Ethylene and Anjou Pears
The Pacific Northwest produces approximately 12 million boxes (44 lb per box) of Anjou pears each year and will continue to increase production within the next five years. This expected increase in the numbers of Anjou pears has forced the fruit industry to evaluate the feasibility of marketing the fruit immediately after harvest. Such marketing, however, is complicated by the fact that Anjou fruit, like most winter pear cultivars, requires a period of chilling to induce normal ripening capacity. Anjou pears harvested at the optimum maturity with flesh firmness of 14 lb (±1 lb) require 60 days of chilling at 30 °F (-1 °C) to generate normal ripening capacity. Anjou pears without sufficient chilling are termed as "under-chilled" fruit. Dr. Eugene Kupferman and his coworkers reported that commercially packed Anjou pears shipped before November did not soften properly after 7 days of ripening at room temperature (Kupferman 1994). In an effort to make ripened (or partially ripened) Anjou fruit a desirable product for consumers shortly after harvest, we have developed three different methods of promoting normal ripening capacity of under-chilled Anjou fruit by ethylene treatment.
Preconditioning of Anjou Pears by Ethylene Treatment
The method of preconditioning Anjou pears involves the warming of stored fruit at 68 °F (20 °C) for 3 days with 100 ppm ethylene in a preconditioning room just prior to the shipment. The preconditioned fruit are then cooled to a temperature of 30 °F
(-1 °C) for further handling and shipping. Usually, the retailers prefer to have Anjou fruit preconditioned at the production site because the facilities at the retail markets may be limited and the labor costs are higher. To be commercially feasible, "preconditioned" fruit must remain green and firm with no risk of bruising during shipping and distribution, ripen swiftly in air at the retail markets, and develop juicy and flavorful qualities upon reaching the consumers.
Fruit firmness (FF) of Anjou pears at optimum maturity was 15 lb. During 8 weeks of storage, a 3-day preconditioning with 100 ppm ethylene softened Anjou fruit to no less than 11.4 lb, which was still quite safe for further shipping, handling, and distributing. Preconditioning treatment had little effect on extractable juice (EJ), titratable acidity, and soluble solids concentration of Anjou fruit, indicating that a 3-day preconditioning period with 100 ppm ethylene induced only the ripening capacity of under-chilled Anjou fruit. Thus, preconditioned fruit were capable of ripening swiftly in room temperature at the retail markets' counter or the consumers' kitchen.
The ethylene-preconditioned Anjou fruit developed acceptable dessert qualities (including texture and flavor) upon ripening on day 7 even though they were preconditioned right at harvest. The dessert qualities of the ripened fruit preconditioned with 100 ppm ethylene after 2 to 8 weeks of storage improved to no less than a score of 7 on a 9-point hedonic scale.
Harvested fruit could be preconditioned with 100 ppm ethylene in the bins immediately at harvest or after being stored in air at 30 °F (-1 °C) for less than 8 weeks. Preconditioned fruit could then be safely packed and shipped to retail markets with minimal risk of blemish since FF of the treated fruit measured no less than 11.4 lb after 3 days of preconditioning under 100 ppm ethylene at 68 °F (20 °C). Anjou pears in packed cardboard boxes (44 lb per box) could also be preconditioned with 100 ppm ethylene for 3 days at 68 °F (20 °C) as long as the packed boxes were stacked loosely with sufficient airiness. Preconditioned Anjou fruit in the packed boxes should be immediately cooled to 30 °F (-1 °C) prior to shipping.
Promote Ripening of Anjou Pears by Ethylene Generated from Bartlett Pears
In most pear-growing districts, the commercial harvest date of optimum mature Bartlett pears (FF 19 to 17 lb) arrived approximately 2 weeks earlier than that of Anjou pears (FF 15 to 13 lb). The chilling requirement to induce normal ripening of Bartlett pears is no more than 21 days at 30 °F (-1 °C). Bartlett fruit harvested at late maturity with flesh firmness of 16.7 lb required 10 days at 70 °F (21 °C) or 13 days at 60 °F (15 °C) to soften to 3 lb if they had not been exposed to prior cold storage temperature at 30 °F (-1 °C). Bartlett fruit harvested at commercial maturity can generate ethylene as high as
20 µL.lb-1.hr-1 at 68 °F (20 °C) even without exposure to cold storage temperature.
