Maintain crunch to maintain profitsHere's the latest information for growers and packing house managers on how to store apples.
By Dr. Eugene Kupferman Maintaining "crunch" is critically important to economic returns, regardless of the apple variety produced. Deciding when to harvest is the single most important decision the grower makes that affects crunch. For fruit going into storage, delaying harvest to obtain color has proven to be a poor strategy. To have crisp apples after storage, one must allow for the inevitable decline in firmness that occurs in storage, packing, and marketing. A combination of maturity indices gives a better indication of appropriate maturity than just using one indicator. However, the storage of apples in which the starch has cleared is ill advised, regardless of skin color or the needs of the marketing department. Apples free of starch need to be marketed immediately after harvest. When considering controlled atmosphere (CA) storage, I divide the varieties into two types: those tolerant of high carbon dioxide and those that are not. GALAS, GOLDENS Gala and Golden Delicious are tolerant of moderate levels of carbon dioxide and benefit from very rapid reduction of oxygen in the atmosphere. However, warm fruit should be cooled to at least 60°F prior to lowering the oxygen. Gala and Golden Delicious have been commercially stored in 1.0% oxygen with carbon dioxide levels of up to 2.5% at 34°F. If the temperature is lower, the oxygen should be raised. The temperature for regular storage should be 32°F. FUJIS, BRAEBURNS, GRANNY SMITHS, ROMES Fuji, Braeburn, Granny Smith, and Rome are generally intolerant of elevated carbon dioxide since the cells are dense, and air circulation around the cells within the fruit is difficult. It is important that the flesh temperature is close to the storage temperature before the oxygen is reduced. These varieties have a tendency to develop internal browning, depending on their natural predisposition (and preharvest factors) and the storage regime. A delay of 10 to 20 days in regular storage (at 32°F) prior to the imposition of low oxygen has been shown to reduce the risk of internal browning. Scientists do not yet know the minimum holding period for long-term CA storage of these apples. Since there is a long holding requirement, fruit should be picked less mature, or the length of storage shortened, to minimize firmness and acid loss. Experiments, and an increasing amount of industry experience with these susceptible cultivars, is showing that they develop less internal browning when the fruit has been treated with diphenylamine (DPA) prior to storage. It is very important that carbon dioxide remains well below the oxygen level at all times. Temperatures should be held slightly elevated during CA storage. For example, fruit stored at 1.5% oxygen should be stored with the carbon dioxide level below 0.5% at 34°F. Air circulation around individual fruit is critical to allow the carbon dioxide to dissipate. Thus, it is not advisable to store waxed fruit, or fruit in boxes with polyliners in CA. Storage can be at 32°F for regular atmosphere or 34°F for CA. Most operators are not using very low levels of oxygen with these varieties. Some are storing fruit at 3 to 4% oxygen with carbon dioxide held below 1%, but this is not optimum for long-term storage. RED DELICIOUS Red Delicious is somewhat tolerant of higher carbon dioxide levels and rapid CA. However, producers have not seen the dramatic positive effects of very rapid CA on Red Delicious that have been seen on Golden Delicious or Gala. Fruit softens more rapidly after harvest than on the tree, so temperature reduction and CA should not be delayed. Regular storage is usually at 32°F or slightly below. Typical regimes for CA of nonwatercored Red Delicious are 1.5% oxygen and up to 2.0% carbon dioxide at 32 to 34°F. Ultralow-oxygen CA storage can minimize storage scald. I have held numerous lots of early season, commercially harvested Red Delicious at 0.7% oxygen with a much greater reduction in scald appearance and intensity than in either regular and CA storage. This low level of oxygen should not be used on heavily watercored apples. To conserve the quality of late harvested Red Delicious, I placed them into CA shock treatments (either very low oxygen or high carbon dioxide) for 14 days prior to holding them in regular storage for 90 days. This didn't work as well as storage in CA for the entire period. Fruit treated with ReTain (aminoethoxyvinylglycine) or DPA were superior to nontreated fruit harvested at the same time, even after storage. WATERCORE Watercored fruit provides a special challenge to storage operators. In an experiment in 1998, I categorized Red Delicious apples nondestructively by the amount of watercore using a hydro sorter developed for this purpose by Dr. Ralph Cavalieri with Washington State University's Department of Biosystems