Modified Atmosphere and Controlled Atmosphere (MA/CA) :
Retarding the respiration rate of fruit and vegetables is an effective means of prolonging shelf life. This can be achieved within a container by modifying the composition of the refrigerated air surrounding the cargo. Specialist containers that can control the critical mix of nitrogen, carbon dioxide and oxygen in the atmosphere are available.
Modified Atmosphere :
A storage atmosphere that is different from that of ambient air, created at the beginning of the storage period, and expected to be maintained without further measurement or active control.
One-shot gas injection: This procedure involves the filling of the container with a pre-determined mixture of gases at the loading point. There is a certain degree of control over oxygen level by using the respiration of the fruit, which reduces oxygen level, and a controlled fresh air inlet port to increase the level. Carbon dioxide can be absorbed by the inclusion of lime, and ethylene by potassium permanganate or similar absorbents.
Membrane systems: They work by passing compressed fresh air over a membrane in which the fast flowing gases such as oxygen, water vapour and carbon dioxide permeate the walls of the membrane. Slow flowing gases such as nitrogen and ethylene pass straight through. In practice this means the container's atmosphere is purged or replaced by clean, dry nitrogen. This purging action can be beneficial in flushing out unwanted gases such as ethylene and carbon dioxide. However, a large number of MA cargos require raised levels of carbon dioxide which cannot be achieved as a result of this action. Because of the dryness of the nitrogen and the purging action humidity control is not possible. Bearing this in mind, it could be argued that these systems produce a nitrogen blanket rather than a truly exclusive container air environment.
Controlled Atmosphere :
A storage atmosphere containing lower concentrations of oxygen and/or higher concentrations of carbon dioxide than ambient air that is regularly measured and its composition maintained during the storage period.
Pressure swing adsorption systems: Use the selective absorption characteristics of certain minerals under pressure. By using more than one absorbent they can not only separate oxygen and nitrogen but also carbon dioxide, as required, and ethylene continuously. Instead of purging it, the gas within the container envelope is processed, thus humidity control and raised levels of carbon dioxide are possible giving a fully controlled atmosphere.
Systems tend to be more complicated and contain more components than membranes however the absorbents have to all intents an infinite life.
There is a small selection of equipment currently available offering a variety of performance and operational features, for both CA and MA. For more information as to whether CA/MA is suitable for your cargoes please contact your local shipping line representative.
Packaging is a fundamental element in the transport of temperature controlled cargos. It is essential to protect cargos from damage and contamination. The correct design and highest quality of materials need to be used to ensure it can withstand the refrigeration process and transit.
Where appropriate, packaging materials must be able to:
- Protect products from damage as a result of 'crushing'.
- Be able to withstand 'shocks' occurring in inter modal transport.
- Be shaped to fit on pallets or directly into the container for stowage.
- Prevent dehydration or reduce the water vapour transmission rate.
- Act as an oxygen barrier preventing oxidation.
- Withstand condensation and maintain its wet strength.
- Prevent odour transfer.
- Withstand temperatures of -30 degree C or colder.
The wide variety of cargos, coupled with the design and quality requirements of packaging materials, listed above, are the reasons for many different packaging types and styles.
Perishable fruits and vegetables require packaging that allows refrigerated air to circulate around the products to remove the gases and water vapour produced by their respiration.
Often cargos are carried in cartons. These cartons must be capable of being stacked to the maximum height allowed in the container of approximately 2.5 mtrs (9 feet) in a hi-cube integral container, higher than most land based palletized operations. Many packaging types used for other forms of transport, for example for road haulage, may be inadequate for sea transport. Shippers are advised to thoroughly test their packaging to ensure it is suitable for transit by sea in a refrigerated container