The capability of Bartlett fruit to exhibit autocatalytic ethylene production provides the potential to induce normal ripening of under-chilled Anjou pears without using external ethylene.
When five Anjou and five Bartlett pears (a total net weight of 5 lb ± 0.2 lb) were packed in a 1-gal Ziploc freezer bag perforated with 10 holes (1/8-inch diameter per hole), Bartlett fruit ripened in 5 days and Anjou fruit in 7 days at 68 °F (20 °C). Ethylene concentration in the perforated bag increased from 25~50 ppm on day 1 at 68 °F (20 °C) to 50~80 ppm on day 5 at 68 °F (20 °C) prior to the removal of Bartlett fruit. Under-chilled Anjou pears stored for 3 to 8 weeks were packed together with Bartlett pears stored for 5 to 10 weeks, which provided time for sufficiently chilling. The Bartlett fruit developed very high dessert qualities on day 5 of ripening at 68°F (20°C) while Anjou fruit softened to no more than 5.2 lb and reduced EJ appreciably on day 7, indicating that ripened Anjou fruit developed soft, juicy, and buttery texture.
The advantages of using ethylene generated naturally by Bartlett pears to promote normal ripening activities of under-chilled Anjou pears packed in the same perforated bag are as follows:
- Packing houses do not need to construct a special room for preconditioning under-chilled Anjou pears during the early marketing season.
- Existing packaging lines for Bartlett pears could be used for packaging the mixture of Anjou and Bartlett pear fruit. The first line could be used for inserting Anjou pears into the bottom half of the bag and the second line for inserting an equal number of Bartlett fruit on top. The net weight of fruit per bag could be between 5 and 6 lb.
- Because Bartlett pears ripened with desirable dessert quality about 2 days ahead of Anjou fruit, a household could consume a 6-lb bag of mixed fruit within 6 days. The consumer could enjoy eating Bartlett fruit first, when its skin color had turned yellow after 4 days at ambient room temperature.
Ethylene generated by ripening Bartlett pears not only promotes normal ripening of under-chilled Anjou pears packed in the same 10-hole bag, but may also improve early marketing of both pear cultivars.
Promote Ripening of Anjou Pears by Ethylene Capsule
The first method, the use of external ethylene treatment to precondition Anjou pears, involves a potential risk: the Anjou fruit may become vulnerable to bruising damage in transit. The second method, packaging Bartlett and Anjou fruits in the same bag, could alleviate the problem of preconditioning with external ethylene. However, it would require two packing lines for the separate insertion of Anjou and Bartlett fruits in each bag, and higher labor costs would hinder the commercial application of this technology. Therefore, we have developed an alternative packaging method for promoting normal ripening of under-chilled Anjou pears by using the ethylene capsule.
The packaging method is quite simple. Anjou pears were harvested at optimum maturity. The harvested fruit were packed in 1-gal Ziploc freezer bags perforated with 8 small holes (1/8-inch diameter per hole). Each bag contained 8 to 10 pears (total weight of 4 lb ± 0.2 lb). A simulated 30-mL ethylene capsule was inserted into each packed bag and the bag was then zipped. Because the fruit in each packed bag could be arranged in one layer, the bags could easily be stacked into commercially existing 44-lb cardboard cartons and stored in air at 30 °F (-1 °C) for further shipment.
The ethylene capsule was made of a 30-mL syringe (B-D Brand #309662, Luer-Lok) with a 23-gauge needle (outer diameter 0.64 mm; inner diameter 0.33 mm) that was shortened to 1 cm in length. The syringe was filled with pure ethylene from an ethylene cylinder with 0.2 atm pressure. After filling, the plunger was replaced by a rubber stopper fitted to seal the syringe barrel. The shortened needle tip was plugged by a rubber septum. When the packed bags were displayed on the counter of the retail markets, the rubber septum was removed from each simulated ethylene capsule; the ethylene capsule was re-inserted into each bag, and the bag was zipped again.
