Introduction
Understanding the factors that influence product temperatures during transit and within refrigerated trailers is essential for successful road transport of perishables. Key factors include:
- Performance of the refrigeration system
- The load: commodities, amount, respiration rates, packaging, temperature at loading, and loading patterns
- Weather: ambient and especially road surface temperatures during transit
- Journey length and whether the trailer and load were pre-cooled prior to loading
Important notes:
- The PPECB has a legal obligation to ensure strict quality and hygiene requirements, as required by importing countries, are met.
- Refrigerated road motor transport (RRMT) is primarily designed to maintain product temperature, not to pre-cool or re-cool products that are not at the optimum temperature before loading.
- Re-cooling products prior to shipping can cause problems at ports, including delays, rejection, and product deterioration.
- In cases of quality loss or waste on overseas markets, PPECB may be asked to submit technical reports for insurance claims. Correct transport conditions must be maintained to avoid such claims.
Minimum requirements specified in this document must always be applied. Registration or certification of a RRMT is not a PPECB requirement but can be done by PPECB or any competent accredited organisation if required.
PPECB Registration/Requirements
If PPECB is requested to register a RRMT, the following requirements apply (captured on PPECB ISO 9001 form Q20):
- Thermocouple (TC) Wires:
- A TC wire with a H110T male plug must be mounted in the delivery air inside the RRMT. The plug is placed in a waterproof box with a sliding lid on the outside of the trailer. The other end is installed ~300mm from the delivery air exit of the evaporator coil, with 300mm slack inside the box. Mark with a blue marker. Lid: “Delivery – Blue”.
- A similar TC wire, marked red, must be mounted in the return air (just before the air enters the evaporator plenum), with the plug in the same box as the DAT plug. Lid: “Return – Red”.
- Temperature Control Sensor:
- Must be mounted in the delivery air, same position as the DAT thermocouple. If mounted in return air, DAT can be up to 7°C lower than setpoint, risking product damage.
- Temperature Control:
- Control in return air is only allowed for deep-frozen cargo.
- Automatic Temperature Recorder:
- Every refrigerated vehicle must have an externally mounted automatic temperature recorder. If only one sensor, it must be in the delivery air. If more than one, a second can be in the return air. The recorder must have an on-site print function, be clean, in working order, and have paper at all times. Accuracy: not more than 1.0°C deviation.
- Portable recorders are unacceptable due to slow reaction and inaccuracy.
- PPECB Assessor will record the temperature history after the trip.
- Arrival at Transfer Depot:
- PPECB assessor will check temperatures, settings, and condition of the unit and insulated box. Unsatisfactory findings result in a warning form (Q25). Repeat faults lead to certificate cancellation.
- Loading:
- Cargo must be loaded so carton holes face front to back, not sideways, to prevent airflow restriction.
- Cargo must always be on pallets if the trailer does not have a T-bar floor, to prevent heat conduction from the floor and ensure airflow.
- Thermostat Accuracy:
- Maximum 1.0°C below setpoint and 1.5°C above setpoint.
- False Bulkhead:
- Required to prevent short cycling of air between delivery and return airflow.
- Insulation:
- Must remain in perfect condition. Holes/cracks allow moisture in, making the trailer heavier and insulation ineffective.
- Repairs:
- Loose panels/plates must be repaired and sealed to prevent cargo damage and contamination.
- Refrigeration Unit Covers:
- Must not be missing, as exposure can cause breakdowns.
- Tyres:
- Must be in good order to prevent delays and accidents.
- Cleaning:
- Trailer must be washed and disinfected before every trip. Avoid strong smells before loading.
- Pre-cooling:
- Trailer must be pre-cooled for at least four hours before loading, especially for fruit and vegetables.
Refrigeration
- The goal is to maintain quality by lowering commodity temperature, slowing deterioration and decay.
- Refrigeration removes heat from the product. For example, to lower from 15°C to 10°C, the unit must remove 5°C worth of heat.
- Heat is measured in kilojoules: the amount needed to raise/lower 1kg of water by 1°C.
- Heat always travels from warm to cold. Main sources:
- Environmental heat (inside and outside the trailer)
- Heat from product respiration
- Heat generated by the cooling system (defrosting, fan motors)
Modern units can maintain desired temperatures but may lack enough air circulation for tight loads. Load pattern must aid, not restrict, air circulation.
- Some systems can provide heat if ambient air is colder than required load temperature.
- Properly designed systems provide high relative humidity (up to 95%).
A truck refrigeration system must remove heat from:
- Residual air inside the trailer (pre-cool trailer and load quickly, keep doors closed, use sealed loading docks)
- Infiltrated heat from outside air (keep body and door seals in perfect condition)
- Heat of respiration from produce (higher product temperature = faster respiration = more heat)
Air Circulation System
- Air must move through and around the load to absorb heat and return it to the evaporator coil.
- If the unit is off, there is no air circulation.
- Defective air ducts or collapsed shutes restrict airflow.
- Newer trailers may not have air delivery shutes; cold air is delivered in the plenum above the pallets. Leave at least 150mm between the load and ceiling.
- T-bar floors and ribbed walls improve circulation; flat floors/walls require careful palletized loading.
Flat Walls:
Keep product away from side walls to prevent heat conduction. Leave at least 150mm between load and ceiling.
Flat Floors:
Heat enters from the road. Always use pallets or T-bars to allow air circulation and prevent return air passage blockage.