Engineering. After sorting, I placed samples from each watercore category into short-term CA for 90 days. There was no internal browning in fruit without watercore, while most of the fruit with severe watercore developed internal browning. Some of the fruit in the moderate category developed browning and some did not. It was not possible to anticipate which fruit with moderate watercore would develop internal browning prior to storage. Some believe that by slowing the cooling and/or establishment of CA, one can allow the fruit to respire away the watercore. The risk here is that fruit with watercore is already soft, and by delaying CA, the fruit will soften further, shortening storage life or risking not being allowed to ship it at all. Maturity management must come first. Not all fruit is suitable for CA. If an orchard has developed watercore to a moderate extent, the risk of storage injury rises. Storage operators may be willing to take the risk. I doubt that it results in quality fruit for the consumer. Fuji also develops watercore. It is a positive attribute of the variety and was selected by the Japanese for this characteristic. However, the reaction of Fuji fruit to watercore appears to be different than that of Red Delicious. Trials and industry experience have shown that Fuji with slight or moderate watercore can be stored. However, severely watercored fruit will deteriorate in CA storage if stored too long. The fact that Fujis should be cooled prior to the establishment of CA may contribute to the disappearance of watercore after storage, and the very firm texture of Fuji at harvest makes it less hazardous to softening. DRENCHING Drenching is a subject that also comes up at this time of year. Like everything else, drenching involves risks. When drenching, it is possible to spread spores from infected fruit. Thus, while applying diphenylamine (DPA) to reduce the risk of scald, the storage operator is increasing the risk of decay. The number of available fungicides is extremely limited. Thiabendazole (TBZ), the most widely used fungicide, is not as effective as Benlate (benomyl) once was. In one test in 1998, as many at 50% of the spores were not susceptible to control by TBZ. HOW CAN THE RISK OF DECAY BE MINIMIZED?
WHICH VARIETIES SHOULD BE DRENCHED? Many storage operators stopped drenching Golden Delicious when they saw burn on fruit in rapid CA. It is not a good idea to place wet fruit into rapid CA. In most blocks, there has not been an increase in decay after storage without drenching during years of dry harvest weather. Thus, most Goldens are not drenched. Granny Smith is highly susceptible to scald and should be drenched, regardless of whether the fruit is stored in CA or regular storage. Most managers use 2,000 parts per million of DPA with TBZ. When Fuji apples were drenched, the incidence of decay was greater than when they were not. Omitting the drench reduced decay. Fujis are particularly susceptible to decay due to their thin skins, knobby stems, and very high soluble solids. However, the finding that DPA can reduce internal browning has prompted a revisiting of this decision. If an operator chooses to drench Fujis, then the fruit should have the stems clipped to reduce stem punctures, pickers should be closely supervised to prevent bruising, and the drench solution should be changed frequently to minimize the risk of decay. Braeburn should be considered for drenching for the same reason, if the solution is carefully monitored. Storage operators have drenched Fuji or Braeburn with DPA at 2,000 ppm and have not seen damage. TBZ should always be included in any drench. A warning under the legal constraints of the label: DPA can only be applied to protect the fruit from scald. Red Delicious placed into CA should be drenched. However, for organic apples, a low oxygen (0.7%) storage atmosphere is used to reduce the risk of scald. CALCIUM Calcium in the drench solution can help reduce nutrient-related disorders, such as bitter pit. However, the most effective place to apply calcium is in the orchard. Postharvest drenching can do very little to reduce disorders, and disposal of chloride under Washington State Department of Ecology regulations is a serious problem for many packers. SUMMARY In summary, these are my observations and thoughts on the storage regimes for Washington-grown apples and pears, which I have gathered from published research, scientific meetings, recommendations from other fruit-growing regions, and my personal experience with the Washington tree fruit industry. Some of these observations may be incorrect, or new information may come to light to change my thinking. I am interested in your observations. Do not hesitate to contact me. My e-mail address is <kupfer@wsu.edu>. Further information can be found on the new postharvest Web site at www.postharvest.tfrec.wsu.edu>.
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