Postharvest Handling of Anjou Pears
Much of the research has been done by Drs. David Sugar (OSU-Medford), Robert Spotts and Paul Chen (OSU-Hood River), Peter Sanderson (WTFRC) and Steve Drake (USDA, Wenatchee). I have just completed a survey of industry practices and fruit quality in cooperation with Dave Burkhart (OSU Extension, retired) and the industry. This research has been funded by the Winter Pear Control Committee and the Washington Tree Fruit Research Commission as well as other industry groups.
This paper integrates information from these and other sources to provide a working platform for adaptation by packers for local conditions. The goal is to communicate what is known (or currently believed to be accurate) about harvesting and handling Anjou pears. This information is subject to correction as new information and experience become available.
Mineral content of Anjou fruit has played a strong role in determining the type and extent of disorders often expressed after harvest. Low or insufficient levels of calcium in Anjou fruit have been associated with Cork Spot (Anjou Pit), Alfalfa Greening, Black End and other Anjou disorders. These disorders increase in severity when fruit nitrogen levels are high.
Growers should be aware that fertilizer application rates and timing, pruning and irrigation affect the mineral content of the fruit and the extent of disorders. Larger fruit is more profitable and growers are tempted to keep nitrogen levels high to increase fruit size. However, excess nitrogen will not improve fruit size and may reduce profitability by increasing fruit disorders.
A recent survey of industry practices shows that almost all packers endorse the spraying of calcium onto Anjou trees during the growing season to prevent mineral content disorders. Calcium must be applied repeatedly and at low enough concentrations to avoid fruit damage. Dr. Raese has published widely on this topic. However, balance between fruit and vegetative growth and reduction of growing season stresses (irrigation) are keys to minimizing disorders.
Work by Dr. Sugar points to the need to minimize nitrogen and optimize calcium, since he found very clearly that these fruit develop less postharvest decay than those grown on high nitrogen or low calcium regimes.
Reduction of fungal spores in the orchard
There is evidence that some types of fungal diseases which appear after harvest have been brought into the storage or packinghouse as spores on bins or fruit surfaces. For example, in August 1994, Dr. Spotts found between 500 and 1,800 Gray Mold (Botrytis) spores on the surface of Anjou pears brought in from the orchard.
Fungal spores found within the core of the fruit may have been spread by wind or rain and entered the upright fruitlet very early in its life. Consideration should be given to the application of an appropriate fungicide during fruitlet development.
Most packers recommend a fungicide spray as close to harvest as is practical; this will reduce the number of spores on the fruit and provide some residual control once the fruit has been harvested. Increased control is especially important during wet harvest seasons when spore counts are elevated and the risk of decay is more likely.
Orchard sanitation is critical in determining the extent and type of decay. Dr. Spotts has done extensive work on Mucor. Mucor is a soil-borne fungus for which there is no effective, acceptable fungicidal control. This pathogen moves into the postharvest arena on dirt particles from the orchard floor. This dirt is found on the skids of the bins, on grounders put into the bin by pickers or even blown onto fruit by the wind. In South Africa, where this disease is prevalent, orchardists keep all bins on low trailers and never allow the bins to touch the orchard floor. Dr. Spotts has shown that the number of spores in the orchard environment increases rapidly as fruit ripens and lands on the floor where it becomes infested with this disease. It is essential to keep fruit off the orchard floor and keep dirt off bins. Everything possible should be done to keep bins clean.
Limbs which allow fruit to be in contact with orchard cover crops should be removed, as this fruit can become populated by other fungal organisms. Under no circumstances should pickers be permitted to place these "grounders" in bins with good fruit.
Wounds are the primary entry site for postharvest decay organisms. Everything possible must be done to minimize wounding fruit. Thick, stubby stems of Anjous can easily wound adjacent fruit during picking and handling. Dr. Sugar counted the number of wounds on Bosc pears immediately after harvest and found that fruit picked by pickers paid by the hour had fewer wounds than when pickers were paid by the bin (4% vs. 14% wounded fruit, respectively).
Dr. Sugar has consistently encountered more postharvest decay on Bosc pears harvested during the later part of commercial harvest than in those at the start of the harvest. Maturity at harvest plays a role in the incidence of postharvest decay.
