New process extends storage life of peaches
By Melissa Hansen
Scientists around the world have studied ways to prevent internal breakdown of stone fruit during storage--a particular
problem in peaches and nectarines. Researchers at the University of Maryland (UM) recently patented a process they say extends peach storage life by suspending ethylene development in the fruit.
The patented system is "not really a magical thing," said Dr. Christopher Walsh, pomologist at UM who is one of three researchers named on the patent. By thinking of peaches as tropical fruit instead of apples, he said they created a mechanism to hold peach ethylene levels very low and ripen it as one would a banana.
The process includes picking fruit several days earlier than traditional harvest, before the climacteric stage is reached in the fruit and significant levels of ethylene are produced. Following an ice water chlorine wash to prevent decay, fruit is packed and stored at 32°F under a controlled atmosphere (CA) of 2% oxygen and 10% carbon dioxide. The ripening process is then triggered when fruit is warmed up to room temperature (around 70°F) and ethylene gas is shot into the holding room.
Within five to seven days of ripening, fruit is soft enough to bite, said Walsh.
Patent collaborator Dr. Donald Schlimme, food science professor at UM, said the process is still in the research and development stage. Under laboratory conditions, researchers were able to hold the Loring peach variety for ten weeks without signs of internal breakdown. "When warmed up, fruit was a little less sweet, but just as juicy and not mealy," he said.
The peach is a marvelous fruit when it is fully mature and tree-ripened, said Schlimme, but growers can't wait until it is tree-ripe because ripe fruit breaks down too quickly during shipment. "If you ask most people which peach they truly like, their answer is probably a peach grown locally and bought through a direct market."
Walsh echoes those sentiments. "Consumers have the least faith in peaches of any commodity in the marketplace," he said. "They wonder, is it really going to be decent inside?"
It was after both Walsh and Schlimme spent sabbaticals studying postharvest fruit quality in New Zealand and Australia, respectively, in the late 1980s and 1990, that they zeroed in on the ethylene relationship to internal breakdown. Much of their research was conducted by former Chilean graduate student at UM, Luis Luchsinger. He is the third researcher named on the patent and now teaches at the University of Chile in Santiago.
Luchsinger became interested in halting internal breakdown of fruit after examining imported Chilean stone fruit at the port in New Jersey. Walsh said the student was embarrassed to find 100% mealiness in some stone fruit varieties sent to the United States by his father's Chilean export company. Fruit looked beautiful on the outside, but was mealy and woolly on the inside.
According to Schlimme, chilling injury in soft fruit can occur from as little as ten days of exposure to cold temperatures, and he identified temperatures just below 50°F as the chill injury point. Storing fruit at 50°F would avoid mealiness and woolliness associated with internal breakdown, but fruit could not be stored for more than a few days, he explained.
The UM researchers believe there is correlation between fruit maturity and chill sensitivity--as fruit matures, chill sensitivity increases. They have also found correlation between background color of the fruit skin and ethylene production by the fruit. Background fruit color is used as a marker by the researchers to determine harvest timing. The patented process is based on picking fruit earlier than traditional dates, before ethylene production within the fruit is initiated.
Researchers used a color meter in the orchard to determine harvest date, picking the Loring peach when it reaches around a negative 12 value on the color meter and when the background color is still green.
The background color will become less green as the peach ripens, explained Walsh, but it will not develop more blush or red color. "The red color stays with what you initially have," he said, adding that after ripening, the fruit may have a slightly more orange color than one normally expects to see in a peach.
The earlier harvest also results in fruit with slightly lower Brix, said Schlimme, with fruit measuring about 10° Brix instead of more normal readings of 11° or 12° Brix.
To deal with the low sugar-to-acid ratio in the immature fruit, researchers have gassed it with ethylene during the ripening process to reduce or lessen the tart flavors. "By whacking them [the peaches] with a shot of ethylene," said Walsh, "we can get around the sugar-acid ratio problem, lowering the acid." Storing or transporting the soft fruit alongside apples may provide enough ethylene to initiate ripening, he added.
Not all in the stone fruit world are ready to embrace the process patented by the Maryland researchers.
The early harvest concept runs counter to years of data collected by the California Tree Fruit Agreement (CTFA), a federal marketing order involved in stone fruit promotion and research. According to field representative Gary Van Sickle, CTFA has supported postharvest soft fruit research for more than 25 years. "The patented process defies all the years of research we have supported," he said, adding that University of California (UC) data shows that more mature fruit, with its higher percent of soluble solids, will hold better in storage and shipment.
Van Sickle, along with other UC researchers, have not examined the data used by the UM researchers to obtain the patent, but they are familiar with the mechanics of the process.
Dr. Gordon Mitchell, retired pomologist at UC-Davis, has spent a lifetime studying postharvest quality of stone fruit.
"I have not seen strong evidence that shows halting ethylene production in stone fruit solves the problem of internal breakdown," he said. "We have shown that fruit at the lower end of maturity at harvest is more susceptible to internal breakdown."
Mitchell said results from internal breakdown research tend to be highly variable, variety specific, and inconsistent. He once tried to replicate research involving CA and an East Coast nectarine variety. "Even after meeting with the researcher, I couldn't get the same results when I used California nectarine varieties," he added.
The UM researchers will continue to collect data; Walsh has plans to scale up testing in 1998, conducting studies in a self-monitoring refrigeration unit about the size of a dormitory room refrigerator. Researchers will study boxes or lugs of fruit under controlled settings before the process is applied to larger storage rooms. "We still need to demonstrate this for one more season before moving to commercial application," he said.
The study will also include additional peach varieties, representing early, mid, and late season cultivars, as well as varieties grown in California. The Loring peach used in the research is an East Coast variety harvested in late July.
Preventing internal breakdown has particular value to soft fruit importers and exporters who must contend with long transit periods. Walsh envisions the process used on fruit grown specifically for the export market.
Copyright, November 1997, Good Fruit Grower,105 South 18th Street, Suite 217, Yakima, Washington
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