A significant complicating factor that comes into play with perishable foods is temperature moderation.
Temperature is essentially the most important factor in preserving the freshness and quality of perishable commodities. This is because the rate of respiration is directly proportionate to the rate of deterioration, which typically increases exponentially as temperatures increase.
As a rule of thumb, every 10°C (18°F) increase in temperature translates to the rate of deterioration increasing two to three times over.
Read our article on “Offshore Reefer Container Temperature Range.”
One way to ensure stable temperature moderation during perishable good transportation is by using an offshore reefer container or offshore refrigerated container.
These unique containers are designed to maintain a stable temperature inside them while controlling humidity and promoting adequate airflow.
Ventilation in reefer containers
Ventilation essentially refers to the replacing of air inside a container with air from the outside surrounding environment. For the most part, the fresh air intake capacity of most reefer containers varies between 0 and 180 m3/hour (50 Hz).
Generally, fresh air ventilation helps to remove accumulated respiration gasses, like carbon monoxide and ethylene, emitted by the cargo as a result of the ripening process of perishables.
However, each cargo maintains its own unique respiration characteristics to accommodate different perishables.
Fruit, for example, can be divided into climacteric and non-climacteric. Climacteric fruits continue to ripen after harvest, while non-climacteric fruits stop ripening after harvest.
As a result, climacteric fruits have a high level of respiration, for example, pears, mangoes, avocados and bananas. Non-climacteric fruits include citrus, grapes, cherry and watermelon.
Fresh air affects the efficiency of refrigeration installation as the evaporator cooling coil is designed to eliminate humidity from the air inside the container.
However, fresh air also means that the evaporator cooling coil must also remove humidity for freshly introduced air.
At temperatures below 10°C, this results in more frequent defrosting periods and frequent interruption of the circulation fans. In turn, less fresh air intake shows that:
- the evaporator cooling coils require to be defrosted less often
- circulation is interrupted less frequently
Therefore, it is safe to say that the respiration gasses emitted by non-climacteric fruit are much lower than climacteric fruit. As such, the need for ventilation of non-climacteric fruit becomes far less.
What is ‘return air’ in reefer containers?
Return air is the temperature of air that has been circulated through the reefer. In low-temperature mode, the majority of common refrigeration units control ‘return temperature’ using a two-point switching method.
So, as long as the ‘return air’ temperature is above the nominal value, the refrigeration unit will cool at full power.
Consequently, once the nominal temperature is reached, the refrigeration unit switches itself off. It leaves only the fans responsible for circulating the air in the reefer container running.
When the ‘return air’ temperature rises to approximately 0.5°C above the nominal value, the refrigeration unit starts up again, and the entire process is repeated.
Do note that the supply air temperature must always be lower than the nominal temperature.
Reefer container ventilation settings
The 8ft & 10ft offshore reefer container by MGS maintains a refrigerated container temperature setting that ranges from -30c to +20c with a humidity controller to meet the requirements of different perishables.
Why is ventilation important in transporting perishable cargo?
Ventilation helps deliver fresh air to perishable cargo that is being transported. It is also the simplest and most cost-effective way to help prevent condensation in your cargo.
However, the amount of fresh air required typically depends on the tolerance of the product to low levels of oxygen and high levels of carbon dioxide and ethylene. Additionally, it also depends on the rate at which the product respires and produces ethylene.
With the ability to facilitate fresh air exchange, ventilation also helps remove unwanted heat, ethylene, carbon dioxide and most gases produced by the cargo.
This, in turn, helps prevent unwanted ripening and accumulation of odours, ensuring longer shelf life for many perishables.
In summary, reefer containers are designed to automatically control refrigeration using the supply air sensor to detect and critically manage temperatures within a 0.5°C (0.9°F) range, or better, under most conditions.
Remember, perishable goods need precise temperature control to maintain their quality all
the way to their final destination. As such, approach reefer unit ventilation settings must be input according to the cargo requirements.
Overall, fresh air ventilation is especially useful for products that produce high levels of ethylene, such as tomatoes and apples. However, excessive ventilation can result in the freezing of the evaporator coils and will necessitate additional defrosting to remove ice build-up.
For more information on reefer containers, their types and efficiency, check out our blog on “What are offshore reefer containers?”
A graduate (Business) from KDU, Jason Tan, is the current Business Director (Sales) for MGS Icestorm and has been associated with the company for the past 10 years.
With over 13 years in the shipping industry, he has had a significant contribution to Malaysia’s oil and gas industry in the engine and boat supply sector.
His expertise includes managing offshore catering business, offshore reefer containers, AI technology, offshore gas tanks, A60 pressurized cabins, etc. His contributions have helped establish MGS, in partnership (joint venture) with Thermo King and Honeywell to produce state of art Offshore Reefer Container products.