.
- Cover the tunnel roof and pallet tops with a tarpaulin.
- Place a suction fan at the end of the tunnel to create a vacuum.
- Cold air moves through the pallet loads, inducing convective cooling and speeding up the process.
In fixed pre-cooling tunnels, exhaust fans are often above the tunnel in the false ceiling, with both ends sealed.
To close gaps on the outside of the tunnel, use vertical custom-made ‘curtains’—solid tarpaulin strips to cover gaps between pallet loads and bases, with netting attached to allow air to move freely through the loads.
4. Factors Influencing Forced Air Cooling Effectivity
Many factors affect forced air cooling. For technical issues, consult a refrigeration engineer or cooling expert.
4.1 Delivery Air Temperature (DAT)
- Target temperature
- Refrigeration capacity
- Defrosting installation design
- Defrost cycle time
- Coil surface
- Air volume
- Air volume bypassing the coils
- Heat generated by fan motors
- Other surfaces (e.g., tunnel insulation)
- 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
- Number of blades
- Blade pitch
- Position
- Cyclone design for air intake
- Tunnel
- Length
- Width between legs
- Distance between tunnel leg and wall or next tunnel
- Design of curtains
- Sealing of gaps
- Tunnel insulation
- Height (number of pallets high, e.g., 2 or 3)
4.3 Packaging
- Carton type
- Carton ventilation design
- Internal packaging:
- Wrappers
- Trays
- Bags
- Punnets
- Other
- Palletisation (neat and square)
- Orientation of pallet (stowed 1M face or 1.2M face)
- Uniformity of packaging
4.4 Fruit Kind / Variety
- Temperature sensitivity
- Moisture loss risks
- Shape
- Size
5. Forced Air Cooling Guidelines
Recommended steps for successful forced air operations:
- Calibrate thermometers prior to the season.
- Regularly compare thermometers against each other.
- Check thermometers at least once a week in 50:50 flaked ice/pure water slurry (should read 0.0°C).
- Ensure workers insert thermocouples correctly when palletising loads (optional).
- Measure fruit temperature at receipt.
- Apply sound forced air cooling principles (see Section 4). Have a refrigeration engineer design the tunnels.
- 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 the fans.
- Check Delivery Air Temperature (DAT) regularly.
- Seal tunnels properly.
- Check sealing effectivity with a manometer (see Annexure 1).
- Be careful with fruit with low sugars—these may freeze just below zero (e.g., grape stems freeze just below zero, berries at lower temperatures).
- Regularly measure and record temperatures of:
- Thermocouples
- Air temperature
- Probes (if any)
- Get to know the cold room—identify hot spots, coldest positions, etc.
- Remember: cooling rate slows as fruit temperature approaches the target. E.g., with DAT at 0°C, it may take 6 hours to cool from 22°C to 5°C, but another 10 hours to reach 1°C.
- Stop forced air cooling as soon as the target temperature is reached—FAC leads to moisture loss. This may require night shifts but pays off in the long run.
- Measure fruit temperature at despatch.
Annexure 1: Description of a manometer
Annexure 2: Troubleshooting cooling problems
Manometer Description
A manometer measures the 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 in the tunnel due to the extractor fan).
- The height difference between the two vertical tube positions indicates the vacuum level.
- A reading above 10mm (preferably 14mm) indicates a successfully sealed pre-cooling tunnel.
- Forced air tunnels are designed for specific pressure differentials (manometer readings).
- Low readings (due to gaps in sealing) can be corrected immediately, saving pre-cooling time.
Annexure 2: Troubleshooting Cooling Problems
Three common problems in forced air cooling systems:
- Room temperature rises as products are added to the cooler.
- Cause: Insufficient refrigeration capacity (under-designed cooler).
- Also occurs when large rooms with multiple tunnels are filled over time.
- Solution: Divide the room into separate cooling bays using curtains or uninsulated walls. This limits the influence of adjacent stacks and allows each evaporator to address one stack.
- Product temperature in the outer cartons of the stack decreases slowly, despite DAT being on spec.
- Cause: Insufficient contact between cooling air and warm product.
- May be due to inadequate ventilation, non-alignment of vents, packaging or product blocking vents, insufficient airflow capacity (too many cartons), or short-circuiting of air past cartons.
- Product temperature in some outer cartons decreases rapidly, but others cool slowly despite DAT being on spec.
- Cause: Non-uniform airflow through the stack due to poorly designed supply or return air channels.
- If supply channels are too narrow (stack too close to wall or adjacent stack), cartons at the base receive inadequate airflow and cool less rapidly than those at the top.
- If the air return channel (plenum) is too narrow, pallets furthest from the fan cool slowly due to inadequate airflow.
- Monitor by measuring pulp temperatures and pressure drops across the stack at different heights and distances from the fan (large pressure drop differences indicate large airflow variation, seen as large pulp temperature differences).
Reference: Thompson, J, et al., 1998. Commercial cooling of fruits, vegetables and flowers. University of California, DANR Publication 21567