Harvest Maturity and Storage Regime For Pears
Harvest maturity and the selection of a storage regime affect both the ripening and quality of pears after storage. This paper was drawn from comments made in March 1988 at the 4th Annual Warehouse Seminar by a postharvest researcher and 2 horticulturists.
Bartlett pears are judged ready for harvest for long-term controlled atmosphere (CA) storage when they reach an average of 19.5 pounds. Bartletts can be picked for CA during a period which lasts between 4 to 7 days, depending upon the weather during the harvest season.
D'Anjou pears are judged suitable for long-term CA storage when 15 to 15.25 pounds firmness is reached. However, ground color, soluble solids and starch are also considered. The picking window varies from year to year; however, it is normally around 7 to 10 days for CA. Storage scald, friction burn and quality of d'Anjou pears are affected by growing temperatures (Table 1).
Bosc pears are harvested for long-term CA when the firmness reaches 15 pounds and the appearance and finish are suitable. Bosc pears can be harvested over a 7-day period for CA.
Precooling is essential for pears. Rooms are cooled before the harvest begins. Pressure cooling or forced air cooling is used when fruit is to be packed and shipped soon after harvest.
Bartletts are placed in rooms with extra cooling capacity and moved to their designated storage rooms when they are cooled. Room temperature during the loading of Bartletts is not allowed to exceed 40°F. Should the temperature rise above that point, additional fruit is loaded into other rooms for cooling. Delays in cooling will shorten storage life and increase the incidence of breakdown in Bartletts (Table 2).
Bosc and d'Anjou rooms are filled quickly, sealed and pulled down rapidly once the fruit has been cooled.
Pears are normally stored loose in bins in CA at Diamond. At Peshastin, most pears are packed before being placed in CA. Some fruit may be stored loose until it can be packed.
Scuffing of pears during the packing process can be a serious problem. When pears are packed soon after harvest the problem is less serious than when fruit is packed after long-term storage. Diamond presizes as many of its pears as possible, while Peshastin packs pears soon after harvest to minimize scuffing. In addition, scuffing can be minimized by limiting the number of brushes, belts and drops, slowing packing lines and waxing pears.
The type of salt used to float pears is an important consideration in the packing process. Warehouse managers must be careful when combining fungicides with flotation salts. There are four major flotation salts currently in use on pears.
Sodium sulfate is supplied as a powder, but it must be well mixed. It is relatively inexpensive and can be used with chlorine since the pH is around 7.0. A surfactant can be added to improve the performance of chlorine. This salt should NOT be used with sodium orthophenylphenate (SOPP) (Stop-Mold).
Sodium silicate is supplied as a liquid. Often called pear float. Helps to reduce belt burn or scuffing. Often used late in the packing season when scuffing is a problem. Can be used with SOPP since the pH is around 11.4. Do not use a surfactant. Do not use chlorine.
Sodium ligninsulfonate is supplied as a liquid. It appears to have some fungicidal properties. Can be used with SOPP. Do NOT use with chlorine. This product is also known as Orzan. There is a similar product called calcium ligninsulfonate which has not been widely tested.
Sodium carbonate is supplied as a powder. Can be used with SOPP. Is also known as soda ash. The pH is about 10.
Regardless of the type of flotation salt used, it is extremely important not to allow pears to sit in the solution for any length of time. Wash pears well immediately after removing them from the flotation salt. Do not combine flotation salts with any type of acid. Seek further information if you have questions.
Temperature has a marked effect on the storage duration of pears. Warmer temperatures can severely limit pear storage and life. Storage temperatures around 30°F for Bartlett, d'Anjou and Bosc extend life over warmer temperatures (Table 3). The freezing point of fruit depends upon the amount of soluble solids (sugars and other compounds) in the pears (Figure 1).
The CA atmosphere selected depends upon fruit maturity and desired length of storage. Bartlett pears of appropriate maturity can be held until the end of October if the oxygen is held at 1.5 to 2.5%, with less than 0.8% CO2 (30°F). Preliminary work by Dr. Meheriuk at Summerland, British Columbia showed no differences in Bartlett quality stored in 1% or 2% oxygen.
Bosc pears are held at 2 to 2.5% O2 with less than 0.8% CO2 (30°F).
D'Anjou pears can be held in Regular CA when the O2 is held at 2 to 2.5% with less than 0.8% CO2.
Low Oxygen CA can be used to extend the storage life of d'Anjou pears if the oxygen is held at 1%, with less than 0.1% CO2. The CO2 level is difficult to control, and about 2 pounds of lime per box is placed in the CA room to keep the CO2 under 0.1% in low oxygen storage. Scald in d'Anjou pears has been effectively controlled in experiments using Low Oxygen CA even after the fruit was removed from CA (Table 4).
High CO2 CA has been successfully used commercially on d'Anjou pears. Fruits should be treated with CO2 immediately after harvest with 11 to 12% CO2 for 10 to 14 days at 30°F. Following this treatment they can either be held in conventional air or Regular CA. The oxygen should not be lowered until after the CO2 has been flushed out of the room.
In summary, d'Anjou pears can be stored in either Regular Cold Storage, Regular CA or Low Oxygen CA. D'Anjous can also be treated with High CO2 CA, after which they can be stored in Regular CA or Regular Cold Storage. Figure 2, provided by Dr. Paul Chen, details the relationship between harvest maturity, storage regime and storage longevity. In CA storage of all types of pears, it is critical that the CO2 be held at least 1% below the oxygen level or storage disorders may develop.
Fruit shrivel can be a problem when storing pears for any extended length of time. In order to reduce the amount of shrivel some packinghouse managers will cover the top bin with plastic sheets. When warehouse personnel have increased humidity in the rooms to extremes, they notice that the moisture on the stems increases the amount of rotting in the neck of the pear. Thus there is a balance between avoiding shrivel and promoting decay.
Carbon dioxide injury can be a problem. As mentioned above, it is critical to keep the CO2 level at least 1% below the oxygen level. See Pithy Brown Core and Skin Speckling of d'Anjou Pears. The growing season's temperature and orchard location can have an effect on a cultivar's response to CA (Table 5). Late harvested Bosc pears were prone to internal problems when stored in 1% oxygen, but not when stored in air (Table 6).
Pink-end of Bartlett is a problem in certain years due to cool temperatures before harvest. When temperatures are likely to promote this disorder, harvest is advanced and during the packing process fruit with pink-end symptoms removed. Finally, particular attention is paid to cooling the fruit and moving it to market rapidly. Fruit with pink-end symptoms are not placed in CA storage.
Dr. Mike Meheriuk(1), Clayton Evans(2), Earl Talley(3), Eugene Kupferman(4)
(1)Agriculture Canada Research Station, Summerland, British Columbia, Canada
(2)Diamond Fruit Growers, Inc., P.O.Box180, Hood River, OR 97031
(3)Peshastin Growers, Ltd., P.O. Box 492, Peshastin, WA
(4)WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 6(3): 11-15