Innovative packaging strategies can improve energy efficiency and space utilisation in shipping containers. By Anna Mouton.
The South African deciduous-fruit industry is built on exports. But soaring shipping costs are squeezing profits while consumers question imported food’s green credentials. Could putting more fruit in a container ease the pressure?
This question was the subject of a Hortgro-funded project led by Dr Tarl Berry, supply-chain researcher in the Department of Horticultural Sciences at Stellenbosch University. Nurayn Adewale Tiamiyu did part of the work for his MEng project.
“We would use fewer containers if we packed them more efficiently,” says Berry. If South Africa exports the same quantity of fruit in fewer containers, the cost and carbon footprint of shipping comes down.
Berry also thinks container airflow can be improved through better carton design and packaging systems, thereby enhancing temperature consistency and control. The container refrigeration units would not need to run as intensively, translating into less power use.
“Europe wants to hear about energy efficiency,” he says, adding that the energy use of containers will likely come under scrutiny as the European Green Deal gains momentum.
So, what are the options to upgrade current packaging strategies, and are big changes feasible?
The container conundrum
The 40-foot high-cube reefer is the most common container for shipping South African fruit. These containers are called high-cubes because they are about 25 cm higher than a standard 40-foot container.
A 40-foot container has internal floor dimensions of 11.58 x 2.29 m. A standard pallet measures 1.2 x 1.0 m. Regardless of how you arrange your standard pallets in a 40-foot container, there will be unused floor space — generally about 2.5 m2 in a container holding 20 pallets. That is the same total area as the footprint of another two pallets!
Space above the stacks represents another missed opportunity. “There’s a red line in the container that indicates the level to which you can pack,” explains Berry. “Carton height was designed for that level, but now the red line has moved up, so if you increase carton height a little bit, you can use that space as well.”
The mismatch between pallet size and floor area also impacts airflow. Reefer-container floors use T-shaped guides that channel cold air from the refrigeration unit along the length of the container.
“The premise is that the cold air is delivered along the bottom of the container,” says Berry. “It’s unobstructed in the T-bar floor and then propagates up slowly through the pallets, displacing the warm air, which returns to the refrigeration system.”
Air takes the path of least resistance, so if there is an uncovered floor area — known as a void — the air will bypass the pallets. Berry recommends plugging the voids by covering the empty floor areas with any impermeable material, even cardboard.
“The void plug is important,” he says. “Not everyone uses it because it was developed through industry experience, and there was some uncertainty about its value. We’ve studied void plugs, and they’re very effective.”
Of course, container designs and dimensions are not going to change to accommodate fruit. “Containers are owned by the shipping lines. They’re not going to make exceptions for us,” says Berry. “So that was the motivation for this study — how do we optimise, given that containers didn’t evolve for apples or stone fruit?”
Rethinking the box
Given that the container dimensions are fixed, the variables become pallet and carton sizes. But, as Berry points out, the 1.2 x 1.0 m pallet is universal and hard to change, even though its shape is suboptimal for shipping containers.
“There is a benefit to current pallets because you can fit several carton configurations on them,” says Berry. For example, apple cartons are typically 500 x 333 mm packed seven to a layer on a standard pallet.
But, as a quick calculation shows, the same apple-carton configuration would fit on a pallet of 1.17 x 1.0 m. In his report, Berry presents a loading design that fits 21 of these smaller pallets in a container, thereby increasing the container-floor utilisation from 88% to 92%.
This is already a substantial improvement, but Berry goes on to demonstrate a hexagon-based — Hex for short — packaging system that utilises more than 97% of the container floor.
Another option, the Tes — short for tessellated or tiled — system allows nearly 100% coverage. The various systems are shown in Figure 1.
Figure 1. Pallet-stack loading strategies. Dark blue is container footprint. Grey shapes are pallets. Adapted from Berry 2017.
Alternative pallet designs call for alternative carton designs, which is an opportunity to improve the efficiency of existing cartons. “The more carton material you have, the less space you’re using for fruit,” says Berry. “By using bigger boxes, you can fit more fruit inside the container.”
Bigger cartons also mean fewer pallet layers and less airflow resistance. More rapid airflow through the pallets improves cooling efficiency. “You can modify the packaging dimensions to influence cooling effectiveness,” notes Berry.
For his Hex pallets, Berry has designed a half-hexagonal carton that would be packed six to a layer, and for the Tes pallets, he fits nine rectangular cartons in a layer. A Hex carton holds about 21.3 kg of fruit, compared to 13.8 kg for a Tes and 16.9 kg for a standard carton. The carton and pallet designs are shown in Figure 2.
Figure 2. Packaging system configurations. Outer measurements are pallet. Inner measurements are cartons. Adapted from Berry 2017.
