—story by Matt Milkovich
—photos by Matt Milkovich and TJ Mullinax

The Michigan State University CA Clinic, held in Muskegon, Michigan, last July, gathered postharvest experts from across North America to discuss the latest research into controlled atmosphere (CA) apple storage disorders.
Controlled atmosphere storage refers to a suite of practices regulating levels of oxygen, carbon dioxide, temperature and humidity that warehouses use to reduce fruit respiration and therefore preserve quality. If there was a takeaway from the clinic, it’s that fruit storage is complicated, and many factors contribute to storage outcomes.
“Everything we do during growing and the cold chain has impacts — some positive, some negative,” said David Rudell, a research plant physiologist with the U.S. Department of Agriculture’s Agricultural Research Service in Wenatchee, Washington.
Michigan
Conference organizer and MSU postharvest specialist Randy Beaudry discussed how controlled atmosphere, dynamic controlled atmosphere (DCA) and other storage treatments affect aroma recovery in apples.
A fruit’s volatile aroma compounds determine its smell and taste characteristics. Storage treatments affect those aroma compounds in complex ways that scientists are still trying to untangle. But in general, the more the oxygen level decreases in storage, the more fruit ripening is delayed, until the stress point at which the fruit starts to ferment and its aromas become unmarketable, Beaudry said.
That low-oxygen threshold differs for each cultivar. DCA storage systems deploy sensors to detect low-oxygen stress and then adjust oxygen and carbon dioxide levels accordingly, allowing storage operators to come as close as possible to the “happy medium” each variety needs to stay sound without fermenting, he said.

Beaudry’s team evaluated volatile aroma compound production in Gala, McIntosh, Jonagold, Golden Delicious, Red Delicious, Fuji and MAIA-1 apples stored in CA or simulated DCA conditions for three to 11 months. Half the fruit were treated with 1-MCP (1-methylcyclopropene) at 1 ppm. They evaluated the fruit for up to five weeks after removal from storage, holding it at room temperature (about 72 degrees Fahrenheit).
According to their data, 1-MCP and DCA each inhibit recovery of volatile aroma compounds. Storage duration also impacts aroma formation. Fruits sold after 11 months in storage, while still acceptable by market standards, don’t have the same character they had earlier in the season. In several cases, treatment effects appeared to be additive: two treatments (DCA and 1-MCP, or DCA and long duration, for example) negatively impacted aroma more than just one or the other; and three treatments more so than two. Every time you manipulate a piece of fruit, something about it changes, Beaudry said.
He also discussed the storage quirks of MAIA-1, the apple marketed as EverCrisp, a newer variety that has garnered a lot of interest from Michigan growers.
“It’s got a lot going for it,” Beaudry said. “It’s one of the best-tasting fruits I’ve ever had.”
But from a postharvest perspective, it’s a “very strange apple,” he said. Noting that it’s incredibly dense and heavy, he told the story of a grower’s tractor-trailer that was pulled over for being overweight because it was full of EverCrisp bins. If you’re hauling EverCrisp, he recommended loading 7 percent to 10 percent fewer apples than normal.
Beaudry also said EverCrisp responds to CA and DCA storage regimes in unusual ways.
“For one thing, it never softens,” he said. On the other hand, EverCrisp keeps “pretty darn well” in air storage, maintaining good flavor after many months.
New York
Cornell University postharvest physiologist Chris Watkins emphasized the importance of applying plant growth regulators at the right times. Preharvest applications of PGRs such as ReTain (aminoethoxyvinylglycine) and Harvista (1-MCP) are critical for long-term storage of Gala, which is subject to storage disorders — including browning, greasiness, stem-end cracking, shriveling and decay, he said.
Storage operators need to know if fruit was treated with PGRs before it was delivered. If they don’t know, it could lead to a lot of “wasted money,” Watkins said.


The ideal storage regime: fruit treated with preharvest PGRs and postharvest 1-MCP treatments. Storage at 38 degrees Fahrenheit reduces flesh browning but is less effective if fruit are not treated with PGRs. Firmness can be compromised without PGRs, too, so Watkins recommended postharvest 1-MCP applications. For long-term storage, he recommended 0.5 percent oxygen levels and 1 percent carbon dioxide levels. For standard CA storage, he recommended 2 percent oxygen and 1 percent carbon dioxide.
DCA “does an incredible job” of wiping out browning in Gala if fruit have been treated with preharvest PGRs and postharvest 1-MCP, Watkins said.
Washington
Washington State University postharvest specialist Carolina Torres talked about the effects of changing seasonal weather patterns on Honeycrisp and Gala storage outcomes. Weather events in Washington, and other growing regions, have become more unpredictable in recent years and appear to be changing fruit behavior in storage. Fruit developed internal browning at different times and intensities in different years, even though the harvest dates varied by only two or three days, she said.
That also suggests the traditional metrics for maturity that growers use to optimize harvest timing could be improved, she said.
Torres is studying weather effects on fruit quality across different technologies and supply chain scenarios.
“We’re trying to figure out what weather events are doing to our fruit,” she said.
So far, they’ve learned that seasonal climatic conditions do play a role in storage outcomes, especially with the fruit’s sensitivity to carbon dioxide, but the cold-chain scenario and preharvest management methods also have an impact, Torres said.
Canada
Jennifer DeEll, fresh-market quality specialist for horticultural crops for the Ontario Ministry of Agriculture, Food and Agribusiness, talked about the storage characteristics of Ambrosia, an apple that’s susceptible to many postharvest disorders.
Soft scald affects Ambrosia in unusual ways and at unpredictable times. During a 2006–08 project in Ontario and British Columbia, growers and researchers noticed Ambrosia apples developing “hard skin bronzing” across all storage regimes, she said. The peels were discolored but the underlying flesh looked healthy, at least at first. The condition occurred more frequently on apples that came from scald-prone orchards. Some said it looked like carbon dioxide injury, but the incidence was not reduced when exposure was eliminated.
The apples eventually started softening, however, and the condition was determined to be soft scald. The unusual version of soft scald disappeared for about a decade after 2008, but then it reappeared in Ontario starting in 2018. No one is sure why, DeEll said.
Ambrosia also is susceptible to lenticel and senescent breakdown, internal browning, mealiness, splits and carbon dioxide injury. Diphenylamine (DPA) controls carbon dioxide injury. Treatment with 1-MCP after harvest improves firmness retention, reduces greasiness and maintains higher acidity and soluble solids, but it has an inconsistent effect on internal browning. Preharvest 1-MCP reduces internal browning, improves firmness retention and reduces stem-end cracking. It also can delay color development, DeEll said.
Harvest maturity is crucial for Ambrosia storage. The apple’s background color should show more green than yellow when picked, she said. •
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