Which packaging to choose when cooling citrus fruit: Open-tops or closed telescopic cartons?

This question I often get from several stakeholders in the citrus supply chain: Should we use telescopic closed-lid or open-top cartons to pack, transport, and store citrus fruit? The underlying question is actually which impact these packaging types have on fruit cooling and its quality. Often the packaging type is chosen (prescribed) by retailers or B2B suppliers, based on a certain rationale. Criteria for choosing a certain packaging can be to optimize cooling, logo placement, the cost of the packaging, or avoiding repacking at the retailer. The choice might seem simple, but it can have a large impact if you ship a lot of fruit. The question is if this choice is always the best for the fruit that is shipped …

Let me give some insights from the past work we did on citrus.

What are we looking at?

For citrus fruit, there are different types of cardboard packaging. Commonly used are closed telescopic cartons and open-top cartons (Figure 1). The cartons differ concerning the vent holes and fruit packing density inside the box. Telescopic closed cartons have vent holes that are well distributed on all six sides of the package, and the fruit is packed very densely. Open-tops often have large openings on the top of the vertical sides, very few vent holes on the bottom of the crate, and no lid. There is an open space above the top layer of fruit. The question is which of these two packaging types is preferred to maximize the cooling rate and the cooling uniformity within palletized citrus fruit?

What we did so far?

We compared these two types of packaging in several studies for different unit operations, such as: (1) precooling with horizontal airflow through the packages at high speeds [1,2]; (2) transport in refrigerated containers with vertical airflow at moderate airflow rates [1,3]; (3) cold storage in storage rooms with horizontal airflow through the pallets, at low airflow rates [1]. For cold storage, the flow is, however, also often omnidirectional and highly dependent on the location of the fans in the room. We did not do any work for a refrigerated trailer yet, comparing these two packaging designs for citrus.

Important for comparing the cooling performance of a package for a specific fruit are the airflow rate (or airspeed) with which we cool, the airflow direction, and the packaging density of the fruit in the box.

I need to put a disclaimer when analyzing and comparing these studies and others that you might find. A fair comparison between studies is often difficult since there are always inherent differences in the alignment of the vent holes, possible airflow bypass, the use of void plugs or other flow-guiding devices, and the nature of the work: experimental or physics-based simulations. In addition, with experiments, for example, we often only have fruit pulp temperatures at limited locations in the pallets and packaging. The results based on a few measurement points are not always representative of the entire cargo. In simulations, we calculate the cooling of each single citrus fruit and can extract volume-averaged temperatures of the fruit or even the full cargo, leading to better statistics.

What can we conclude?

Let us first have a look at what the impact of airspeed is. We cool the fruit faster and more uniformly within the package at higher speeds [1] (Figure 2). The airflow rate also changes the airflow field through the pallet of fruit (Figure 3). The more open the package is on the side where the airflow is coming from, the more uniform it cools in principle. However, the ventilation openings should be uniformly distributed over the sides. Open-tops have the issue that the fruit of the bottom layer cools less fast as it is sheltered from the airflow.

For precooling, open-tops seem to cool faster in our experiments but not necessarily more uniform (Figure 4a,b,c). Our simulations contradicted the faster cooling of the open-top packaging (Figure 4d). The reasons are that simulations are more 'perfect' and do not suffer from imperfect stacking, blocking of vent holes, or possible airflow bypass. These all occur in reality, but the extent depends largely on the handling practices. In addition, the experiments were performed on similarly sized citrus fruit, but lemons were used for the closed-lids and mandarins for the open-tops.

Why did we not just choose the same fruit for both experiments (Figure 4)? Well, we did not have a few 10 kUSD to spare to buy fruit for our experiments and place them in a precooler. So we had to do experiments on precoolers filled with commercial cargo. Here, we could not 'choose' the fruit that was coming from the field and needed to be precooled and shipped. The availability (cost) of fruit for such full-scale experiments can be a stumbling block for many researchers and engineers.

Apart from fruit size, it is also possible that the airflow rate was not exactly the same for the experiments of both packages: the fan's operating point – so airflow rate – is determined by the pressure resistance of the package. Open-tops have a much lower aerodynamic resistance to airflow. In our simulations, all parameters were kept the same – obviously – as it is a simulation. Theoretically, simulations showed that the low cooling rate of fruit in the bottom layer of the open-tops is responsible for slower cooling.

