A balancing act between crispness & carbon dioxide

Two research teams are tackling one of the trickiest puzzles in apple production. In the presentations “EverCrisp, but not everlasting: Addressing internal browning issues in ‘EverCrisp’ apples” and “‘Honeycrisp’ internal browning: Chilling or CO2 injury?” Özge Horzum (Ankara University), Nobuko Sugimoto (University of Georgia), Randolph Beaudry (Michigan State), Emily Lavely (West Central Research Station, Michigan) and Nikki Rothwell (Northwest Horticulture Research Station, Michigan) explored why two premium apple varieties can develop disappointing internal browning during storage. Their work offers fresh clues that could help growers protect fruit quality from harvest to consumer.

The challenge begins long before an apple reaches the grocery store. Modern consumers expect crisp texture, sweet flavor and flawless flesh no matter when the fruit was picked. Meeting those expectations often means storing apples for months under carefully controlled conditions. Those same storage systems preserve quality but can also create hidden hazards that remain invisible until the fruit is cut open.

EverCrisp® perfectly illustrates the problem.

Released through the Midwest Apple Improvement Association breeding program, EverCrisp quickly earned a reputation as an exceptionally firm, extremely sweet apple that barely softens after harvest. Those qualities make it an attractive choice for both fresh markets and processing. The catch is that the variety may need to spend as long as a year in controlled atmosphere storage to satisfy year-round demand. The longer the stay, the greater the risk that internal browning will spoil an otherwise outstanding apple.

To better understand those risks, researchers first searched for the lowest oxygen concentration EverCrisp could safely tolerate. Their experiments showed the fruit could withstand oxygen levels of about 0.7%. They then tested a range of oxygen and carbon dioxide combinations that simulated commercial controlled atmosphere storage.

The results revealed a careful balancing act. Lower oxygen slowed ethylene production and helped preserve firmness while CO2 had little effect on those traits. Acidity, which contributes to the bright, tart flavor consumers enjoy, also held up slightly better under very low oxygen conditions.

Unfortunately, the same storage environments influenced several physiological disorders including CO2 injury, core browning, watercore dissipation and internal browning. Although ordinary air storage produced fewer disorders, the fruit became bland and overripe, losing much of the quality that makes EverCrisp so appealing.

The search now turns toward plant growth regulators that may encourage beneficial ethylene activity while limiting browning.

Honeycrisp presents a different but equally puzzling problem.

For years growers have debated whether its notorious internal browning is caused mainly by exposure to chilling temperatures or by CO2 during storage. Untangling those competing explanations could help orchard managers make smarter storage decisions.

The evidence increasingly points toward CO2 as the chief culprit.

Researchers demonstrated that CO2 injury rises steadily as storage concentrations increase and that the damage is largely complete within five or six weeks after harvest.

They also discovered something even more surprising. The antioxidant diphenylamine successfully suppressed CO2 injury even when apples were stored in ordinary air with virtually no CO2 added to the storage room. That finding suggests apples may generate enough internal CO2 on their own to trigger injury, making the disorder far more complicated than previously thought.

The team also examined the role of chilling. Conditioning fruit before cold storage or storing it at slightly warmer temperatures effectively reduced classic chilling disorders such as soft scald and soggy breakdown. Yet those treatments did not eliminate internal browning. Measurements taken in commercial orchards showed that autumn temperatures rarely fall low enough during harvest to mimic damaging storage conditions. Together, those observations strengthen the case that CO2 injury rather than chilling is the greater threat for Honeycrisp.

Although the two apple varieties behave differently, both studies point toward the same practical lesson. Successful storage requires more than simply lowering temperature or adjusting oxygen. Fruit physiology is remarkably complex and even subtle shifts in storage atmosphere can influence flavor firmness and hidden disorders.

For growers, packers and consumers, the payoff could be substantial. Better storage strategies would reduce waste, improve eating quality and help premium apples live up to their promise months after harvest.

By peeling back the mystery of internal browning, these research teams are helping ensure that crisp stays crisp, sweet stays sweet and great apples remain great long after they leave the orchard.

by Enrico Villamaino