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Distinctive bitter pit spots develop on Honeycrisp apples as part of an experiment at the U.S. Department of Agriculture Agricultural Research Service Tree Fruit Research Laboratory in Wenatchee, Washington. The experiment, led by Jim Mattheis, seeks to understand what combination of postharvest practices best reduces development of bitter pit in the notoriously finicky variety. <b>(Kate Prengaman/Good Fruit Grower)</b>

Distinctive bitter pit spots develop on Honeycrisp apples as part of an experiment at the U.S. Department of Agriculture Agricultural Research Service Tree Fruit Research Laboratory in Wenatchee, Washington. The experiment, led by Jim Mattheis, seeks to understand what combination of postharvest practices best reduces development of bitter pit in the notoriously finicky variety. (Kate Prengaman/Good Fruit Grower)

Honeycrisp production continues to boom, and more apples are being stored in warehouses, increasing the likelihood of postharvest disorders.

While there’s still no consensus on the best postharvest strategies to protect quality and prevent disorders for the finicky fruit, new research suggests a combination of common strategies could lower the risk of challenging bitter pit.

Storage strategies for Honeycrisp differ from those for other varieties because it is susceptible to chilling injuries if placed in cold storage right after harvest.

As a result, most warehouses temperature condition the freshly picked apples by holding them around 50 degrees for a week before moving them to cold storage, but that practice also appears to exacerbate development of bitter pit.

However, new research suggests that the tradeoff is not necessarily so stark. Adding controlled atmosphere (CA) or Smartfresh treatment to Honeycrisp during that temperature conditioning period — rather than waiting a week or more for the fruit to be cooled — seems to reduce the development of bitter pit.

“By turning on the CA when the fruit is relatively warm, we are seeing positive — most of the time but not all of the time — bitter pit control and no negative consequences related to low oxygen or high carbon dioxide injuries,” said Jim Mattheis, plant physiologist for the U.S. Department of Agriculture in Wenatchee, Washington, and lead author of a forthcoming study in HortScience.

In data collected over three years in several orchard lots, Mattheis found that apples treated with both 1-methylcyclopropene upon arrival at the warehouse and placed in CA the next day, while still warm, developed the lowest amount of bitter pit.

The highest level of the disorder was found in the control apples, which were stored in regular air after temperature conditioning.

Relative to that control, bitter pit was also reduced in apples that were placed in CA or treated with MCP after the week of temperature conditioning — a practice many warehouses currently employ — but it wasn’t as good as when the CA and MCP were done right away, Mattheis said.

While much of the conversation about bitter pit prevention has focused on orchard management to ensure the developing fruit have the right nutrients, adding a postharvest strategy will give growers and packers more options to protect their Honeycrisp.

“This is an opportunity to at least partially manage it from a postharvest standpoint,” Mattheis said. “It’s good to have more tools available to manage the problem and to realize that some things we can do, we were already doing.”

His data shows that practices already being used — MCP treatment and CA storage — do reduce bitter pit compared to apples stored without any method to dampen ethylene receptors and slow ripening.

But for at-risk Honeycrisp, initiating those practices during the temperature conditioning appears to be even better. Because Honeycrisp is so susceptible to so many disorders, the best postharvest strategies may come down to those nuances of timing, Mattheis said.

“In some ways, this is really not new. The notion that if you establish CA rapidly on a bitter pit-prone variety you can prevent the disorder has been known since the 1970s,” Mattheis said. “This is kind of an illustration of how we have a number of postharvest technologies that are effective in different ways — temperature, CA, MCP — but it turns out that it’s not as simple as when you get that new technology, you can just add it on to what you are already doing and get a better result at the end.”

The finding, that initiating the CA or MCP on the warm apples offered the most benefits and no negative consequences, was not a surprise to Randy Beaudry, a postharvest researcher at Michigan State University who is also looking at Honeycrisp storage.

In his own studies, he recently found no impacts from carbon dioxide injuries on Honeycrisp held with high CO2 during five days of temperature conditioning. (See “Safeguarding Honeycrisp”)

“Up until last year, based on a different variety studied in New York, we would have expected doing those treatments at higher temperatures were really deleterious,” but doing them at cold temperatures actually resulted in worse outcomes for Honeycrisp in his research, Beaudry said. “Honeycrisp is such a bizarre creature nobody knows what to expect.”

The USDA research was funded by a $210,000 grant from the Washington Tree Fruit Research Commission and ongoing support from the U.S. Department of Agriculture. Mattheis will present these findings at the Research Commission’s Apple Horticulture and Postharvest Research Review Jan. 25 in Yakima, Washington. •

Finding the best storage strategy

When it comes to high-maintenance Honeycrisp, the devil is in the details.  Jim Mattheis, plant physiologist for the U.S. Department of Agriculture, said many warehouse managers are using temperature conditioning to prevent chilling injury and MCP or CA to slow ripening, but they all do things their own way.

“Most places anymore are putting increasingly more of their fruit into CA because they can’t market it fast enough. But how they get it there is not consistent warehouse to warehouse,” he said. “Everybody does it, but how they do it isn’t the same. They don’t use the same atmosphere regime or temperature management strategy. There’s lots of iterations.”

For this study, all the apples were held at 10 degrees C for seven days before being cooled to 3 degrees for storage. The SmartFresh treated apples were exposed to 42 micromoles per liter of 1-MCP the day they arrived at the warehouse.

The controlled atmosphere for the study was set at 3 percent oxygen and 0.5 percent carbon dioxide for two days and then adjusted to 2 percent oxygen and 0.5 percent carbon dioxide.

Mattheis said that while the results have so far been positive, including his own tests and what he’s heard from warehouses that are using this strategy, he believes more research is still needed to find the optimal storage strategy for Honeycrisp.

– by Kate Prengaman