Observations on Storage Regimes for Apples and Pears, 1997
There has been much discussion lately about the current state of knowledge about desirable Controlled Atmosphere (CA) storage regimes for Washington grown apples and pears. Unfortunately there is no working group which meets to formulate recommendations for CA storage. The following are my observations gathered from published research, formal and informal scientific meetings, recommendations from other fruit growing regions, and my personal experience with the Washington industry. They are my best "educated" guess as to how to store Washington grown apples and pears.
Anjou pears developed skin marking on handwrapped fruit stored in long-term CA in cartons in a number of sheds in 1996-1997. There has been some evidence that shorter-term storage, or storage in bins or on trays, does not induce marking. There is some suspicion that the speed with which the CA regime is imposed may affect fruit marking, and by slowing the rate of CA establishment marking may be reduced. This harkens back to the "original" recommendations for CA which stated that the fruit must be at storage temperature before the oxygen is lowered.
A large scale experiment is being done this year with packed Anjou pears at the Washington Tree Fruit Research Commission's (WTFRC) storage facility to evaluate different regimes for packed Anjou pears.
Anjou pears should be stored with the oxygen at least 1% higher than the carbon dioxide at all times, especially when the fruit are held at 31.5° or 32°F. There has been some research that fruit stored at higher temperatures (34°F) can tolerate slightly higher CO2 levels.
When thinking about CA for apples, I divide the varieties into two types-those which are tolerant of high CO2 and those which are not. Obviously this is a broad generalization.
a) Gala and Golden Delicious are the varieties which I place in the CO2 tolerant category. These varieties also benefit from rapid reduction of atmosphere. Thus, fruit with moderate temperatures can be placed into a low oxygen environment rapidly without problems.
How rapid is rapid? This question has taken on new meaning as currently available equipment can remove oxygen more rapidly than ever before. Ten years ago "rapid CA" was defined as having the first harvested fruit in a room down below 3% oxygen within 7 days. Now we are talking about filling several rooms each day and having them down to 3% within hours of harvest. In my opinion, fruit with core/flesh temperatures greater than 60°F should be cooled prior to lowering the oxygen level. Certainly fruit which is harvested at 80°+ should be cooled to at least the 50s (?) or lower prior to lowering the oxygen level.
Rapid CA is valuable in that it helps retain fruit firmness and acidity better than slowly established CA on these varieties.
Gala and Golden Delicious can be stored as low as 1.0% oxygen with CO2 levels up to 2.5% at 34°F. As the temperature is lowered below this point, the oxygen should be raised.
Regular storage should be 32°F.
b) Fuji, Braeburn and Granny Smith are varieties which I place in the CO2-intolerant category. The cells appear to be dense and air circulation around the cells within the fruit is difficult. I would probably include Rome in this category.
These apples should have the flesh temperature close to the storage temperature before the oxygen is reduced. These varieties have a tendency to get internal browning (BBD, etc.) which is associated with the natural predisposition of the apple (and other preharvest factors) as well as the storage regime.
New information this year indicates that, in limited experiments, these varieties may develop less internal browning when the fruit has been treated with diphenylamine (DPA) prior to storage. How and why this happens is still unknown and research is underway to define conditions under which this occurs.
Carbon dioxide should remain well below the oxygen level at all times with these varieties. Temperatures should be held slightly elevated during CA storage.
For example, fruit stored at 1.5% oxygen should be stored with CO2 below 0.5% at 34°F if the fruit is not overmature at harvest. It is not advisable to store waxed fruit in boxes with polyliners in CA, as this can hinder air circulation within the fruit.
Storage in regular atmosphere can probably be at 32°F or 34°F. The fruit should be subjected to good air circulation and not in polylined boxes or waxed.
c) Red Delicious, Washington's dominant apple variety, is somewhat CO2 tolerant and is also tolerant of rapid CA. However, producers have not seen the dramatic positive effect of very rapid CA on Red Delicious that is seen on Goldens or Galas. Storage operators should realize that this fruit softens more rapidly in a bin than on the tree, so CA should not be delayed after harvest.
