Cold chain pallets work in damp, cold and frequently changing conditions that wear out standard dry-warehouse pallets quickly. The main design questions are how the pallet handles moisture, whether it permits cold air to circulate through the load, and whether it can be kept clean. Plastic pallets are the most common choice for deep-frozen and chilled operations, but wood remains widely used, particularly in fresh produce and in export.
Conditions that stress pallets
A cold chain covers chilled products (roughly 0–8 °C), frozen products (commonly −18 °C or lower) and sometimes ambient transport with temperature protection. Pallets in this chain face:
- Moisture and condensation. Moving from a cold store to a warm dock causes condensation on every surface. Wet wood gains weight and swells, and can support mold if stored in poor conditions; see mould and blue stain.
- Freeze-thaw cycles. Water in wood freezes, expands and thaws; repeated cycles loosen nail joints and split boards.
- Washing and defrosting. Cold stores and food plants wash floors and pallets, and ice builds up at doors and under loads.
- Low-temperature brittleness. Some plastics become more brittle at low temperatures, and a pallet's impact resistance depends on the polymer and design.
- Heavy dynamic loads. Frozen goods are dense and often handled by rapid automated systems.
Airflow and the pallet
Cooling works by moving cold air around and through the load. A pallet influences this in several ways:
- Deck design. A solid or closely spaced top deck blocks air flowing upward through the load. Open decks with gaps, or vented plastic decks, let air rise through cartons with ventilation holes.
- Bottom clearance. Fork openings and the space under the load carry air in the T-shaped floor channels of refrigerated containers; see below.
- Stack pattern. The load itself must leave gaps. Cartons stacked tightly in columns, wrapped in film, can behave like a solid block.
Pre-cooling is crucial for fresh produce, which is cooled before loading and then kept cold, since reefer units are designed to maintain temperature rather than remove heat from warm cargo.
Refrigerated containers
Reefer containers are ISO containers with a refrigeration unit on the front wall. Cold air is blown along a T-shaped slotted floor under the cargo and returns over the load. For this to work, pallets and cartons must not block the floor channels or the space beneath the roof.
Loading rules from container operators and carriers include:
- Load only to the red line or load limit marking on the container walls, so the air return space remains open.
- Keep cargo clear of the doors and unit so air circulates.
- Use pallets suited to the floor: cargo should rest on the T-floor, not on a solid sheet that blocks the channels.
- Do not use plastic sheet between pallet and floor which seals the T-floor.
Pallet footprint also determines utilisation: the container's internal width fits 1200 × 1000 mm pallets in a specific pattern, while 1200 × 800 mm pallets fit differently. See pallets in shipping containers.
Materials for cold chain pallets
| Material | Advantages | Drawbacks |
|---|---|---|
| Wood (kiln-dried softwood) | Low cost, easy repair, good stiffness, absorbs minor condensation | Takes up moisture, risk of mold and splinters, needs ISPM 15 for export, harder to clean |
| Plastic (HDPE/PP) | Does not absorb water, can be washed and sanitized, consistent size, long life | Higher cost, may be slippery, performance varies at low temperatures, hard to repair |
| Metal | Strong, washable | Cost, weight, condensation on surfaces |
| Moulded wood fiber | Light, low cost | Absorbs moisture, limited strength when wet |
| Paper and corrugated | Light, recyclable | Weak when wet, mostly unsuited to freezer use |
The choice of material is covered in wood vs plastic pallets. The argument favouring plastic in cold stores is mainly about moisture and hygiene, while the argument favouring wood is cost, repairability and availability. In practice, many chilled supply chains use returnable plastic pallets or pooled pallets; see pallet pooling.
Wood used in damp conditions benefits from moisture control: pallets should be stored dry, kept off the floor, and well-dried before use. Consult wood moisture content for the relation between moisture and fungal risk.
Operating practices
- Dedicate pallets to the cold zone. Pallets that have been in cold stores should be dried and stored under cover before reuse.
- Use plastic or sealed pallets in wet areas such as blast freezers.
- Inspect for splits and ice-affected boards before racking.
- Choose pallet height and rack beams to match the cooling equipment's air return.
- Monitor product temperature, not just room temperature, for cold chain compliance, because pallets in the center of a block may stay warm while the air is cold.
For pharmaceutical products the rules are stricter; see pallets in pharmaceutical logistics, and for food hygiene, pallets and food safety.
Frequently asked questions
Are wooden pallets allowed in cold storage?
Yes. Wooden pallets are used in cold stores worldwide, but they absorb moisture and need to be dry, in good repair, and handled hygienically. Many freezer operators prefer plastic for the washing and moisture advantages.
Why does airflow matter for pallets?
Cold air must reach the product. Pallets with solid decks or tightly packed loads can block circulation, creating warm spots, especially in reefer containers that depend on airflow through a T-shaped floor.
Do reefer containers cool warm goods?
No. They are designed to hold the temperature of pre-cooled cargo, not to lower the temperature of warm products, so cargo should be at the target temperature when loaded.
Sources
- Proper loading of reefer containers — BWS
- Cargo consolidation and packaging — Identec Solutions
- FDA pallet requirements: finding a pallet for sanitary transportation of food — iGPS
- Wood pallets and the FDA sanitary transport rule — WoodPackGlobal