Skin Speckling and Pithy Brown Core of D'Anjou Pears
The 1987-1988 storage season brought with it a significant amount of Skin Speckling and Pithy Brown Core of d'Anjou pears. Northwest Wholesale, Inc. has sponsored and coordinated several meetings to stimulate the sharing of research and industry experiences.
Warehouse personnel made the following observations this season on Skin Speckling. Dr. Olsen has compiled the data.
Skin Speckling appeared on D'Anjou pears in varying amounts on both wrapped and unwrapped fruit. Fruit wrapped in paper containing copper had more serious speckling than either plain paper wrapped or unwrapped fruit. The speckling appears as discrete spots, rather than the generalized browning of scald or belt burn. It has been found both in pears with and without Pithy Brown Core (PBC) (Figure 1).
Fruit generally free of the disorder were harvested in the early part of the season, cooled promptly and held in CA with low carbon dioxide (CO2) (less than 1%), relatively high oxygen (O2) (more than 2%), and with no or little operation of the burner during the storage period after initial pull down.
Pears are more sensitive to CO2 under the following conditions:
Harvested at advanced maturity
Cooled slowly (results in advanced ripening)
Held at temperatures above 32°F (results in advanced ripening)
Held in atmospheres with CO2 above 1.0%
Held in atmospheres where the difference between O2 and CO2 concentration was less than 1%
Held under CA where condensation could occur on fruit surface (as during the operation of a nitrogen generator during defrost after initial pull down).
Condensation on stored fruit in the presence of elevated CO2 can cause some CO2 burn. If the fruit is wrapped in paper containing copper, a transfer of copper from the wrap to the pear can occur under these conditions. Normal concentration of copper in the pear skin is less than 7 ppm. Analyses run during the 1987 season indicated severe speckling occurred on fruit when 12 ppm or more copper was present on the skin. More severe speckling appeared on fruit wrapped in the lightweight paper (14 #) than on fruit wrapped in the heavier standard (19 #) paper. Possibly the heavier paper was capable of more water holding capacity.
Skin Speckling caused by carbon dioxide or copper should not be a problem if the fruit is dry when wrapped, if condensation in storage is prevented, if carbon dioxide is controlled, and if fruit is of the appropriate maturity. Research has shown that a shorter time elapses from bloom to harvest when warm temperatures occur during the first 30 to 50 days after bloom. The somewhat earlier bloom period in 1987 further advanced maturity. In 1987, it was difficult to detect the change in the maturity of d'Anjou pears due to a lack of normal softening during the harvest period. Pears retained firmness because of the higher than normal harvest season temperatures. It is recommended that growers add soluble solids and starch tests to firmness tests to judge harvest maturity.
The increased sensitivity of pear skin to damage in 1987 is thought to be related to less natural wax due to the higher than normal harvest temperatures.
Pithy Brown Core
Pithy Brown Core (PBC) appears as a disorder of the fruit flesh, especially in the core region. Brown, dry areas develop first in the core and, in serious cases, may encompass the entire core extending into the surrounding flesh. The tissue will eventually collapse and form cavities. This disorder develops dry tissue, not wet tissue as in core breakdown (Figure 1).
Scientists have associated the appearance of PBC with a number of orchard and postharvest factors.
Scientists have reported that cool growing conditions increase PBC. However, the 1987 growing season was extremely warm. Cooler locations did NOT have as much PBC as warmer sites in 1987. We must better understand the influence of postbloom, growing season and harvest temperatures on fruit quality, especially for fruit destined for long-term storage. Research workers will be looking at growing season temperatures and their effect on storage disorders. There has been some work on this in the past.
Scientists have shown that late harvest increases the tendency to PBC. Currently, harvest maturity is determined in large part by firmness. Early maturity fruit (15 to 14 lbs) can be placed in long-term CA if CA conditions are established within 14 days after harvest.
In 1987, fruit firmness levels remained constant, while the fruit continued to mature. It may be useful to begin to look at additional maturity indicators, such as starch and soluble solids. Dr. Paul Chen (OSU Mid-Columbia Research-Station, Hood River, Oregon) has provided a photograph of the progression of starch in d'Anjou pears during the 1987 harvest season (Figure 2). When pears are tested at regular intervals, the starch index appears to relate well to the decline in firmness (Figure 3). It must be emphasized that this work is still in the development stage, and that no recommendation is currently being made regarding the use of the starch test other than the suggestion to take a look at it. Another harvest indicator may be a sudden increase in the rate of accumulation of Soluble solids, which in 1987 occurred around 11 to 12%. Using a refractometer at regular intervals might give additional information about the progression of maturity.
