Mark Mitchell, Chairman of the Australian Food Cold Chain Council (AFCCC) explains the issues around the way a refrigerated load is loaded , and its effect on the efficiency of the refrigeration, because air flow can be tricky.
Food carriers might have the best refrigeration system for their transports, but its efficiency will be compromised unless those who pack the truck understand the principles of packaging, packing, stacking and wrapping.
These are four very different things, but any one of them can be responsible for blocking airflow sufficiently to negate convection and introduce conduction.
What does this mean? Good air flow, created by the refrigeration unit in a truck, trailer or intermodal container produces heat convection, or heat transfer, and this is crucial for maintaining product temperature in transport. The convection in a loaded container under refrigeration occurs within the air, as it travels in the gaps between pallets and the wall. This moving air is removing heat from the environment, via convection.
Most cold chain foodstuffs are typically carried in boxes, stacked on pallets. But unless these loads are strapped down so that they don’t move and block air flow, the product can’t be expected to be maintained at a compliant temperature.
The minute an untied stack of boxes on a pallet collapses and part or all of the load touches the wall of the truck, thereby closing any gap, convection disappears, and conduction takes over. The heat coming through the wall is conducted from the outside environment, through the wall and into the stock. The stock becomes the victim of heat conduction.
You don’t need a lot of air flow for efficient forced air convection. As long as there are gaps between pallets and between pallets and the walls, convection as low as half a metre a second is good enough. So some air flow is better than no air flow.
If there is no air movement, such as the free air or inadequate forced air that is typically found down the back of the trailer in a poorly stacked load, this means that this stagnant air is not being returned to the refrigeration unit, so obviously, the refrigerating effect must be diminishing.
Packaging can make or break the cold chain. As the food’s first point of contact, proper packaging is essential if the integrity of the food is to be maintained. But this is a separate issue, to be discussed in more detail in future issues.