Temperature Control System
- Accurate temperature control is essential. Includes thermostat (heating/cooling), defrost, and fan speed control.
- Thermostat sensor must be in the delivery air stream for chilled cargo. Wrong positioning can cause cold/freezing injury.
- Some trailers have two sensors (delivery and return air) with a switch for “chilled” or “frozen” mode. The switch must be clearly marked.
- Modern units may use DAT/RAT modulation.
Refrigerated Trailer Condition
- Damaged insulation allows heat in, especially if moisture enters.
- Stainless steel plates must be sealed to prevent contamination.
- Wash inside with clean water (80°C+) and food-grade disinfectant before every load.
- Check cooling coil plenum before each trip for cleanliness.
- Remove strong odours before loading.
- Regularly check outside insulation and skin for cracks or loose panels.
- Keep the outside clean for better heat reflection and appearance.
Humidity
- Most perishable products require 85–95% relative humidity to prevent dehydration.
- Relative humidity is the percentage of water vapour in the air relative to saturation at a given temperature.
- In mechanical refrigeration, moisture is condensed from the product to replace what is lost via the evaporator.
- Some moisture loss is inevitable but can be minimized by:
- Thorough pre-cooling of product and trailer
- Keeping the evaporator coil only a few degrees colder than transit temperature (requires proper calibration)
- Controlling air temperature at the delivery side of the evaporator for chilled product
- Ensuring thermostat accuracy (temperature graph should be as straight as possible, variance ideally no more than +1.5°C or -1.0°C from setpoint)
- Using the largest available evaporator coil
- Waxing product or using semi-permeable wraps (with caution, as airflow may be restricted)
Pre-cooling the Product
- Lower temperature and remove field heat as soon as possible after harvest.
- Pre-cooling methods:
- Room cooling (slow, may not reach optimum temperature)
- Forced air cooling (efficient, fast, even cooling)
- Pressure cooling (like forced air, but in tunnels; cost-effective for large volumes)
- Hydro cooling (direct contact with chilled water; fast and effective, but requires skilled operation)
Problems:
- Trailers not properly pre-cooled before loading
- Mixing pre-cooled and warm product in the same load
Best practice: Only load properly pre-cooled products.
Freezing and Chilling Injury
- Sometimes the heating cycle is needed if ambient temperature is lower than the required carrying temperature.
- Heaters must be checked regularly.
- Loading warm produce and setting delivery air temperature too low can cause chilling injury, especially to product near the evaporator.
- Always control temperature in the delivery air for chilled produce to prevent chilling injury.
Loading
- Ensure product is at the correct temperature at loading. Record pulp temperatures of all pallets (at least 5 per load) and send with the truck.
- If local market fruit is loaded with export produce, it must also be pre-cooled to the export temperature.
- Best practices:
- Educate drivers on proper loading and temperature checks
- Stand by during loading to prevent damage
- Ensure carton holes face front and back
- Check the trailer at every offloading point
- Ensure the unit is functioning and set correctly
The Load
- If no T-bar floor, always use pallets to create air return passage and keep load off the floor.
- Maximum load height (including pallet): 2.1m. Leave at least 150mm between load and ceiling.
- Keep loads away from walls (min 25mm gap).
- Load pallets with minimal gaps to force air through the load.
- When packing loose cartons, leave small gaps for airflow.
- Inspect the unit regularly during transit.
Compatibility of Mixed Loads
- Only mix loads if commodities are compatible in:
- Transit temperature
- Relative humidity
- Emission of gases (e.g., ethylene)
- Odour absorption
- Packaging type and airflow
Never mix different packaging in the same load.
Offloading
At the port transfer depot, PPECB will log:
- Air delivery and return temperatures (from thermocouples)
- Chart recorder temperature (or copy if variation is too high)
- Product pulp temperature in at least 3 positions (door, centre, front)
- Pulp temperatures in the centre of pallets
- All relevant dates and times
All data is available to exporters and used for pre-shipment history and claims.
Calibration
- Use an ice-water mixture (0°C) to calibrate thermometers and sensors.
- Adjust instruments or note errors for correction.
- Refer to PPECB ISO Work Instruction TWI02 for details.
Summary and Recommendations
- Only load pre-cooled products.
- Pre-cool trailers before loading.
- Record product temperatures during loading.
- Do not push product up against the evaporator.
- Load on pallets away from side walls.
- Do not load higher than 2.1m (including pallet); leave 150mm to ceiling.
- Allow at least 80mm between door and last pallet.
- Ensure carton holes face front to back.
- Regular maintenance and calibration.
- Drivers should have and know how to use an electronic thermometer.
- Obtain a PPECB certificate and comply with minimum specifications.
Buying or Contracting Refrigerated Trailers
When purchasing a new refrigerated trailer, ensure it can maintain necessary product temperatures. Key features:
- 50mm deep T-bar floor for air circulation
- Grooved/fluted side walls for airflow
- False bulkhead with open bottom entry for air return
- Curved air deflector at base of inside front wall
- Strong inner wall skins to prevent damage
- Smooth, preferably white, outer skin for heat reflection
- Refrigeration unit with fan capable of circulating at least 0.944 m/s air against 50mm water gauge static pressure
- Large evaporator coil for high humidity and constant temperature
- Solid state temperature controller (thermostat)
- Thermostat with switch for delivery/return air control
- Thermograph or temperature recorder for trip history
- Thermocouples in delivery and return air streams for checking and calibration