Timing harvest for Anjou pears has been based primarily on fruit firmness. However, in some seasons the rate of softening slows or even appears to stop. At the same time, starch continues to clear out of the flesh of the fruit, indicating that maturation continues. Starch clearing is more difficult to judge in pears than in apples as the starch pattern is more variable. Starch clearing should be evaluated as an additional determinant of maturity. Charts of starch clearing of Anjou pears are available.
Anjou pears gain size during the maturation season and growers are tempted to delay harvest to allow the fruit to gain an additional box size. Delaying harvest for size must be evaluated against the reduction in storage life. More mature fruit has a shorter storage life than less mature fruit.
Storage scald is the darkening of the skin of Anjou pears which appears after storage. Packers apply an antioxidant (ethoxyquin) to prevent scald, but its effectiveness is limited. Of importance to the grower is the fact that scald is more severe on early harvested fruit. Scald has been studied extensively on apples where scientists have found that the number of hours of cool temperature immediately prior to harvest influences the fruit's susceptibility to scald. This cold temperature relationship has not been defined on pears, yet it may provide some useful information.
Early season temperatures influence the rate of maturation of Anjou pears. Warm weather in the early growing season reduces the number of days to harvest. Walt Mellenthin (OSU, retired) developed a predictive model in which the number of heat units above 45°F during the first 9 weeks after bloom is added and, by consulting a chart he developed, the days to harvest can be determined.
The maturity of Anjou Pears at harvest determines the length of time the fruit can be stored. Dr. Paul Chen (OSU) developed a graph which illustrates storage options based on the maturity of Anjou pears at harvest (Figure 1). This is an excellent summary of a great deal of information and should be studied carefully.
Figure 1. Storage options for Anjou pears (developed by Dr. Paul Chen).
Growing season climate
Growing season climate predisposes the fruit to disorders and diseases. One extreme example has been the uneven ripening of pears from orchards in the Wenatchee and Entiat Valleys adjacent to the forest fires in 1994. Ethylene was produced by the burning wood and this may have predisposed the fruit to ripen unevenly.
Under more normal circumstances, growing season climate affects the rate of maturation and post-storage quality. The work which has been done in this area shows that Anjou pears apparently have a very narrow optimum growing temperature. Using temperature-controlled limb cages, Mr. Mellenthin determined that the daily hourly average (DHA) temperatures during 6 weeks prior to harvest influenced the quality and ripening capacity of Anjou pears after storage. Fruits which had experienced 57°-63°F DHA ripened with higher acidity and sugar levels compared with those grown in areas with higher or lower temperatures. Those grown at lower temperatures (53°F) were more susceptible to friction discoloration, while those grown at higher temperatures (68°F) developed more superficial scald. However, more work needs to be done to understand the role of growing season climate on postharvest performance.
Drenching Anjou Pears
However, the benefits of drenching include the opportunity to apply a fungicide immediately after harvest to control Blue and Gray Mold while the fruit is in storage. In some instances, an antioxidant is included in the drenching solution to control scald. Dr. Chen has found that if 1,000 ppm ethoxyquin is incorporated in the drenching solution, the incidence of scald can be controlled effectively for at least 3 months. During packing, 1,700 ppm ethoxyquin can be applied to extend control of scald for up to 5 months. (Note: This "split application" is not legal in Washington State at this time.)
Several packers have been drenching pears successfully and storing them in bins for packing later in the season. They point out that it is cheaper to remove any decayed fruit while the fruit is passing over the packingline than it is to repack. Industry experience shows that Anjou pears packed after the first of the year should not be stored after packing, rather the fruit should be marketed as soon as possible after packing.
Trials using two fungicides (TBZ + Captan) after harvest resulted in improved decay control, but fruit from some orchards was damaged. These orchardists had sprayed oil during the latter part of the growing season. Wrapping the pears in paper containing oil also caused fruit marking when Captan had been applied after harvest.
Storage of Anjou Pears
Rapid temperature reduction
Rapid reduction in temperature is important in the retention of quality and prevention of ripening. The freezing point of fruit in storage depends upon its soluble solids level; soluble solids act as an anti-freeze. Figure 2 is a generalized model of the freezing point of stone fruits plotted against the soluble solids level. It is important to remember that it is not only the delivery temperature but the air velocity which determines the rate of cooling.
