Controlling pear scald
MCP and preconditioning show promise as an alternative to antioxidant treatment.
Kristi Deschuytter is working to find new scald
A popular ripening inhibitor used on apples can control superficial scald in pears, but getting the pears to ripen when removed from storage has been the tricky part.
Oregon State University scientists, working to improve control of superficial scald in d'Anjou pears, are making progress in developing a program that combines treatment of the ripening inhibitor 1-methylcyclopropene (MCP) and ethylene or temperature preconditioning to regain ripening, reports Kristi Deschuytter, postharvest physiology research assistant.
From research conducted during the last two storage seasons, she said they have determined appropriate rates of MCP that give scald control, yet allow fruit to ripen. Regaining the ripening capacity isn't as easy as simply removing the MCP-treated fruit from storage and expecting it to ripen on its own, Deschuytter said. Fruit also needs to be preconditioned after storage or treated with ethylene to trigger the ripening.
Additionally, the OSU postharvest research team has learned that the MCP rate needed depends on the growing area from which the pears originated. For example, pears grown at the OSU Mid-Columbia Agricultural Research and Extension Center in Hood River, which is 500 feet elevation, need a lower rate (65 parts per billion) to ripen than fruit grown at the higher elevation of 2,000 feet in Parkdale, which can ripen when exposed to 100 ppb. In 2006–2007, they completely controlled scald in d'Anjous grown at the lower elevation, but the fruit did not ripen, despite temperature preconditioning at 50°F.
Research during the last two years was conducted on fruit placed in wooden boxes and lined with a perforated polyethylene bag and treated with MCP for 24 hours. Fruit were then stored for three or five months in regular air storage or six and nine months under controlled atmosphere.
The scientists then used a temperature preconditioning program that kept fruit at 50°F for five to ten days, followed by further ripening the fruit at 70°F for seven days. After all treatments, fruit were evaluated for superficial scald, color, firmness, Brix, and titratable acidity.
Deschuytter reports that the preconditioning seems to effectively control scald and allow the fruit to ripen.
In the second year of the scald project, they began investigating whether the addition of ethylene could replace the need for the temperature preconditioning. Two temperature regimes (60° and 70°F) and two lengths of ethylene exposure (24 and 48 hours) were tested.
In all storage regimes, there was a general trend of increased yellowing with longer exposure to ethylene. However, fruit treated with MCP were significantly greener than those not treated, she said. Also, the MCP-treated pears were significantly firmer than the untreated, but there was an effect from the ethylene exposure.
"In all cases, the 48 hours of ethylene, even when fruit was not treated with MCP, appeared to have better quality," she observed. Furthermore, fruit treated with MCP was able to soften to eating quality of six pounds of pressure after 48 hours of ethylene at 70°F.
Although the OSU researchers were unable to completely control scald, the scald observed in fruit treated with MCP was slight enough that the fruit could still have been marketed, Deschuytter said.
The 2007–2008 storage season was considered a high-risk year for scald, and more scald was observed in all storage conditions.
Deschuytter is encouraged by the results of the MCP and preconditioning treatments in controlling scald. "We've been able to get success when normal preconditioning and ethylene are combined with MCP, and we think it will be a good option for those who want to move away from ethoxyquin," she said, adding that the industry is already accustomed to using ethylene and preconditioning to ripen pears before shipment, so it's not really an added burden.
The OSU postharvest research team has been tinkering with storage temperatures and preconditioning as a way to get around the "chemical issue," she said.
While the industry standard for controlling scald has been the use of the antioxidant ethoxyquin, Deschuytter believes there is room for alternatives. Ethoxyquin, while effective in controlling scald, often causes surface chemical burning. That burning makes the fruit unmarketable.
"And, there's the concern about what Europe will do in the future with ethoxyquin," she added.
The chemical has passed reregistration review of the U.S. Environmental Protection Agency and Codex Alimentarius Commission, but, according to Dr. Mike Willett of the Northwest Horticultural Council, it is still under review by the European Union.
Willett said that there isn't a problem of losing a tolerance for ethoxyquin in the United States or by Codex, but he is unsure about the outcome in Europe. "The EU system is very different than ours."
During the 2008–2009 season, the OSU research will be done on pears that are in bins instead of boxes.