The ocean leg gets blamed for a great deal of damage it did not cause. A reefer container is a very capable machine for holding a cargo at a temperature it already has. It is a poor machine for pulling heat out of pallets that arrived warm, because the air it moves is sized for maintenance, not for pull-down.
Everything that follows comes out of that one distinction.
The container does not cool the cargo
Product has to reach the set point before the doors close. Frozen lines leave the cold store already held at −18°C; fresh lines leave pre-cooled to the temperature written into the booking. A container asked to remove field heat or storage heat during the first days at sea will run hard, hold an uneven profile through the stow, and still deliver a cargo whose warmest cartons never quite came down.
The corollary is simple and unpopular: a stuffing that runs late is not a scheduling problem. It is a quality problem.
Airflow is a stow decision, not a machine setting
Delivery air enters along the T-bar floor, travels the length of the container and returns over the top of the cargo. That circuit only works if the stow lets it:
- Pallets sit square on the T-bar so the floor channels stay open.
- Cargo stays below the load line — anything stacked into the return air path short-circuits the circuit, and the far end of the container is what pays for it.
- Frozen cargo is blocked tight, so cold air is forced through the floor rather than finding a free lane around the stack.
- Fresh cargo needs the opposite: ventilated cases, aligned vents, and space for air to pass through the produce rather than around it.
A container loaded against its own airflow will produce a temperature record that looks acceptable at the return sensor and unacceptable inside the cartons at the doors.
Temperature is not a service applied to the cargo. On a frozen programme it is most of the product.
Set point, not average
A trace that averages correctly can still be a failed shipment. What matters is the shape of the curve: how long the cargo spent above the set point, when, and where in the container. That is why the record travels with the lot rather than being summarised into a single number on a delivery note.
What the paperwork has to show
- Pre-trip inspection on the unit before it is released for stuffing
- Container pre-cooled, and the set point matching the booking
- Loading recorded lot by lot, so the stow can be reconciled to the bill of lading
- Data logger placement noted, not assumed
- Seal number captured at the point of sealing
Where integrity is actually lost
Rarely mid-ocean. Far more often it goes at the transitions: a container standing at a gate with the unit off, a transhipment yard where the plug-in is delayed, a delivery door held open while a receiving bay is cleared. The sea passage is the best-monitored part of the journey. The handovers on either side of it are where a programme is genuinely at risk — which is why they are worth writing into the contract rather than leaving to habit.
More on the stages before the container in cold chain, packing and shipping.