Dr. Stan Porritt (Agriculture Canada) showed that room cooling of boxed, palletized Anjou pears is actually very slow. He placed thermocouples in the center of selected cartons of both tray packed and tight filled boxes of Anjou pears. When fruit was tray packed and boxes were palletized with 6 boxes per layer with space around each box, it took between 8-10 days for core temperatures to reach 32°F in the boxes on the second layer. When the fruit were tight filled and the boxes were palletized with 7 boxes per layer (without space around each box), it took 31 days for the fruit in boxes in the same position to reach 32°F. Obviously, cooling is slower than we realized. I am working right now to determine actual cooling rates within the industry and will report on the results when the project has been completed.
The storage regime used on Anjou pears depends in part upon the desired length of storage. In general, CA stored Anjous are held at 1.5% oxygen, under 0.5% carbon dioxide at 30°F).
However, recent research modifying the oxygen and carbon dioxide levels has come up with surprising results. For example, Dr. Drake (USDA-ARS) has done some short-term CA studies in which Anjous were held at 1.0% oxygen with up to 3.0% carbon dioxide at 33°-34°F. This fruit was greener and had a better skin finish and less scuffing when packed than fruit from the standard CA regime. However, concern has been voiced that pithy brown core might be increased at this high carbon dioxide level.
Dr. Chen developed regimes to extend the storage life of Anjou pears. His multi-year research has shown excellent results in improving fruit quality using stepwise storage regimes in which the oxygen starts at 0.5% for 3 months, then rises to 1.0% for the next 3 months, and then up to 1.5% for the final 3 months. Carbon dioxide must be held below 0.1% since, when carbon dioxide rose to 0.5%, fruit injury appeared. The temperature was held at 30°F throughout the storage period. Obviously, very careful control of atmospheres is essential to success.
High carbon dioxide treatment
Experiments using high carbon dioxide treatment were reported in the early 1970s. Mr. Mellenthin found that Anjous treated immediately with 12% carbon dioxide for two weeks and then stored in regular storage were superior in quality to those in regular storage without carbon dioxide treatment. However, carbon dioxide injury is always a risk. This fruit was not compared to that in rapid CA.
Ethylene in CA pear storages can be a problem. Although there is no research on pears exposed to ethylene in modern CA storages, industry experience has been convincing enough so that pears are not stored in rooms with apples which might give off ethylene. CA rooms in which pears are stored are plumbed such that the atmosphere of apple rooms does not mix with that of pear rooms.
Storage and Packing Strategies
- Anjou pears to be marketed during the first 3 months
after harvest should be cooled and packed when convenient.
Fruit can range in firmness from 12-14 lbs. at harvest.
Anjou pears shipped 30-45 days after harvest will not ripen uniformly since their postharvest chilling requirement has not been met in storage. Drs. Drake and Chen have been working to determine whether warming coupled with ethylene treatment might overcome this lack of chilling. Although the details of this treatment have not yet been worked out, initial results are encouraging. It appears that the application of ethylene to warm fruit will be superior to the use of ethylene in trucks on cold fruit.
- Pears to be marketed 3-6 months after harvest should
be cooled and stored in CA immediately after harvest. Fruit
should be above 13.5 lbs. at harvest.
Packers can choose to store loose in bins or as packs depending upon the capacity of the packingline. Depending upon the humidity in the CA room and fruit finish, pears are subject to shriveling in storage. Increasing the time between defrosts may reduce shrivel. Capping the top of the top bin in each stack and using poly curtains in the bins may also help. Fan cycling and using coils with very large surfaces may also help eliminate shrivel. On the other hand, having the humidity too high may induce an environment conducive to decay, especially when water droplets are present. Care must be taken to change from brushes to belts on the line to prevent scuffing. Packers may wish to change flotation material to one less abrasive. Line speed should be reduced and water used as a lubricant where appropriate. Wax or other compounds can be used.
Packingline dump tanks must be cleaned often to reduce the number of spores. Chemicals must be carefully monitored since the susceptibility to decay increases in this time frame.
- Pears to be marketed 6-9 months after harvest become
challenging, as scuffing during packing, scald, shrivel and
decay become very serious threats to quality. Most packers
in the Wenatchee River Valley pack fruit intended for
marketing in this time period immediately after harvest.
