Responses of Apple and Pear Fruit to
Many of the postharvest management practices used to prolong storage life of apples and pears act by reducing the effects of the plant hormone ethylene. Refrigeration and controlled atmosphere (CA) storage both slow ripening in part by reducing ethylene production and activity. The discovery by Drs. Sylvia Blankenship and Ed Sisler at North Carolina State University that 1
Factors That Influence Fruit Responses to MCP
A number of factors influence apple and pear fruit responses to MCP. These include MCP concentration during treatment, duration of exposure, fruit maturity, temperature at the time of treatment, and the interval between harvest and when the treatment is applied. Like ethylene, MCP is a gas and can be applied as a fumigation in a closed chamber or room. One of the many desirable characteristics of MCP is its activity at very low concentrations. The active concentration range for apples and pears treated at harvest is 10 ppb to 1 ppm. Within this range, the duration of responses induced by MCP increase with concentration. Responses to MCP can be induced by exposures of as little as 1 hour under laboratory conditions using relatively small volume chambers for treatment. In large storage rooms, longer treatment durations (i.e., 24 hours) may be needed to ensure adequate distribution of MCP throughout the room and sufficient contact time with fruit. In general, fruit temperature during treatment is not critical, assuming treatment concentration and duration are sufficient. However, riper fruit treated at low temperatures for relatively short periods may respond less compared to the same fruit treated at warmer temperatures. The duration between harvest and treatment within which maximal responses can be induced may vary with cultivar and fruit maturity. For example, 'Granny Smith' apples, treatment within 2 weeks of harvest induced maximal responses including complete control of superficial scald. However, fruit treated 4 or more weeks after harvest developed superficial scald and had lower firmness and titratable acidity compared to fruit treated at or within 2 weeks of harvest.
The duration of responses induced by MCP can also be influenced by an interaction between fruit maturity and treatment concentration. Riper fruit may require treatment using concentrations at the high end of the range (~1 ppm) to achieve maximum duration of MCP-induced responses. We have also observed that fruit previously stored in CA may require concentrations in excess of 1 ppm to induce detectable responses.
Effect of CA Storage and MCP Treatment on Ethylene Production
Both CA storage and MCP treatment reduce fruit ethylene production (Figure 1). Comparisons of untreated apples stored in CA with MCP-treated fruit treated indicate no differences in fruit quality for the first several months after treatment. For 'Gala' apples, values for firmness and titratable acidity were similar through 3 months after harvest. As storage duration increased beyond 3 months, fruit stored in CA had slightly higher firmness and titratable acidity compared to MCP treated fruit; however, MCP treated fruit remained firmer with more titratable acidity than fruit stored in air. We have observed the combination of MCP treatment at harvest then storage in CA can provide an additional benefit in titratable acidity retention after long-term storage (7 months) of 'Gala' apples.
Figure 1. Ethylene and CO2 production by 'Delicious' apples. Fruit were treated at harvest with 1 ppm MCP for 4 hours, then stored at 68 °F.
Ethylene regulates production of volatile compounds that contribute to apple and pear aroma and flavor. Long-term CA storage tends to reduce the capacity of fruit to produce these compounds, and MCP treatment induces the same response. The duration of this MCP response is determined by treatment conditions, particularly treatment concentration. Fortunately, when the effects of MCP begin to fade, volatile production resumes and occurs at higher rates compared to fruit stored in CA. This and other effects of MCP are not reversible by exposing treated fruit to ethylene.
Effects of MCP on Ripening and Fruit Quality
While MCP can be used to reduce the rate of ripening of apples harvested at an advanced maturity, the responses obtained are relative to fruit condition at harvest. Experiments conducted using 'Delicious' apples indicate fruit harvested past the optimum for long-term CA based on starch (index > 2.5) and internal ethylene concentration (> 1 ppm) can benefit from the use of MCP. While some measure of ripening control can be expected when late harvest fruit are treated with MCP, the potential for long-term storage of these fruit remains low compared to fruit harvested earlier.
'Delicious' apples harvested 175 days from full bloom with an average starch index of 4.9 and firmness of 15 lb were treated with 0.1 or 1 ppm MCP for 12 hours the day after harvest. After 3 months storage in air at 32 °C, treated fruit were 2 to 2.5 lb firmer compared to controls. Firmness of controls was less than 11 lb, making the treated fruit unlikely to meet the 12-lb standard for out-of-state shipment.
Treatment with MCP results in reduced respiration rate and ethylene production (Figure 1) for all apple and European pear cultivars we have tested. These physiological responses are accompanied by impacts on fruit quality including reduced firmness and titratable acidity loss (Figure 2), control of superficial and soft scald, core flush and senescent flesh browning, and reduced development of peel greasiness and senescent decay. The reduction in greasiness indicates there is less production of cuticular components following treatment with MCP. As these compounds play a role in reducing moisture loss, an increased potential for shrivel exists for MCP-treated fruit. This appears to be a manageable situation as we have observed minimal shrivel when MCP-treated apples or pears are stored in perforated box liners after treatment. While the development of decay is slowed by MCP treatment, MCP does not have fungicidal activity. As fruit ripening progresses, decay may eventually develop in treated fruit. Additionally, fruit wounded, then inoculated with pathogen spores are not protected by a pre-inoculation treatment with MCP, indicating decay resulting from injuries during harvest and packing may still require the use of other decay control technologies.
