A few million reefer containers are transported across the globe each year, carrying millions of tonnes of chilled and frozen cargoes. It comes as no surprise that some cargoes are unfit for consumption upon arrival. Cargo interests are often quick to blame the operation of the reefer containers. But are they right?
Cargoes transported in reefer container include fruit and vegetables, pharmaceutical products, fish and shellfish, fresh meat, flowers and flower bulbs. The values represented by these cargoes range from tens of thousands to millions of euros for just one container.
To understand the cause of damage to chilled and frozen cargoes, we need to understand how reefer containers work and what they’ve been designed for. And that means understanding the nature of the produce that is carried.
The first major difference in handling is determined by whether the cargo is alive, like fruit, or inert, like frozen fish and meat products. Bananas, for instance, use oxygen, give off CO2 and ethylene and produce heat. This means fresh air supply is needed, while CO2 and heat must be removed during the voyage. Inert and frozen produce, on the other hand, just need a constant temperature.
These differences are also reflected in the packaging. For frozen cargoes sealed packaging can be used, whereas chilled cargoes require ventilated packaging. And not just that, the box vents must be carefully aligned when stacked, to secure the vertical air flow. Suffice to say that chilled cargoes must not be covered.
How does a reefer container work?
Depending on the setpoint temperature, the reefer automatically switches to chilled or frozen mode. In chilled mode, the supply air temperature is measured to avoid chilling. In addition, air circulation will be at high speed. Whereas in frozen mode, it’s the return air sensor that is measured and air circulation speed will be low.
To cool, chilled air is distributed from the floor. The baffle plate directs the air flow into the T-bar floor and produces a consistent and uniform upwards flow across the container. However, to ensure optimised air exchange with the goods, correct stowage is key. If there are too many gaps, of if gaps are too large, the air will follow the easiest path, not reaching the entire cargo.
The cooling mechanism resembles your own fridge: compressing, condensing and evaporating the gas and liquid states of the refrigerant. And as in a freezer, humidity will form ice on the cooling element. To remove the ice, circulation is temporarily interrupted from time to time as a heater melts the ice and the water is drained away.
Don’t cool while stuffing
A mistake often made, is stuffing the reefer with the cooling unit running. Why is this a mistake? As cold air is heavier than warm air, the cold air will escape through the open doors, to be replaced by warm air drawn in over the top of the cargo. So having the cooling unit running during stuffing has, in fact, the opposite effect of drawing in warm air. In addition, this results in increased humidity inside the reefer, causing excess ice formation on the cooling element, which activates the heater and interrupts circulation unnecessarily often during the first part of the voyage.
Another important thing to note is that the reefer is not designed to rapidly reduce cargo temperatures. And due to the sealed packaging and the low-speed air circulation, reducing the temperature of frozen cargo is particularly difficult. The reefer’s function is to maintain the pre-cooled cargo temperature.
QUEST: improving reefer energy efficiency
QUEST is a system designed to reduce energy consumption while safeguarding the quality of the chilled cargo.
Chilled cargoes are particularly active at the beginning of the journey, requiring high-speed circulation. However, during the voyage the cargo is basically put to sleep and will then generate less heat. As a result, the evaporator fans can be switched to half speed. As soon as the return air sensor detects deviations from the required temperature, the evaporator temporarily goes back to full speed. In addition, the compressor is operated intermittently. And finally, there is less need for defrosting the cooling element using the heater, because much of the defrosting will take place when the compressor is inactive.
Over the years the QUEST system has been constantly monitored, evaluated and improved.
Controlled atmosphere extends shelf life
Controlled atmosphere is mostly used for fruit and extends shelf life. I mentioned that the fruit is put to sleep by reducing the temperature. It could be said that the fruit can be put to hyper-sleep by reducing O2 and increasing CO2. This needs to be done carefully, as too little O2 will suffocate the fruit, resulting in discolouration, failure to ripen and off-flavours. While too much CO2 results in premature softening, poor texture and an awful taste.
To achieve controlled atmosphere, the reefer container must be completely air-tight. A vacuum pump filters and removes excess CO2, while the fresh-air intake introduces O2 when needed.
Suffice to say that when it comes to airflow and atmosphere requirements, there’s no one size fits all. Each commodity requires different settings.
Identifying the cause of damage to fruit
As said, cargo interests are quick to blame reefer operation. We distinguish between pre-shipment causes and damage caused by reefer malfunctioning. Typical types of pre-shipment damage include:
- Abrasion and bruising.
- Antrachnose: a group of fungal diseases, the symptoms of which include discoloured sunken spots and dead leaves.
- Cigar or crown end rot, resulting in ripening and heat generation, affecting surrounding healthy fruit.
- Penicillium italicum and penicillium digitatum, in other words: mould. Mould may always be present. Here the question is whether damage to an entire consignment is caused by the fruit, or by reefer malfunctioning, as the latter would exacerbate the spread of mould.
Types of damage often caused by a malfunctioning unit are:
- Advanced ripening.
- Chilling damage.
Interpreting reefer data: a case study
Today, reefers generate a wealth of data throughout the voyage. Temperature setpoint, supply, return and ambient temperatures, O2 and CO2 setpoints and readings, humidity levels, heater, evaporator and vacuum pump operation: everything is recorded. In addition, errors and events are continuously logged. But it takes specialist interpretation to make sense of the data.
Let me illustrate. A consignment of shrimps showed excess frost inside the packaging, indicating that the correct temperature had not always been maintained. As a result, the shrimps were unfit for consumption. The shipper had placed a temperature logger inside the unit, near the doors. Based on its readings, showing that the temperature had been too high during the voyage, the shipper blamed the damage on the reefer. This is how we disproved the claim:
- The cargo was leaning against the doors and had, therefore, been stuffed incorrectly, preventing a correct airflow inside the container.
- The logger had been placed on the aft-stowed pallets, against the doors. With the cargo leaning against the doors, the temperature recorded by the logger was not representative for the temperature inside the cargo compartment. The shipper had failed to place a second logger in the front of the unit, which would have offered more relevant information.
- Both the reefer data and the logger showed that the cargo temperature during stuffing was far too high: -10 °C return air temperature with a supply air temperature of -18 °C indicates that the cargo temperature was well above -10 °C when stuffed. In fact, the shipper’s logger showed a cargo temperature of -5 °C at the time of stuffing! And reefer containers are not designed to rapidly cool down cargoes.
- After the first three days, the reefer data showed that the return air temperature had gone down to and had remained at requested -18 °C for the remainder of the voyage.
- The reefer data also showed an abnormal high number of defrosts during the first days of the voyage, indicating that the container had been stuffed with the reefer unit running.
As a result of these mistakes made by the shipper, it took three days for the return air temperature to reach -18 °C. And by that time, the centre of the stow was certainly still too warm, as the cold air could not reach the aft part of the stow due to incorrect stuffing.
Interpretation of the logged data, showing correct supply air temperatures throughout the voyage, the slow decrease of the return air temperatures and excess defrosting, combined with the visual inspection of the stowage, resulted in the conclusion that the shipper was clearly to blame for the state of the consignment. And that’s how expert cargo surveyors make their mark!