The plus side is that the pears have a protective layer of
fungicide and antioxidant and each pear is individually
wrapped. The risk is that the pears will have to be
repacked to remove pears which develop disease or
Pears which are not packed immediately after harvest and are to be marketed out of late CA are usually presized or drenched and stored in bins after treatment with protective chemicals. These pears are shipped immediately after packing.
Control of Diseases and Disorders
Another article in this issue of the Tree Fruit Postharvest Journal contains information on control of postharvest expressed diseases. Control of postharvest disorders is often a problem as many of the disorders are similar in appearance. We are in the process of establishing a library of photographs of pear disorders which we will distribute to packers when it has been completed.
This disorder appears as brown to black discoloration on the skin of the fruit and the flesh is not initially affected. As it progresses, the flesh of the fruit deteriorates and falls apart without ripening. The problem is not controlled by oiled paper wraps or antioxidants. Senescent scald usually affects mature fruit, but it can affect immature fruit held under higher than optimum temperatures or for longer periods in storage.
Also known as Anjou Scald, superficial scald appears as a brown discoloration of the skin; the flesh is not affected. Although it may appear when the fruit is taken out of storage, superficial scald develops rapidly after fruit has been ripened or warmed. It can be controlled by oiled wraps or an antioxidant.
Pithy Brown Core
This is a disorder of the flesh of pears following storage. It starts at the core area but may extend into the surrounding flesh. It may be restricted to brown flecks in the flesh or may be so extensive as to cause tissue collapse. The affected tissue is dry and pithy; it is not watery. Pithy Brown Core is associated with higher than optimum levels of carbon dioxide, especially when oxygen levels are low. The susceptibility of fruit varies from season to season (climate?) and from orchard to orchard. Advanced fruit maturity, slow cooling and delayed storage all contribute. Fruit affected with severe PBC may also develop skin speckling.
Also known as Belt Burn, this condition appears as brown, irregular areas of the skin, possibly extending into the tissue underneath. It results from abrasion of fruit by a belt, brush or another fruit. It can occur from vibration in transit when fruit is insufficiently immobilized. Pears packed soon after harvest are less susceptible to friction bruising than those packed later in the season.
Impact or Pressure Bruises
Damage to tissue can result from dropping (impact) or compression forces. Impact damage may appear as brown spots under the skin and may be visible only when the fruit is peeled. In other instances, evidence may be present on the skin. These spots may become dry and corky over time. Compression forces lead to flattened areas of skin and flesh which may disappear over time.
Ammonia fumes can injure pears. At first, reddish-brown rings form around lenticels which develop into black spots. Pears with liquid droplets are more susceptible than when the skin is dry. This type of damage is often seen all around the pear and, when severe, can be found on fruit throughout the bin.
Paper wraps impregnated with copper compounds can under some circumstances leach copper onto wet fruit, causing the development of small black spots. Modern copper papers are less likely to cause this damage than older types. Dry fruit is less likely to develop this problem than fruit packed moist or fruit in boxes in which condensation forms between the fruit and the paper. When fruit is packed wet, at times the pattern of a thumb or other finger may appear on the skin.
Sodium orthophenylphenate (SOPP) when used as a fungicide can cause injury if the fruit is not rinsed following treatment, if the solution is heated, or fruit is left in the solution for a period of time. If SOPP is used, it is important to clear out the dump tank at breaks or during equipment failures. This damage appears as a darkening of the lenticels and breaks in the skin; the flesh may also be damaged.
Several researchers have found that, when the fungicide Captan is applied after harvest to Anjou pears which have also been exposed to oil, dark blotches on the skin may develop over time in storage. The blotches can be as large as 0.5" in diameter, affecting only the skin. It is recommended that, if Captan is needed, the fruit be shipped soon after application.
ConclusionAnjou pears are a difficult fruit to store, pack, and ship. Quality fruit requires that grower and packer make no mistakes. Growing season climate and harvest maturity predetermine poststorage quality. Storage management is critical and requires knowledge of the storage potential for each lot of fruit. Packing requires attention to details about chemicals as well as the potential for fruit injury. The right fruit must be sent to the right market or disaster may strike. Good luck!
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Tree Fruit Postharvest Journal 6(2):3-8