Transporting frozen or chilled cargo in reefer containers - stuffing requirements

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means.

Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, air conditioning, and heat pumps. Transporting refrigerated cargo in reefer containers can be classified as food or non-food items and as chilled or frozen items.

Frozen products
Fig:Frozen meat products

Frozen cargoes

Stored below minus 10°C, these are `dead' cargo, ie no chemical processes or reactions take place within the product and no gases or heat is released.

It is a general misconception that for all refrigerated cargoes, the lower the temperature, the better they are preserved.

For chilled cargoes, the temperature must be maintained very close to the set-point. If the temperature falls below the set- point, the cargo can be damaged by overcooling.

Frozen cargoes are not damaged even when the temperature falls below the set-point for prolonged periods of time. If, for example, the temperature of ice cream is maintained at minus 25°C instead of minus 20°C, there is no damage or deterioration. Because there is no heat produced by a frozen cargo, it is much easier to maintain the temperature.

When there is a power outage or a planned shut-down of the ship's power generation, it should be remembered that chilled cargoes are more vulnerable to damage and deterioration due to heat generation and build-up inside the container.

Where possible, chilled cargo reefer containers should be clustered together and connected to one power source or circuit breaker. Power should be restored as soon as possible to chilled cargo containers, before the frozen cargo containers. The frozen cargoes are not as easily damaged since they do not produce heat and the temperature rise is very slow.

Chilled Cargoes and chilled operation mode

During chilled operation the circulating fans run at maximum rpm and the intake air temperature is regulated constantly. Chilled cargoes, also known as perishable cargoes, are stored above minus 10°C. Some chilled cargoes are live and respiring, with chemical reactions and processes taking place within the product and a continual output of gases and heat.

Chilled Fruit products
Fig:Chilled Fruit products

Chilled cargoes are temperature sensitive. The heat some chilled cargoes generate through chemical reactions needs to be removed outside the cargo space quicker than it is released to prevent heat build-up and a rise in temperature. Some cargoes require that the gaseous products of the chemical reactions also be removed to avoid damage and deterioration in the quality of the product.

Chilled cargoes include fruit, vegetables, dairy products, chilled meat, flower bulbs, electronic equipment and medical supplies.

The temperature of a chilled cargo must be maintained within a very narrow band of ±0.5°C around the set-point. Temperatures outside this can cause the cargo to deteriorate.

Chill damage is caused when the storage temperature goods of vegetable origin drops below the specified limits. This process means that fruit suffers irreparable metabolic damage which renders it incapable of ripening.
Packaging requirement of chilled and frozen cargo
Fig:Packaging requirement of chilled and frozen cargo

Type of packaging & stuffing methods

Each commodity has different airflow requirements. Inside a reefer container the airflow is influenced by the type of packaging and the method of stuffing used. In the case of pre-cooled frozen goods, air only has to flow around the cargo, since no heat has to be dissipated from the goods themselves. Only the heat which penetrates the insulation from outside has to be removed. When transporting chilled goods such as fruit and vegetables, however, air also flows through the cargo, as it generates respiration heat internally which has to be dissipated.

There are two standard loading patterns for perishable products in reefer containers:
block stow of break bulk cargo (e.g. loose cartons), and palletised cargo stowage (e.g. cartons on a pallet). With both stuffing patterns, cargo must cover the entire T-floor to ensure proper distribution of refrigerated air. This is possible to arrange in most cases when, for example, loose cartons are stuffed into the reefer container.

However, not all palletised cargo or, say, drums, can be stuffed in this way. In that case, when the cargo does not cover the entire T-floor, heavy cardboard or dunnage must be placed where no cargo is stuffed. This will avoid short-circuiting the circulating air and ensure proper refrigerated air distribution in reefer containers with bottom air supply. Improper stuffing, and thus respective by-passing of the circulating air, initiates a larger spread of different temperatures within the cargo and can lead to severe cargo damage.

The height of the cargo must not exceed the red cargo load line, which shows maximum allowed cargo height, so that ample free space is left above the stow to ensure proper air circulation around the load.

Stuffing chilled products

The significant difference when stuffing chilled products such as fruit and meat is that refrigerated air must be circulated through the entire load. This is because heat in the reefer container is not generated just from the outside, but may also be produced by the cargo itself. The respiration process of fruit and vegetables, for example, requires air circulation both around the commodity and through the load to remove respiratory heat, water vapour and gases such as carbon dioxide and ethylene.

Correct cargo packaging is essential in maintaining product quality during transportation and marketing. In addition to protection, packaging in the form of bins, boxes, crates, etc. serves to enclose the product and provide the means of handling.

The packaging must withstand:
  • rough handling (stuffing and unstuffing)
  • compression from the accumulated weight of stacked packages
  • impact and vibration during transport
  • high humidity during pre-cooling, transit and storage

The most commonly used types of packaging are cartons, crated boxes and bags. The material used for this packaging depends on the product, packing method, pre-cooling method, strength and buyer’s specification.

Cartons for fresh fruit and vegetables require airflow holes in the top and bottom so that when stacked they align with adjacent cartons. The number, placement, size and shape of the air holes are determined by the product being packaged. Wax-impregnated cardboard or other materials that will not lose strength in high-humidity environments are to be applied. The strength of a carton is its corners. Stacking cartons directly on top of each other is recommended to minimise crushing of the cartons below.

If loading cargo on pallets, the cartons on the pallets should be placed so that air fl ows up into the cartons unrestricted. The corners of each carton should be supported directly by the pallet, and if pallets are wrapped in plastic to provide stability, the bottom and top of the pallet/cartons must not be covered.

Stuffing frozen products

If frozen cargo is pre-cooled to the correct carrying temperature as prescribed, it is only necessary for air to circulate around the periphery of the load. A block stow, i.e. one that has no deliberate spacing between any of the packages or pallets, is all that is required. It is, of course, necessary to ensure that air can circulate under, over and to each side and end of the stow.

Related guideline:

Handling frozen meat - Risk factors and loss prevention

Temperature monitoring guideline for frozen and chilled cargo

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Reefer cargo
Why refrigerate?
We choose to refrigerate commodities such as fruits and vegetables because we want to prolong their “practical shelf life” – the time from harvest until the product loses its commercial value.

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