Typical regimes for CA of non-watercored Red Delicious are 1.5% oxygen and up to 2.0% CO2 at 32-34°F. I have done trials using early season commercially harvested Red Delicious at 0.7% oxygen with good results. This low level of oxygen should never be used on watercored apples.
Regular storage is usually at 32°F or slightly below.
d) Watercored fruit provides a special challenge to storage operators. Experience has shown that Red Delicious with watercore, when stored in CA, will develop internal browning within 3 months or so--especially when the watercore is moderate or severe at harvest.
Discussions have revolved around the best way to remove watercore from the fruit. Watercore is liquid sugar/alcohol (sorbitol) which surrounds the cells of the fruit. This interrupts the exchange of gasses (oxygen going into the cells and CO2 out of the cells) and the fruit develops internal browning.
Obviously the best way is never to place watercored Reds in CA. However, given the current rigors of marketing, this may be difficult.
I do not know how to remove watercore from severely watercored ("pineapple watercore") fruit. On the other end of the scale, slightly affected fruit may respire away the watercore and may not develop internal browning.
The risk comes in dealing with moderate watercore. This is fruit with slightly coalesced watercore. Some in the industry believe that, by slowing the cooling and/or slowing the process of 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 a block 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 that variety and the apple 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. 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. The very firm texture of Fuji at harvest makes it less hazardous to softening.
e) Drenching is a subject which also comes up at this time of year. Like everything else, drenching is subject to risks. When drenching, it is possible to spread spores onto fruit from infected fruit. Thus, while applying DPA to reduce the risk of scald, the storage operator is increasing the risk of decay. Our ability to choose from a number of fungicides is limited. Most packers will not use Captan due to restrictions in export markets. TBZ, the most widely used fungicide, is not as effective as Benlate once was. It has been estimated that as many as 20% of the spores may not be susceptible to control by TBZ.
How can the risk of decay be minimized?
Here are some thoughts:
- The greater the number of bins that go through each batch of solution, the larger the risk that decay organisms will accumulate. Thus, I am opposed to the extension of the number of bins drenched by a batch of solution to beyond the label rate-regardless of the type of filtering system used. Filtering may be an excellent way to keep the solution clean (of dirt) while drenching, but it is not a substitute for new solution.
- Do not combine dirty residue from one tank with that of a fresh tank. Get rid of dirty or spent solutions.
- Check the levels of chemicals on the fruit to be sure the correct amount is arriving on the fruit. Do not assume that because the solution in the tank is at the correct concentration the fruit has received the correct amount.
- Are the trucks remaining under the drencher long enough? Do not let the need to turn trucks around be more important than a long enough dwell time under the spray.
- Clean tanks between batches. Remove all residual debris.
Which varieties should be drenched?
Many storage operators stopped drenching Golden Delicious when they saw burn on fruit from rapid CA. In most blocks they have not seen an increase in decay after storage during years of dry harvest weather.
Granny Smith is highly susceptible to scald and should be drenched, regardless of whether the fruit is stored in CA or regular storage.
It is not a good idea to place wet fruit into rapid CA.
Industry experience has been that, when Fuji apples were drenched, the incidence of decay was greater than when they were not. Fujis are particularly susceptible to decay due to their thin skins and very high soluble solids. Omitting the drench reduced decay. However, the new information that DPA may possibly reduce internal browning (see above) prompts revisiting this decision. IF an operator chooses to drench Fujis, then the fruit should have the stems clipped to reduce stem punctures, pickers closely supervised to prevent bruising, and the drench solution should be changed very frequently to minimize the risk of decay.
Braeburn should be considered for drenching 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 be applied only to protect the fruit from scald.
Red Delicious placed into CA should be drenched, unless low oxygen storage is used to reduce the risk of scald (at 0.7% oxygen) for organic apples.
Calcium in the drench solution can assist with the reduction of nutrient-related disorders, such as Bitterpit. 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 Department of Ecology regulations is a serious problem for many packers.