Supply Chain

Container Atmosphere Control Options

By Jim Taeckens, senior product manager, Carriers Transicold

Jim Taeckens, the senior product manager at Carrier Transicold, Container Products Group, talks about some of the options available to shippers with regard to container atmosphere control technology.

Q. Why regulate container atmosphere?

Taeckens: By using container atmosphere control technologies, shippers can control the atmosphere within refrigerated containers, permitting fresh fruits and vegetables to arrive at their destinations in optimum condition as never before possible.
Refrigeration plays an important role in controlling on-board ripening. The technique of adjusting the container’s atmosphere can add days or even weeks to the potential storage and transit time, allowing produce to travel longer distances, while bringing greater variety to consumers around the world.

Q. How does the technology work?

Taeckens: As produce shippers know, atmospheric composition is important to fruits and vegetables, because they “breathe” or respire as they mature, consuming oxygen (O2) and producing carbon dioxide (CO2) and a hormone called ethylene. At a certain point in its maturity, the fruit undergoes an increase in respiration, after which it goes through a stage called senescence, softening very rapidly as it ripens (Fig. A).
In the enclosed space of an unventilated container, fruits with rapid respiration rates can quickly change the atmosphere, reversing the normal levels of O2 and CO2 in only a day or so. That could be disastrous. That’s where atmosphere control comes in.
Properly managing the O2 and CO2 concentrations through atmospheric control can actually retard produce respiration and delay ripening beyond what refrigeration alone can do. In some cases, adjusting the balance of O2 and CO2 can even be used to thwart the effects of post harvest pathogens, decay, and insect infestation.

Q. Are there different ways to regulate the air inside a container?

Taeckens: Yes. The term “controlled atmosphere” is often erroneously used interchangeably to describe three different techniques — Controlled-Atmosphere, Modified-Atmosphere, and Fresh-Air Exchange (also called Fresh-Air Management). Understanding the differences and limitations can help shippers make the best decisions for their applications.

Q. What are the differences between the three techniques?

Taeckens: The Controlled-Atmosphere technique is built into the refrigeration unit. A Controlled-Atmosphere system draws outside air and also uses nitrogen (N2) taken from it to fine-tune the balance of O2 and CO2 inside a container. The Controlled-Atmosphere system is the only one with the ability to control and maintain specific O2 and CO2 levels over a wide spectrum of potential settings. Ethylene absorbers can be used with extremely sensitive cargoes. If needed, supplemental CO2 can be added.
The Fresh-Air Exchange technique invokes mechanical ventilation of the container to provide some level of control over CO2 build-up, but to a lesser degree than Controlled-Atmosphere. This is an energy-efficient alternative to the common practice of continuously ventilating refrigerated containers to address cargo respiration, and it can be useful for high respiring cargoes with shipping durations of up to three to four weeks.

With the Modified-Atmosphere technique, the container is injected, prior to shipping, with a specific composition of gases appropriate to the perishables. Then the container is sealed-off for shipping. During transit, the environment will change within the container due to off-gassing of the produce. To make a container suitable for Modified-Atmosphere use, it needs to be retrofitted with a “purge port” assembly. Additionally, the container must be very well sealed.

While all three techniques aim to adjust the atmosphere, only true Controlled-Atmosphere systems provide real-time control capability covering the full range of possibilities.

Q. How do you select the most appropriate technology?

Taeckens: It all depends on what is being transported, and how many varieties will be shipped in the same fleet. Shipping lines that want to maintain the greatest flexibility will choose solutions that can accommodate the widest variety of produce.
Modified-Atmosphere systems using gas injection have their place in applications where the transit time is relatively brief, and products are not high-respiring. High value and extremely perishable produce, such as white peaches, can benefit from the ability to achieve an atmosphere immediately. The problem with a gas-injected atmosphere is that it can’t maintain a desired balance over a lengthy journey, and while the initial investment for Modified-Atmosphere is the lowest, there is a built-in costly expense for gas injection with every shipment.

Deciding between the two mechanical methods, Fresh-Air Exchange and Controlled-Atmosphere, becomes a little more complex and requires analysis of the tolerances of the produce being shipped.

Q. In practicality, how wide of a range of atmosphere control does a shipper need?

Taeckens: It all depends. Shippers who deliver over the longest routes, or who need versatility to handle the widest variety of produce, will always find Controlled-Atmosphere systems to be the most accommodating.
If a shipper only handles a specific crop that can be accommodated by Modified-Atmosphere or Fresh-Air Exchange, then those methods may provide satisfactory results, although, as with any major capital purchase, a careful long-term cost/benefit analysis should always be done in advance.