Consumers love Honeycrisp apples, so growers are planting more, especially in establishing new high-density orchards. While a substantial portion of the Honeycrisp crop is marketed shortly after harvest, the growing popularity of this variety means storage will have to improve.
Dr. Chris Watkins, postharvest researcher in the Cornell University Department of Horticulture, has been conducting studies on Honeycrisp storage throughout New York's three growing regions - western, Champlain and Hudson Valley - each with unique growing situations. "Champlain apples had higher acid and higher incidence of soft scald, and negligible bitter pit," Watkins said. "Hudson Valley apples had lower levels of soft scald, but much higher acid."
At a certain point during maturation, there's a rapid increase in autocatalytic ethylene production, Watkins noted. "That means a little bit of ethylene today means a lot more tomorrow, and much more the day after," he said. "The ethylene just keeps rising, which is normally associated with apples becoming soft, but also associated with nice things such as flavor development."
Although starch content is variable from region to region, Watkins said that in western New York, Honeycrisps are typically harvested with next to no starch. "Despite the fact that we aren't able to detect changes using standard measurements like ethylene or starch, firmness, soluble solids or titratable acids, there are lots of things going on in those apples," said Watkins. "We know that the flavor is changing, volatiles are changing, and the fruit is changing in its susceptibility to a range of physiological disorders."
Watkins listed bitter pit, soft scald, soggy breakdown, greasiness, skin wrinkling, blotch-like necrosis, and senescent breakdown as disorders common in postharvest Honeycrisp. As a postharvest physiologist, Watkins says he'd prefer working with a variety that has somewhat consistent issues, something Honeycrisp doesn't exhibit. "Another very strange feature of Honeycrisp is that we get the formation of volatiles such as ethanol," he added.
Part of the problem is that Honeycrisps are harvested close to or at maturity. "Honeycrisp is an apple that just doesn't soften on the tree. Because Honeycrisp maintains its texture, it remains harvestable much later than it should be," Watkins explained. "The solution would be early harvest; we have sufficient color, but we don't have that Honeycrisp flavor that the consumer wants and is willing to pay a fairly high price [for]."
While the apple is red, for Honeycrisp that doesn't mean it's fully ripe. The enzyme polygalacturonase is responsible for the softening of many apples, but that enzyme isn't present in Honeycrisp, and that's why more creative methods are needed to successfully store Honeycrisp.
Watkins said that holding the fruit at 50 degrees Fahrenheit for seven days prior to storage helps prevent soft scald. "There's nothing magical about 50 degrees. When we discovered this accidentally, it was because I had a 50-degree cold room available," he noted. "Many other storage operators are using slightly higher temperatures for a shorter period of time, or slightly cooler temperatures for longer periods." Watkins added that storage at 50 degrees Fahrenheit for a week essentially eliminates soft scald disorder.
Another issue is bitter pit. "Anything that will not cool the fruit down rapidly exacerbates bitter pit development," he said. "That is the dilemma. We thought the trade-off was worth it, because we thought we could control bitter pit in the orchard, but that is not the case. Even more of an issue is that nine months in storage resulted in a bland apple, likely due to loss of acidity."
Apples treated with SmartFresh showed little difference after three months in air storage, but after six months there was a significant improvement in flavor. SmartFresh also helps maintain acid levels.
In comparing controlled atmosphere (CA) storage to air storage, there were no significant changes after three months, while CA apples were superior after six months. However, CA storage isn't ideal for Honeycrisp due to the risk of internal carbon dioxide injury. "When you see a disorder where there are cavities in the fruit, that's a CA-related issue," said Watkins. "Honeycrisp in any region of New York will have a high risk of internal CO2 injury, which has prevented us from making that recommendation. This is also the case in Michigan, Maine and Pennsylvania." He added that preharvest growth regulator treatments, such as Harvista or ReTain, raise the risk of internal CO2 injury.
Watkins said that until more research is completed, growers' best bet for optimal storage is the use of SmartFresh with air storage. "There's very little effect of SmartFresh with CA storage, but air plus SmartFresh can be roughly equivalent to CA storage."
In other research, Watkins tried using DPA (diphenylamine) and DPA with SmartFresh in CA storage to help prevent superficial scald and other injury that comes with CA storage. "Honeycrisp doesn't respond like the 'normal' apple, in the sense that it maintains very low levels of ethylene compared with treatments where we used DPA or no treatment at all," explained Watkins. "We looked at firmness, and it increased slightly over time with delay.
"The unexpected result is that with DPA, we wiped out any carbon dioxide injury, eliminating the major limitation to using CA storage," said Watkins. "DPA may permit CA storage, and with no bitter pit problems."
The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.