Ethylene gas in the ethylene capsule gradually emitted into the packed bag, thus promoting the ripening activities of Anjou pears when displayed on the counter of the retail markets. Ethylene concentrations emitted from the ethylene capsule in the perforated bag increased to greater than 450 ppm on day 1 of ripening at 68 °F (20 °C) if the fruit were treated at harvest or after 2 weeks of storage. The accumulation of ethylene concentration in the perforated bag on day 1 at 68 °F (20 °C) decreased accordingly if the fruit had been stored at 30 °F (-1 °C) for a longer period of time but still maintained at no less than 250 ppm. Ethylene concentrations in the perforated bag maintained at no less than 100 ppm even after 4 days of ripening at 68 °F (20 °C).
Our previous study had shown that a 3-day preconditioning treatment with 100 ppm ethylene was sufficient to trigger the normal ripening of under-chilled Anjou pears. In this study, ethylene released from the ethylene capsule could be maintained at the sufficient level (i.e., above 100 ppm) in the perforated bag to trigger the normal ripening of packed Anjou fruit. Our preliminary experiment had identified that the orifice of each ethylene capsule should have an inner diameter of 0.33 mm in order to maintain the ethylene concentration of 100 ppm for at least 3 days in the perforated bag.
Fruit packed in the perforated bag with the ethylene capsule softened normally with concomitant reduction of EJ during 7 days ripening at 68 °F (20 °C). Anjou fruit required at least 8 weeks of chilling at 30 °F (-1 °C) in order to develop its own ripening capacity, and this chilling requirement could be offset by external ethylene released from the ethylene capsule.
The measurement of EJ in ripening pear fruit is an objective method for the evaluation of texture quality. Ripened pear fruit with a buttery and juicy texture usually have a higher reduction of EJ than those partially ripened fruit with a chewy, coarse, and dry texture. Fruit in the perforated bag with the ethylene capsule ripened with high texture quality (>7.0 scores) regardless of storage intervals. However, the flavor quality of ripened fruit did not reach an acceptable level (i.e., 5.0 scores) until the fruit had been stored for 6 weeks or longer. Further study is required to improve the flavor quality of ripened Anjou fruit induced by external ethylene at the early period of cold storage.
The ethylene capsule is another effective means to promote normal ripening capacity of under-chilled Anjou pears for early marketing. The advantages of using the ethylene capsule are as follows:
- The ethylene capsule developed in this study could be redesigned to form an attractive 30-mL plastic capsule (such as a pear-shape capsule with a screw cap to seal the orifice), which would be small, light, safe, and user-friendly.
- The existing packaging lines in the packinghouses could, without any modification, readily be used for packaging Anjou pear fruit with the insertion of ethylene capsules.
- The 1-gal (3.8-L) package with 4 lb (1.8 kg) Anjou pears could be arranged to form a one-layer flat surface, which could easily be packed into the existing 44-lb (20-kg) standard cardboard carton for shipment and distribution.
Chen, P.M. and Varga, D.M. 1999. Ethylene from 'Bartlett' pears promotes early ripening of 'd'Anjou' pears when packed together in modified atmosphere bags. HortTechnology 9, 190-194.
Chen, P.M., Drake, S.R., Varga, D.M. and Puig, L. 1996. Precondition of 'd'Anjou' pears for early marketing by ethylene treatment. Food Quality 19, 375-390.
Kupferman, E.M. 1994. Anjou pear quality. Tree Fruit Postharvest Journal. 5(1):3-16.
Ma, S.S., Chen, P.M., Varga, D.M. and Drake, S.R. 2000. Ethylene capsule promoted early ripening of 'd'Anjou' pears packed in modified atmosphere bags. Food Quality (In Press).
This study was supported by funding from the Winter Pear Control Committee.
Dr. Paul M. Chen
Mid-Columbia Agricultural Research and Extension Center, Oregon State University
3005 Experiment Station Dr., Hood River, OR 97031
FAX: (541) 386-1905
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000