It has become increasingly apparent that apple growers today are facing a variety of challenges related to determining how ripe their fruit actually is. Successfully picking fruit at the optimal time, depending on whether it will be sold direct to consumer for immediate eating, to supermarkets or other outlets where the traditional handling time is about three weeks of shelf life, or put into long-term storage, is the key to quality apples and to avoiding storage losses.
Christopher S. Walsh, professor of pomology, University of Maryland, and Tara Baugher, Penn State Extension tree fruit educator, are in the second year of a grower-funded study in which they are monitoring a variety of parameters, ultimately to provide growers with more accurate tools for determining ripeness.
The primary goal, Walsh said, is “to avoid storage losses,” a problem that is becoming more prominent as growers switch to smaller rootstocks, newer cultivars and high-density orchard systems. Combined with the ever-changing climate patterns being seen each season, as well as changing marketplace demands, growers are often challenged to determine when, exactly, to pick their fruit.
Apple maturity assessments
Recent apple maturity assessments, started in 2016, strive to identify physiological maturity of the apples. The hypothesis is that ethylene production spurs coordinated ripening of the apple, and that the normal standards for determining ripeness, such as soluble solids, flesh firmness and red color development, are not able to accurately predict internal ethylene levels.
Ethylene begins to form in the core area and increases slowly; it is believed to trigger genes in the nucleus of the cells. These genes then activate ripening-related proteins. Because ethylene levels in the fruit can only be measured with sensitive, expensive equipment, growers need other tools to help assess the actual physiological ripening happening in the fruit.
These other assessment tools “tell you things about the quality of the fruit, but they don’t necessarily tell you about the true maturity,” Walsh said. Soluble solids or red color are “not very good predictors” of ethylene levels and fruit ripeness. Over the past two decades, the industry has increasingly relied on starch testing, which “is probably the best correlated with ethylene.”
Coordinated fruit ripening happens when all the of cells of the fruit ripen simultaneously. In apples, “some varieties work better than others,” as far as a coordinated response to ethylene is concerned, Walsh said, complicating the issue of when to pick that fruit.
First in/last out
When picking apples, growers are often picking for direct sales of tree-ripened fruit. However, if they happen to have more tree-ripened fruit than they actually can sell for immediate sales, problems arise.
“People start picking tree-ripe fruit in August, but then if they can’t sell it, what’s left on the trees is already fully ripe, and doesn’t have a lot of storage life,” Walsh said.
The primary commercial storage market for these early-season apples doesn’t open up until April or May, when the price increases for these early-maturing varieties. These early varieties normally have the longest storage ability if picked before fully tree-ripened. Those early season Galas, if tree-ripened, will need to be sold by November, Walsh emphasized.
Tools such as Smart Fresh, which binds with ethylene receptors to prevent ripening in storage, won’t work if the fruit is too ripe when the product is applied, he said. After a certain point, the ethylene response isn’t going to be shut down, and fruit will be too soft for commercial sales after long-term storage.
Changes in ripening times
Part of the issue with ripeness today comes from changes seen in the market. Supermarkets used to offer only a handful of varieties but now want more choices. Some new favorites, such as Honeycrisp, ripen prior to Labor Day in the Mid-Atlantic region, competing with August peach sales, when consumers aren’t yet thinking about apples.
Growers are also experiencing changes in cultivation practices. Newer rootstocks and high-density orchard systems speed ripening. Combined with climate change – generally bringing hotter summer temperatures as well as changing rainfall patterns – and apples just aren’t ripening on the same timeline today as in the past. And today’s apple cultivars are often firmer than they used to be, too.
“As we’re making trees smaller, they are getting more sunlight in the field. They are getting more heat units,” Walsh said. Add in a hotter climate, and “all the new selections that people have been putting into these intensively managed systems,” and determining actual fruit ripeness can be a challenge.
One apple variety in particular, the Honeycrisp, has a reputation for causing confusion with ripening times, at least in the Mid-Atlantic region. Weak trees will never size apples properly, Walsh said, and if growers attempt to compensate by over-fertilizing trees, bitter pit will develop. Uneven ripening, often requiring four or five pickings per tree, also complicates the equation. Honeycrisp, developed in Minnesota and now the state fruit, is not well-adapted to Mid-Atlantic growing conditions.
Other apple varieties, such as the many Fuji strains, can have some quirks of their own. Some Fuji strains can be picked early, when very hard, and eaten immediately, with good taste and quality. They can also hang on the trees for another month and be tree-ripened with good results, Walsh said.