Figure 2. Firmness and titratable acidity of various apple cultivars (see key below) treated with 1 ppm MCP for 12 hours at harvest, then stored in air at 32 °F for 6 months followed by 7 days at 68 °F.
Cultivar Key for Figure 1.
While effective for managing many undesirable consequences of apple fruit ripening, MCP treatment does not provide solutions to all postharvest problems for apple and pear fruit. The dissolution of watercore is slowed following MCP treatment compared to untreated fruit. Watercore loss is also slowed by CA storage; however, our results indicate MCP-treated fruit lose watercore at a faster rate compared to non-treated fruit stored in CA. MCP does not reduce development of internal CO2-induced disorders. Fruit treated with MCP then placed into an environment containing 3% CO2 and 1.5% O2 developed internal browning similar to non-treated controls.
A single MCP treatment at harvest can effectively slow ripening for several months, and repeat applications during storage have potential to further prolong the response. For some apple cultivars, the combination of MCP treatment followed by storage in CA may provide storage benefits superior to the use of either technology alone. While retention of firmness is critical for maintaining quality of apples, European pears do not develop full dessert quality without firmness loss. The development of soft, buttery texture is critical to optimal eating quality of 'Bartlett', 'Anjou', 'Bosc', and other pear cultivars. As the duration of MCP induced responses is determined in part by treatment concentration, a low concentration treatment at harvest (10 ppb) slows ripening initially but ripening resumes in 1 to 3 months depending on the cultivar. Reapplication can then be used to prolong the response. This type of protocol for pear storage may allow the benefits of MCP to be realized while also increasing the potential for predictable ripening throughout the storage period.
Pilot-Scale Trials of MCP Treatment
Several pilot scale trials have been conducted using the Washington Tree Fruit Research Commission's CA research facility at the Stemilt Growers plant in Wenatchee, WA. The first test was conducted in March 1999. 'Delicious', 'Golden Delicious', and 'Fuji' apples previously stored in CA were exposed to MCP for 24 hours. The results indicated MCP moved rapidly from the site of generation and was detectable throughout the room. Although the target concentration was not reached in this test, most likely due to generation of MCP at a cold (32 °F) temperature, reduced firmness and titratable acidity loss were detected in treated fruit. Subsequent trials in the fall of 1999 were conducted using a heat source to maintain the liquid solution near 70 °F. A closed system was used whereby MCP was generated outside the room and pumped into the room via gas tubing. Gas samples were collected from five locations in the room and indicated a rapid distribution of MCP to all sampling locations. Initial evaluations of fruit after 3 or 4 months storage in air or CA at 32 °F indicated MCP treated fruit were firmer and had higher titratable acidity than non-treated controls. While these tests indicate MCP treatments can be successfully conducted under large scale conditions, development of efficient and reliable technology to generate and introduce MCP into storage rooms will be important for commercial development of this material.
Research conducted to date worldwide indicates the use of MCP can be an effective management tool for storage of apple and pear fruit. While more work is required to establish treatment conditions that maximize MCP responses while affording management flexibility for marketing, several potential benefits of MCP use are likely based on the results of research trials. Application of the material as a gas avoids the use of water and the potential risk of decay from inoculation via a re-circulating water drench. The low concentration at which MCP is effective also ensures a low residue present in fruit after treatment. The reduction in firmness and titratable acidity loss following a single application of MCP is comparable to that resulting from short- to mid-term CA storage. Control of superficial scald and other physiological disorders following MCP treatment may allow reduced use or elimination of other scald control technologies. While these benefits are likely, other management issues related to the use of MCP will also need to be addressed. The increased tendency for shrivel in treated fruit will require proper humidity management. Reduced production of volatiles that contribute to aroma can impact fruit quality; however, for apple fruit this response is comparable to what is induced by CA storage. Our work with apples as well as 'Bartlett' and 'Anjou' pears indicates the capacity to produce these compounds can develop over time as the effects of MCP lessen. Development of treatment protocols, particularly for pears, that utilize multiple applications of MCP may provide a means to increase management flexibility to allow predictable ripening while maximizing storage duration.
Rohm and Haas Company have submitted an application for registration of MCP for use on apples to the U.S. Environmental Protection Agency.
Technical assistance of David Buchanan and Janie Gausman, USDA, ARS, is gratefully acknowledged. We also thank Dr. Nate Reed, Stemilt Growers and Rohm and Haas Company for their assistance in conducting pilot tests, and the Washington Tree Fruit Research Commission for providing funds to support this research.
J. Mattheis, X. Fan, and L. Argenta
USDA, ARS Tree Fruit Research Laboratory
1104 N. Wenatchee Ave.
Wenatchee, WA 98801
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000