Controlled Atmosphere Storage of Apples and Pears

As we move into the new year, most all of the apples and pears being shipped will come out of Controlled Atmosphere storage (CA storage). Prior to January regular refrigerated storage is used for much of the fruit marketed in the Fall and early Winter months. When the proper growing and harvesting techniques are used, many varieties of apples can store for 12 months or longer in CA. Most of these apples are shipped to market between January and September.

CA storage as we know it got its start in England before World War II when farmers discovered their produce kept longer if stored in an airtight room. Commercial CA storage of apples in the United States had its early start in New York State and almost entirely in the Hudson Valley with the McIntosh variety. The first three CA rooms were put into operation in 1940. By late 1940, the Hudson Valley area had a 24,000 bushel CA capacity, and by 1949 this had increased to a 100,000 bushel capacity.

Buying crisp, sweet fruit year-round is possible due to controlled atmosphere storage. CA storage involves careful control of temperature, gases (oxygen, carbon dioxide and nitrogen) and humidity; all kept at specified levels. Apples take in oxygen and give off carbon dioxide as starches in the flesh change to sugar. In sealed CA rooms, this respiratory process reduces the oxygen level to slow the ripening process.

CA storage is a non-chemical process. Oxygen levels in the sealed rooms are reduced, usually by the infusion of nitrogen gas. Oxygen levels drop from approximately 21 percent in the air we breathe to 1 or 2 percent. Temperatures are kept at a constant 32 to 36 degrees Fahrenheit. Humidity is maintained at 95 percent and carbon dioxide levels are also controlled. Exact conditions in the rooms are set according to the fruit variety. Researchers develop specific regimens for each variety to achieve the best quality. Computers help keep conditions constant.

Timing of the harvest is critical for good storage results. Fruit picked too early will not store well in CA nor will those that are picked past their proper maturity. In mid-August, apple growers start testing the maturity of their apples to accurately predict when to harvest their crop to put in CA rooms so the apples are mature, but not too ripe. Pear growers soon follow with similar tests. Firmness, skin color, seed color, sugar level and flesh chlorophyll are all tested.

Most of the storage apples and pears come out of Washington state. Washington law places requirements on the length of time fruit must remain in CA conditions to qualify as CA-certified. Then state inspectors check every lot of fruit as the lot comes off the packing line to make sure the apples meet maturity requirements, the same requirements the U.S. Department of Agriculture uses for apples being exported. Only then will the carton be stamped with the warehouse number and the "CA" symbol.

Fruit meeting these standards must be shipped within two weeks or be re-inspected to meet the same requirements. If they don’t pass, the CA designation cannot be stamped on the carton. These standards apply to all apples shipped under Washington Extra Fancy and Washington Fancy grades and all US No.1 grade pears.
Washington has the highest concentration of CA storage of any growing region in the world.