The Many Faces of CA Storage
Controlled atmosphere (CA) storage has long been established in most of the world's apple growing areas as a means to lengthen the marketing period of apples. Until recently, CA facilities and strategies could usually be expected to provide apples in reasonably good condition for six to eight months after harvest.
The increases in apple production in most areas, but most notably in Washington and in France, have created the need for lengthening the apple marketing period to 12 months. Recently, several new CA strategies have been developed to assure good fruit condition late in the prolonged marketing season. Some of these strategies may also be useful for the six to nine months storage period.
A treatment of 15-17% carbon dioxide lasts four to five days. Compressed carbon dioxide is introduced into the room because the moisture introduced by oxygen burners may condense on apples, causing carbon dioxide burn of the apple skin. After the treatment period the room is flushed with air, and then the normal CA atmosphere is established. The commercial use appears to be limited to Golden Delicious grown in Washington, Virginia, and Australia. Elsewhere, including New York, and with other varieties, this treatment causes unacceptable amounts of carbon dioxide injury.
Rapid CA is a strategy developed for Golden Delicious in British Columbia where prestorage carbon dioxide treatment causes unacceptable amounts of carbon dioxide injury. With this strategy the delay between harvest and establishment of the final oxygen concentration in CA is held to a minimum. In commercial practice this means filling the CA room in three or four days, then reducing the oxygen immediately with a catalytic oxygen burner or nitrogen gas flush. Rapid CA is also being used commercially for Washington Delicious.
Research in British Columbia shows that McIntosh response to Rapid CA is not as dramatic as the response of Golden Delicious. In the Northeast we have seen apples ruined by slow cooling which resulted from loading CA rooms too rapidly.
Zero Initial O2
"Zero" Initial Oxygen has been used successfully under commercial conditions for Jonathan in Australia. In this strategy there is a minimum delay (seven to ten days) between harvest and the establishment of nil oxygen (less than 0.5%) with less than 1% carbon dioxide and 33°F. core temperature. To attain these conditions 5 pounds of lime per bin are placed into the CA room, which must be very tight. The oxygen is burned to 2%, and the apples then take about four days to respire the oxygen down to less than 0.5%. Ten days after the beginning of the nil oxygen period the oxygen is slowly brought up to the concentration used for the remainder of the storage period. The strategy has increased the storage life and greatly reduced storage scald and breakdown.
Ultra Low O2 CA
Ultra-low Oxygen CA was developed in England, where it has been used in a number of seasons. Storage operators use up to three different CA regimes for this variety: 3% oxygen with 5% carbon dioxide for rooms opened until mid-February; 2% oxygen with less than 1% carbon diox.ide for rooms opened from mid-February through March; and Ultra-low Oxygen (1.25% oxygen with less than 1% carbon dioxide) for apples sold after April first. The oxygen concentrations in the Ultra-low Oxygen rooms are analyzed and adjusted with an electronic or a computerized oxygen controller. This equipment prevents the occurrence of dangerously low concentrations of oxygen (below 1%).
If the oxygen concentration goes too low in a CA room, the apples will turn brown and develop serious "off-flavors". These apples cannot even be sold for juice--they must be dumped. These observations have made us very hesitant to suggest CA operators use oxygen concentrations below 2.5-3%.
Low O2 Damage
In our research program we make use of what we and others have learned about low oxygen damage to apples:
Low oxygen damage is aggravated by low storage temperature.
For all varieties the carbon dioxide should be kept below 1% if the oxygen is held below 2%.
Downward fluctuations in the oxygen concentrations must be avoided. We very strongly feel that oxygen controllers should be used when operating at below 2% oxygen.
Best responses to low oxygen are achieved with early picked apples.
Response often requires the use of Rapid CA (outlined above).
Low oxygen damage varies with the variety and the orchard block.
Ethylene scrubbers for CA rooms are the most recent and the least commercially developed of the new innovative strategies. We are interested in ethylene scrubbing as a safe method to improve the condition of McIntosh and Empire apples. The Europeans are interested in ethylene scrubbing as a nonchemical method to control storage scald. They fear that the use of all chemical storage scald inhibitors will soon be banned in many EEC countries.
The commercial development of ethylene scrubbing in CA rooms will probably progress rapidly when ethylene scrubbers become available on the market. Only one company (Italian) has produced an ethylene scrubber that has held the ethylene below 1.0 ppm for the entire storage period in a commercial size (25,000 bushels) CA room.
New York Strategies
At this time we cannot know which strategies are best suited for our apple industry. The research program at Ithaca (funded by New York and New England Apple Institute and Western New York Apple Growers Association) has concentrated on low oxygen and ethylene scrubbing for CA. We will incorporate Rapid CA into the program this next season. Evidence appears to be accumulating that some form of Rapid CA with either low oxygen and/or ethylene scrubbing will add substantially to the condition of our apples. The immediate implication to those planning construction of CA is the likelihood of the need for smaller CA rooms to permit a minimum delay between harvest and the establishment of the CA atmosphere. The longer range implication is that fruit condition will be an even more important factor in the marketplace.
G. D. Blanpied, Department of Pomology
Cornell University, Ithaca, NY 14853
Post Harvest Pomology Newsletter 1(4): 3-5