Storage Factors Influencing the Quality of Apples
Early investigations into CA storage of McIntosh, Delicious, and Golden Delicious apples concluded that prompt cooling was more important than rapid establishment of the CA atmosphere (2-3% oxygen and 2-5% carbon dioxide) for maintaining high fruit quality. Consequently, the conventional CA practice in British Columbia (before 1978) involved an 8-10 day loading period to facilitate rapid and thorough cooling, and atmospheres containing 5% or less oxygen were obtained by fruit respiration within 20 days from the time of sealing. This "slow CA" storage procedure had not, however, been consistently effective in maintaining a desirable level of flesh firmness, particularly in Golden Delicious and McIntosh apples. This deterioration in condition was accentuated by extended storage, and it was the main impediment to the successful storage of these cultivars much longer than February or March. Apples with mealy texture are not only undesirable from the consumer's viewpoint; they are also highly susceptible to breakdown and bruising during storage, packing, and distribution.
Storage quality of CA-apples in a standard 2.5% oxygen atmosphere can be markedly improved by shortening the time between harvest and attainment of the desired storage temperature and atmosphere (Rapid CA, Tables 1-3). Reducing loading time (2-3 days) and oxygen reduction time (2-3 days) to a total of 6 days or less yields best storage results. It is not enough to reach desired storage conditions eventually. Rapidity of achieving desired storage conditions is of considerable importance.
The "soft" apple cultivars such as Golden Delicious and McIntosh respond more to the rapid CA procedure than do the "hard" apple cultivars such as Delicious and Spartan (Table 3). Rapid CA is effective in maintaining flesh firmness of apples picked over a wide range of maturities. Also, it is as effective as the prestorage high carbon dioxide treatment and superior to the postharvest calcium chloride treatment in controlling firmness loss in Golden Delicious apples without causing any fruit injury (data not shown).
Lower Oxygen Storage
Golden Delicious, Delicious, and Spartan apples from British Columbia can be safely stored in low-oxygen atmospheres (1.0% to 1.5% oxygen), with a higher poststorage firmness and acid level than apples stored in the standard commercial atmosphere of 2.5% oxygen (Tables 4 and 5). Apples stored in 1.0% to 2.5% oxygen with a rapid CA storage procedure are always firmer than those stored in a similar atmosphere with a slow CA storage procedure (Table 4).
While Golden Delicious responds more to the rapid CA storage procedure than do either Delicious or Spartan in 2.5% oxygen storage (Tables 1 to 3), it responds less to the 1.0% oxygen atmosphere established by rapid CA than Delicious or Spartan (Tables 4 and 5), indicating that there is an enormous variation in the responses of apple cultivars to rapid CA and low-oxygen CA.
A Word of Caution
The drive to retain fruit quality longer in CA storage has led to a delicate balancing of cultural (e.g., tree nutrition, growth regulator), physiological (e.g., maturity, ethylene, watercore), storage (temperatures and atmospheres), and climatic factors, in which the margin for error has progressively narrowed. Although a 1.0% oxygen atmosphere has yielded the best storage results, an oxygen level of less than 1.5% is not to be recommended at this time. More work is needed to identify the factors that may render the fruit more prone to low-oxygen injury. An effective quality control program is also needed to exclude susceptible fruit lots from low-oxygen CA storage.
Temperature & Oxygen Level are Related
Storage of apples over an extended period (e.g., beyond May) is critically dependent upon correct storage temperatures, storage atmospheres, and fruit maturity. For example, McIntosh apples stored in 1.0% oxygen at 35°F [1.7°C] (a standard storage temperature employed in B.C. for 2.5% oxygen storage) and below, develops a new type of "corky" flesh-browning disorder (Table 6). The occurrence of this disorder varies greatly with fruit lots and seasons (data not shown), and appears to be higher in fruit of a more advanced stage of maturity (Figure 1, fruit with little starch, high internal ethylene, and low firmness). This disorder can be reduced greatly, however, by storing the fruit at a warmer temperature (e.g., 37.4°F (3.0°C), Figure 1, Table 6). Results suggest that apples stored at low levels of oxygen are more sensitive to low-temperature/low-oxygen disorders than those stored at a higher level of oxygen. Consequently, a warmer storage temperature should be used for storing apples in low-oxygen CA.
Likewise, development of flesh-browning in Delicious apples is a low-temperature disorder and is related to advanced fruit maturity (watercore), low storage temperature (e.g., 31.1°F [-0.5°C]), and the level of carbon dioxide (e.g., 2.0% or above, Table 7). The incidence of this disorder is low in fruit stored at a warmer temperature (e.g., 35.6°F [2°C]) and a lower level of carbon dioxide (e.g., 1%, Table 7).
In conclusion, low-oxygen CA is not a substitute for good cultural practice, proper handling procedure, and correct storage condition. A properly managed fruit selection program to eliminate susceptible and overmatured fruit lots, an optimum storage temperature, and a correctly maintained storage atmosphere are all needed to minimize fruit injury in apples kept in low-oxygen CA storage.
Dr. O. L. (Sam) Lau
B.C. Tree Fruits Ltd., Kelowna, B.C. Canada
Post Harvest Pomology Newsletter, Vol. 3, No. 1