A 40-foot high-cube container takes 1 120 standard 500 x 333-mm cartons containing 18.9 tonnes of fruit. The 21-pallet configuration and the Tes packaging system would both increase this to 19.9 tonnes. But the biggest potential gains are with the Hex system, which can pack a container with 22.5 tonnes of fruit in 1 056 cartons. The pallet configurations are shown in Figure 2.
Berry clarifies that he is not telling everyone to switch to Hex cartons. “The idea was to see what you can do with a different shape — to see what improvements are possible and if you can do something a little differently.”
Optimising packaging systems is not just about fitting more fruit into shipping containers. Any packaging system must allow air movement to maintain fruit temperatures. “You have to remove the heat coming into the container and the respiration heat of the fruit,” says Berry.
He uses computational fluid dynamics to model airflow in containers — air is a fluid, and physicists have devised equations that describe fluid behaviour. “It’s incredibly complex and can only be solved using supercomputers,” says Berry, “but there are software packages that allow us to set these simulations up and run the calculations efficiently.”
The software applies the equations to a virtual representation of the container and its contents. Berry compares a pallet to a sponge because it has pathways for airflow in a matrix of solid components like fruit and cardboard. “We call it a porous medium, and that’s how we model it in the simulation. Currently, it’s not computationally possible to simulate every fruit in a container.”
Figuring out airflows did not rely entirely on simulations. Parameters for the porous media were derived using wind tunnels. Additionally, before simulations could be used to explore new scenarios, the modelling approach was validated using airflow and temperature data from several full-scale containers.
Figure 3. Air flow in different packaging systems. Arrows indicate direction of flow. Colour indicates air velocity. Uniform airflow — neither very high or very low — is associated with better air distribution within the pallet stacks. Adapted from Berry 2021.
The simulations showed that about half the standard pallets in a container receive low-velocity air because cold air is only delivered at one end of the container and slows as it travels along the container length. As illustrated in Figure 3B, pallets closer to the door tend to have lower airflows, even though they are at higher risk of temperature fluctuations, as the door is often less well-insulated than the rest of the container.
Airflows and cooling consistency were better in the Tes and Hex systems than in the standard system. The air slowed less along the container’s length, so pallets near the refrigeration unit and the door received comparable airflows.
Computer simulations like those in Figure 3A showed similar trends for the vertical movement of air. Vertical air velocities were poor nearer the door in the standard system. Upward airflows were much stronger in the Hex and especially the Tes systems.
Airflow resistances under 2 500 kg per m3 are necessary for air to move through pallets and optimally cool fruit. Pallet-stack designs should make it as easy — or easier — for air to go through a pallet and its cartons than under or around them.
Coming back to the sponge analogy, this means that interconnected spaces for flow must make up about 5% of the pallet. In previous research, Berry found that aligned edge vents — holes along cartons’ top and bottom edges — are the most efficient way to channel air through cartons.
Is change possible?
Shipping containers are not going to change, but pallets might. “You can do something different and meet the same forklift requirements,” says Berry, “but have a different space utilisation.”
The easiest win, though, is changing carton designs. Berry is seconded to the Department of Horticultural Sciences by Citrus Research International, and he recently witnessed a revolution in citrus cartons. “We needed a citrus carton with better vent alignment that cools significantly better to meet new phytosanitary requirements.”
The citrus industry rose to the challenge and redesigned their cartons — about 80 million of the new cartons were exported last year. “It was a considerable challenge for everyone, but market access requirements made the decision for us,” he says. More efficient cooling in the new cartons had the extra benefit of improved fruit quality by allowing the application of higher set points.
He also thinks more interesting and sustainable packaging could offer South African exporters a competitive edge.
“There are a near-infinite number of potential packaging solutions,” says Berry. “I only selected two interesting options, to illustrate the potential directions one could go.”
While Berry thinks significant improvements are possible, he acknowledges that exporters will be thinking about the cost-benefit implications. Changing to new packaging will incur costs, some of which may be unforeseen, and the benefits may be hard to quantify, making it difficult to know whether a design is optimal.
All these uncertainties may stifle innovation, as no one may be prepared to be the first to take a risk.
Carton design also presents challenges. “Let’s say that the carton is larger. You need less packaging overall, but you’ll probably have to use stronger board,” says Berry. “There is no easy route to figuring out what specs should be used.”
Nonetheless, Berry is confident that all these aspects can be overcome. “But it will take some dedicated attention. You need the right skill set — effectively a team of industry specialists — to tackle this problem.”
What are people saying about this research?
“This is basically about getting more fruit in a container. Tarl Berry looked at carton configurations to get more cartons — therefore more kilos and lower cost — in a container. It’s an easy win.”
Henk Griessel. Quality Assurance Manager, Tru-Cape.