For refrigerated transport in containers, the cargo is typically precooled before being loaded in a refrigerated container, and the same holds for trailers. However, sometimes, citrus is cooled down in a refrigerated container without prior precooling [4]. The reason is to speed up logistics or just need-based if the available precooling facilities are limited or maxed out. Airflow in a container is vertical, bottom-to-top. Open-tops seem to cool slightly faster and more uniformly than closed-lid packages in our experiments (Figure 5a,b). However, the packaging density was different: we often can't pack the same amount in closed-lid and open-top packaging. This aspect is key for a refrigerated container (or trailer) as the cooling capacity is much more limited than in a precooler. Cooling down a smaller amount of fruit in a container will be much faster. However, simulations on a single pallet of fruit predicted a similar cooling behavior for both packages (Figure 5c). Here, the gaps between the pallets in a container were not included. Since open-tops have limited vent holes on the bottom, these gaps could play a key role in speeding up the cooling process.

For refrigerated transport in trailers, the entire story would change as the airflow is less unidirectional. The cold air is blown in from the top and is directed, often through a chute downwards, towards the cargo to the outlet. In contrast to a container, also a large horizontal velocity component exists. The open-tops will likely benefit from their open head space to cool, but we do not have data here yet.

Cold storage in large storage rooms is often used to cool down the fruit or store the warm fruit before it can be precooled. Here the fans are often at the ceiling of the room. In such rooms, the differences in cooling between closed-lids and open-tops seem less relevant (Figure 2a,b). Due to the low airspeeds, the airflow field changes. Due to the lower momentum, the air can better reach the sheltered fruit at the bottom of the package for open-tops.

The wrap-up

The bottom line is that the package, the airflow rate, its direction, and the packaging density are all relevant when cooling the product down. So independent on how you precool the fruit (in a precooler, container, trailer, or cold store), the package will have a large impact. Open-tops seem to cool down often faster, but not necessarily more uniformly. The fruit on the bottom of the open-tops is less exposed to the cold horizontal airflow. Open-tops are typically packed less dense, so you often have less citrus fruit per pallet and less fruit per shipment. They also have a different resistance to airflow, which can change the airflow rate of the fans. So all and all, the answer is not that simple.

If you asked me, I would prefer open-tops, if you can choose, for precooling, storage, and refrigerated trailers, as this gives the citrus a bit more space, and there is a horizontal airflow component. For refrigerated transport in containers, I would be more cautious as the airflow is vertical. Here, make sure to always choose a carton design with sufficient vent holes at the bottom or at the edges to promote vertical airflow.

However, keep in mind that we talked about cooling down fruit so far. Once the cargo is fully (pre)-cooled, subsequent transport or storage just needs to keep the temperature. As citrus fruit is not respiring really, no extra heat needs to be removed. In this case, the packaging type is not that important for keeping the temperature. So, in the end, proper (pre)cooling is essential, and packaging plays a key role here. And here, remember the cargo needs to be precooled fully, so also deep inside the pallets (where you often can't measure easily). Here, be honest with yourself: if you did not measure in key locations, for example, in the pallet's center, you don’t know for sure that your cargo is fully precooled.

#supplychain #food #freshproduce #coldchain

References

[1]      W. Wu, P. Cronje, P. Verboven, T. Defraeye, Unveiling how ventilated packaging design and cold chain scenarios affect the cooling kinetics and fruit quality for each single citrus fruit in an entire pallet, Food Packag. Shelf Life. 21 (2019) 100369.

[2]      W. Wu, P. H?ller, P. Cronje, T. Defraeye, Full-scale experiments in forced-air precoolers with 40 pallets for citrus fruit: impact of packaging design and fruit size on cooling rate and heterogeneity, Biosyst. Eng. 169 (2018) 115–125. https://doi.org/10.1016/j.biosystemseng.2018.02.003.

[3]      T.M. Berry, T. Defraeye, W. Wu, M.G. Sibiya, J. North, P.J.R. Cronje, Cooling of ambient-loaded citrus in refrigerated containers: What impacts do packaging and loading temperature have?, Biosyst. Eng. 201 (2021) 11–22. https://doi.org/10.1016/j.biosystemseng.2020.11.002.

[4]      T. Defraeye, P. Verboven, U.L. Opara, B. Nicolai, P. Cronjé, Feasibility of ambient loading of citrus fruit into refrigerated containers for cooling during marine transport, Biosyst. Eng. 134 (2015) 20–30. https://doi.org/10.1016/j.biosystemseng.2015.03.012.