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Jim Mattheis, USDA physiologist. (Geraldine Warner/Good Fruit Grower)

Jim Mattheis, USDA physiologist. (Geraldine Warner/Good Fruit Grower)

Firmness is one quality aspect of Honeycrisp apples that producers don’t need to worry too much about, but this can lull them into a false sense of security, Dr. Jim Mattheis warned during a Honeycrisp Fruit School in Washington in December.

“One of the great things about Honeycrisp, from a postharvest management perspective, is that firmness really is not what we’re worried about,” said Mattheis, a postharvest physiologist with the U.S. Department of Agriculture in Wenatchee. “We’ve seen this consistently where fruit that’s picked at a relatively low firmness of around 12 pounds doesn’t tend to fall apart when it’s in storage.

“That’s certainly a strength from the marketing perspective, but it can also provide you with a false sense of security,” he added. “If you’re relying on a firmness estimation to tell you whether or not you have good quality, you can fool yourself because other components of quality of Honeycrisp are very important and they can go away before you see a drop in firmness.”

David Bedford, breeder at the University of Minnesota, where the variety originated, said Honeycrisp is one of the longest-storing apples in existence. “I know it has the capacity to store like none I have ever seen,” he said.

But the apple is susceptible to a whole host of postharvest disorders. These include soft scald and soggy breakdown, which are related to exposure to low temperature as the fruit comes off the tree and goes into cold storage.

It’s important not to cool them too rapidly, Mattheis said, so holding Honeycrisp apples at a temperature of 50°F for a week before cold storage is the key to managing such disorders.

The apple is also subject to low-­oxygen or carbon-­dioxide injury. And it can develop core flush just from being stored too long.

Soft scald

Susceptibility to soft scald or soggy breakdown varies significantly from orchard to orchard. Mattheis and his colleagues studied fruit from a number of orchards to try to find out if susceptibility was related to certain maturity indicators or orchard factors, but they were unable to discover why fruit from one orchard might develop the disorders while fruit from another did not.

Scientists are still trying to get a better handle on how likely a particular lot of apples is to develop chilling injuries, but, in general, fruit harvested more mature seems more susceptible.

So, if the grower has delayed harvest waiting for the fruit to color, even if the fruit is held at 50°F before going into cold storage, the disorders might still develop. And, delaying cooling can lead to earlier development of bitter pit and greasiness and loss of titratable acids, which affects the flavor.

Both CA (controlled atmosphere) storage and treatment with MCP (1-methylcyclopropene) can slow acid loss and development of greasiness. Where ­carbon-dioxide injury is a problem, the antioxidant DPA (diphenylamine) can reduce but not eliminate the risk of injury in CA, Mattheis said.

Maturity indicators

Fruit with a higher acid level tends to taste better when it comes out of long-term storage. But Mattheis stressed that this does not mean the fruit should be picked ­immature, because fruit picked too early will not taste good either before or after storage.

Acid levels can vary greatly at harvest, in the range of 0.3 to 0.6 percent. Where fruit lots have equivalent soluble sugars, starch levels, and color development, those with the higher acid levels are the ones that will hold up better long term. The internal ethylene level is not a good indicator of long-term storability.

At harvest, growers should look for a change in the background color from pure green to a more yellowish color that indicates that the fruit has matured and is beginning to ripen, Mattheis said. If the fruit is harvested on the green side, the color will turn to yellow in regular storage, given enough time, but that doesn’t necessarily mean it will taste good, he said.

Long-term storage

As production of Honeycrisp increases and exceeds packers’ capacity to ship it all early in the year, they will have no choice but to store the variety for longer, Mattheis noted.

He offered the following tips to producers in order to have the best outcome from long-term storage:

• Make sure the fruit has enough red color at harvest. The background color should be breaking from green to yellow.
• Starch conversion should be around 4.5 to 5.0 on the six-point scale.
• The acid level should be fairly high, preferably at least 0.5 percent.
• Although most packers would be more comfortable with a higher firmness level, Honeycrisp at 12.5 pounds pressure will retain firmness and texture if handled properly in storage.
• There is a difference in how the apples perform in storage, depending on if they’ve been treated with MCP before harvest (Harvista) or after harvest (SmartFresh). Because Harvista tends to delay maturity, it can reduce sensitivity to chilling injury, which Mattheis has not seen from a SmartFresh application.

The sooner SmartFresh is applied after harvest, the better the response in terms of eliminating greasiness and maintaining the acid levels, he said. He’s seen no negative effects of applying it right after harvest.

• It’s essential to condition the fruit for a week at 50°F before putting into storage at 37° to 39°F to avoid chilling injury. However, low temperatures in the orchard before harvest can also induce these disorders. If the chilling has happened in the field, there is nothing that can be done postharvest to prevent the disorders.

• The oxygen level should be 2 to 3 percent. Low oxygen is not necessary because the variety retains its firmness well, and there can be fruit injury from very low levels.

• The carbon dioxide level should be between 0.5 and 1.0 percent. Honeycrisp is not as sensitive to carbon-dioxide injury as some other apples.
The Fruit School was presented by Washington State University. •