VIDALIA ONION RESEARCH LABORATORY

UNIVERSITY OF GEORGIA

TIFTON CAMPUS

What is Controlled Atmosphere (CA) Storage?

The best atmosphere for humans and animals is 21% O₂ (High oxygen), .03% CO₂ (Low Carbon Dioxide), 78% N₂ (High nitrogen) and other gasses 1%. The best atmosphere for fruits and vegetables is a low oxygen concentration and a high carbon dioxide concentration.

How does CA work?

In order to extend the storage and shelf life of fruits and vegetables, storage cells must maintain low oxygen content. By lowering the oxygen level in the storage cell the rate of respiration of the fruits and vegetables are reduced. The higher the respiration rate of a product the faster it will breakdown.

Lower oxygen levels reduce sprouting. A cut or injured onion will sprout faster than a solid onion with all scales intact because more oxygen is entering the onion; therefore, increasing the rate of respiration.High carbon dioxide levels inhibit fungal growth.

What are the requirements for CA?

You must have a gas-tight construction. If a room has a leak and ambient air is allowed to enter, there is no way that you will be able to establish and maintain a low oxygen atmosphere.

As the air inside the storage chamber changes temperature (temperature increases, the air expands; temperature decreases, the air contracts) there must be a relief valve to prevent an explosion or implosion.

The atmosphere must be able to be modified in order to change the oxygen and carbon dioxide levels. Also, there must be a means of controlling and changing the relative humidity in the storage cell.

The ability to raise and lower the temperature in the storage cell is also critical. Lowering the temperature reduces the rate of respiration, therefore extending the storage and shelf life of a product.

A means of monitoring the gas levels, temperature and humidity in the storage cell is necessary.

Due to the low oxygen concentration in a CA cell, you need a self-contained breathing apparatus if you enter the room during a time of a low oxygen concentration.

How is the CA generated?

Product Generation: when a product is placed in storage in an airtight container, oxygen is consumed and carbon dioxide is produced (the basic respiration process). As the product respires, the oxygen level will reduce and the carbon dioxide level will increase.

Catalytic Burner or Converter: Catalytic burners and converters are used to establish CA. This method removes the unwanted oxygen by burning it off and reduces the carbon dioxide and other gasses with scrubbers. Three things are essential for something to burn.

You must a fuel source, heat source and oxygen. As the oxygen is burned it produces carbon dioxide. One example of scrubbing excess carbon dioxide is by placing lime inside the storage cell. The lime will absorb carbon dioxide. Methods for scrubbing other unwanted gasses such as ethylene are necessary.

Liquid Nitrogen: Liquid nitrogen is brought to a vapor state and pumped into the storage rooms to flush out any unwanted gasses.

Gas Separators: Methods for removing nitrogen from ambient air such as Pressure Swing Absorption units and Hollow Fiber Membrane Separators are used. The method that we use is the Hollow Fiber Membrane Separator method. The way this method works, ambient air is compressed and pushed through the hollow fibers at high pressure. As the air passes through the fibers oxygen, carbon dioxide and various other gases are removed and stored in an oxygen enriched air tank. The end product is nitrogen and it is stored in a separate nitrogen tank. The purity of the nitrogen is dependent upon the speed that the air is allowed to pass through the membranes. The faster the air travels the lower the purity of the nitrogen. A purity of better than 99% is possible.

Why was CA needed?

CA was needed in order to extend the marketing window of the Vidalia Onion season. The harvest season window is a 6 weeks period. All onions would have to be harvested and marketed during this window. With the addition of CA, onions could be marketed for several months after this period. Since the addition of CA, production has increased to approximately 5 million bushels (50 pound bags) and facilities for storing 2.4 million bushels in CA. Since the introduction of CA, the industry has experienced a 4 fold per year growth in production and money. At the start of the Vidalia Onion season, the price for a bushel of onions ranges from $14 to $16. After a couple of weeks into the harvest season, the price drops to $8 to $10 per bushel. Onions that are stored in CA upon removal and sold bring a premium price of $22 to $28 per bushel. A pack out of 85% is good. Some growers have a higher pack-out while other growers have a lower pack-out. Operating cost for storing onions in CA is $0.10 per bushel per month.

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Last modified: February 13, 2002