The caller was beside himself. He thought he had done everything according to the book, but this longtime Michigan grower and packer had lost a whole roomful of Honeycrisp apples to chilling injuries.
“He had never seen anything like it before,” recalled Randolph “Randy” Beaudry, the professor in the Michigan State University Department of Horticulture who received the call.
That is just one example of postharvest problems with this variety, said Beaudry, who last year saw about $1 million in Honeycrisp lost at a large shipping house in Yakima, Washington, and it came as a complete surprise to the storage manager.
“Again, the storage operator was fit to be tied. He didn’t understand what he’d done wrong,” he said.
Honeycrisp apples are prone to interior browning, surface irregularities and other damage at multiple stages after picking, but with care and some new recommendations based on MSU research, even these especially finicky apples can run the postharvest gauntlet damage-free, Beaudry contends.
He and his research group have been studying Honeycrisp storage in earnest since 2008, when the apple industry chipped in to help Beaudry’s laboratory acquire a 32-chamber system that allows testing of various combinations of temperatures, gas concentrations and postharvest treatment options.
Through continued experiments, including one earlier this year that replicated the presumed conditions in the Yakima shipper’s facility, the research group is constantly fine-tuning its guidelines for Honeycrisp storage.
Based on the findings of those experiments, Beaudry said one of the first things growers must do after harvest is to desensitize the fruit to the low temperatures used in storage by conditioning them in a temporary holding area or in the storage room prior to cooling. “Because Honeycrisp is an unusual apple in that it doesn’t seem to soften much at all in storage, we can hold them at an elevated temperature for several days, and they’re not going to go soft,” he said.
A common method for conditioning, holding the fruit at 50 degrees Fahrenheit for between five and seven days, provides chilling desensitivity. The MSU experiments in 2016 confirmed that intense conditioning, utilizing a temperature of 70 degrees F for three to five days, is also effective.
At the same time, growers also must be careful to keep carbon dioxide levels below 1 percent. Levels of this gas can increase rapidly because the apples are still respiring — taking in oxygen and giving off carbon dioxide.
“If you close the door on that room overnight and the carbon dioxide levels build up, then you can destroy your crop in the conditioning phase before it ever goes into storage,” he said. “We’ve done some work this year trying to characterize that, and we found that in as little as 12 hours at a carbon dioxide concentration that is known to be damaging, you can lose a very significant portion of your fruit.”
Warm fall weather conditions can also affect conditioning. An unusually warm autumn in Michigan in 2016 caused many growers to leave their Honeycrisp crop on the trees longer than normal so the apples could develop color. As a result, the apples were a bit over-mature at harvest, and that causes them to become even more sensitive to chilling and to not store as well.
For such late-picked Honeycrisp, Beaudry suggests three things:
—Condition the fruit at 50 degrees F for the full seven days.
—After a day or two in the conditioning room, treat them with 1-methylcyclopropene (1-MCP) at 1 part per million for 24 hours to slow the ripening process.
—Maintain carbon dioxide levels below 1 percent throughout conditioning.
Following conditioning, fruit then goes into air or controlled-atmosphere storage at 36 to 38 degrees F. The addition of 1-MCP is useful to inhibit ripening. “Because 1-MCP is a gas, you have to seal up the room.
That’s an issue because the fruit are still comparatively warm coming as they are from the conditioning treatment, which means that their respiratory rate is still very high so they’re releasing carbon dioxide,” Beaudry said.
Hydrated lime can absorb carbon dioxide and it works very well, but he has seen many instances where someone has been rushed and either forgotten to add it or left it in the wrong room.
“For the first 30 days, you have to keep carbon dioxide levels low (below 1 percent) and oxygen at 1.5 to 3 percent to avoid those sensitivity issues,” he said. “If you treat Honeycrisp like any other apple and give it 3 percent carbon dioxide — a level that is perfectly fine for most apple varieties — you will get at least 20 to 30 percent losses.”
Even with hydrated lime, carbon dioxide levels can sometimes creep high enough to cause browning in Honeycrisp.
The only surefire way to prevent that injury is to use the antioxidant material diphenylamine (DPA), but financial constraints or market demands for organic fruit may discourage this option, he said.
Once Honeycrisp has reached 30 days postharvest, Beaudry says carbon dioxide levels can be increased to 3 percent with oxygen concentrations of 1.5 to 3 percent. His storage-temperature recommendations remain at 36 to 38 degrees F for now, but preliminary experiments suggest that slightly lower temperatures may be suitable following some conditioning scenarios.
The bright side of Honeycrisp’s delicate nature is that it is showing growers and shippers the importance of postharvest diligence, a factor that may prove critical for understanding the best way to condition and store the many new and soon-to-come varieties that have Honeycrisp in their parentage. While storage damage in these new varieties hasn’t been documented yet, he said, time will tell.
Until then, Beaudry’s research is helping growers and shippers learn how to avoid making missteps that lead to Honeycrisp losses.
His group replicated the conditions that preceded the $1 million apple loss at the Yakima shipper’s storage facility, and it’s believed the problem was a very short span of elevated carbon dioxide.
“Just a few hours of carbon dioxide at a slightly high level were enough to induce the same type of damage that we saw in Washington. In that case, treatment with DPA would have completely protected the apples,” Beaudry said.
For the Michigan grower who called about losing a roomful of Honeycrisp after he felt he had done everything right, faulty monitoring equipment was to blame.
“We made a couple of suggestions of what he might do next year, and that includes making sure that his darn temperature equipment is accurate,” Beaudry said. “I have a feeling they were just a little below the threshold, and if you’re off by just 1 degree, you can lose hundreds of thousands of dollars of fruit.”
Research on Honeycrisp is far from over, especially since growers continue to add it to their orchards.
“As production increases and additional fields are planted, we have more variables whether that be the weather or the soils, or the individual treatments from one grower to another,” he said. That all adds up to uncertainly in terms of storability, and offers new avenues for research and ultimately better fruit.
In addition to growers and packers, financial supporters of this project include the Michigan Apple Research Committee, the Michigan State Hort Society, MAES (now AgBioResearch) and Project GREEEN, Michigan’s plant initiative, among others. •
– by Leslie Mertz