Delayed cooling at harvest
PBC susceptibility is increased when cooling is delayed. Pears respond well to rapid cooling and forced air cooling or to spread out bins in several cold rooms before they are grouped for storage. These techniques can help retain fruit quality and reduce disorders. Hydrocooling of d'Anjou pears is a method to rapidly reduce fruit temperature. However, several warehouses have had trouble with severe skin spotting, possibly from the addition of chemical in the hydrocooler.
Boxed palletized fruit cools extremely slowly (Figure 4). It is important to allow for air circulation throughout the pallet, even if fewer boxes of fruit can be placed on each pallet.
Poly box liners
Although polyliners help pears retain moisture, it is important to allow for air circulation within each box. Polyliners slow cooling and can hinder gas exchange. Polyliners should have sufficient holes to allow for air exchange and reduce CO2 buildup. Holes should be well distributed and completely punched out. Hand holds in the box should remain clear and should not be blocked by the extra folds of the polyliner. Pull the polyliner down over the long sides of the box, rather than over the narrow ends.
High carbon dioxide levels greatly increase the possibility of PBC. Storage operators who had a great deal of PBC in 1987 had higher CO2 levels than those who did not. It is important to keep the O2/CO2 levels in balance with one another. As the O2 level is lowered, the CO2 level must be reduced. Pears are far more susceptible to CO2 induced disorders than apples are, and the CO2 level should be held lower than it is for apples, even if more lime or scrubbing is required. The industry wants longer storage times for d'Anjou pears; therefore, lower O2 levels are required. This forces even lower CO2 levels. Operators storing pears at 1 to 1.5% O2 should hold their CO2 levels below 0.1%. If the CO2 cannot be held at this level, raise the oxygen. At 2% O2 the CO2 should be held below 1.0%. If the fruit has been cooled slowly, reduce the CO2 level even further and shorten the storage time.
Lower oxygen levels (1%) will provide longer storage and better firmness retention than higher oxygen levels. However, the lower oxygen levels require careful control of carbon dioxide. Fruit in CA storage is more susceptible to PBC than fruit in regular storage.
Length of storage
Longer storage times increase the possibility of fruit injury from PBC.
Fig.4 . Effect of stacking pattern on cooling rate of d'Anjou pears (Porritt, S.W. and E.D. Edge). Cartons and their arrangement in a stack influence the cooling rate of pears. The method of stacking and its effect on cooling rate of d'Anjou pears was carefully examined. Thermocouples were placed in the center of selected cartons and rates of cooling were calculated from the readings
D'Anjou pear quality is influenced by seasonal temperatures, and the harvest maturity window for long-term storage may be very short in certain seasons. It may be useful to keep track of growing season temperatures and incorporate additional maturity indicators such as starch and soluble solids.
Once fruit has been harvested, it must be handled, packed and placed under CA conditions very rapidly. Rapid cooling is absolutely essential at harvest and after packing. Palletization and box configuration must be done with air circulation in mind. Polyliners must permit gas exchange.
Storage regime is important with extremely careful attention paid to oxygen and carbon dioxide levels. Avoid CO2 levels above 1.0%. Do not permit the levels of carbon dioxide and oxygen to be closer than 1.0. Avoid conditions in which condensation can occur on the fruit.
Do not prolong the length of storage beyond the capability of the fruit. Take samples out of storage and ripen them at regular intervals during the season.
As a result of the seriousness of these problems in 1987, several research projects have been initiated and increased emphasis has been placed on others. We will keep you informed of the progress made by these researchers as results become available.
Dr. Chen has been investigating the usefulness of additional maturity indices to predict pear maturity; he will be joined by Drs. Steve Drake and Jim Mat theis (USDA-ARS, Wenatchee). They will also look at delayed cooling in relation to PBC and speckling. Dr. Chen is studying the basic chemical and physiological events that occur in these disorders. Drs. Drake and Mattheis will continue their work on waxing of pears and its effect on quality.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 6(3): 5-10