Controlled atmospheres are used increasingly to supplement good refrigerated storage practices. In a controlled-atmosphere storage, a beneficial combination of reduced oxygen and increased carbon dioxide in the atmosphere is used with refrigeration. Refrigerated storage of apples at temperatures of 31° to 32°F effectively slows the rate of ripening, but it is harmful to certain varieties because it is too cold for their normal metabolism. For example McIntosh, Twenty Ounce Pippin, and Northwestern Greening are subject to brown core, Yellow Newtown is subject to internal browning, and Jonathan, Grimes Golden, and certain other varieties are subject to soft scald and soggy breakdown if stored at 31° to 32° for long periods. These diseases can be prevented by storing at 36° to 40°, but these higher temperatures shorten both the period apples can be stored and their shelf life after removal from storage.
Properly maintained controlled-atmosphere (CA) storage at about 38 °F controls these low-temperature disorders and also slows the rate of ripening so that apples can be stored longer and remain firmer than similar varieties stored at 38° or even at 32°. By retarding the ripening, controlled-atmosphere storage also controls Jonathan spot, a senescent disease, on susceptible varieties and prolongs the life of the apples during retailing. Controlled atmosphere storage at 31° to 32° further prolongs the life of most varieties of apples, and its use is on the increase for varieties that can withstand storage at those temperatures.
Controlled-atmosphere storage can be obtained by placing apples in a refrigerated, gas-tight room and allowing apple respiration to reduce the oxygen in the air and build up the carbon dioxide to the desired levels. Additional oxygen then can be added as needed by daily ventilation, and excessive carbon dioxide can be absorbed in various types of "scrubbers."
A more modern method is to control the atmosphere with a generator located outside the storage room. Such a controlled atmosphere generator creates an atmosphere with the desired oxygen and carbon dioxide levels and flushes it through the refrigerated storage.
Since about 1942 about 50 percent of the McIntosh apples grown in the Northeast have been stored in controlled atmosphere. Controlled-atmosphere storage is especially important to McIntosh apples because they soften faster than most other long-term stored apples, and they cannot be successfully stored at 31° to 32 °F because they are very susceptible to brown core. For successful controlled-atmosphere storage, the gases must be carefully controlled. For McIntosh in New York, for example, the oxygen should be maintained at 3 percent and should never be allowed to drop below 2.5 percent. Carbon dioxide should be maintained at 2 to 3 percent for the first month and then maintained at 5 percent.
The success of controlled-atmosphere storage in the Northeast has created interest in other apple-growing regions and with other varieties. As research continues on the subject, it is safest to assume that each apple variety has its own optimum requirements for controlled-atmosphere storage. The recommendations of the State agricultural experiment station should be followed for each variety.
If the oxygen and carbon dioxide levels are not carefully controlled, injury to the fruit can occur. Injury may be due to too-low oxygen, too-high carbon dioxide, or a combination of both.
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