A rethink of 1-MCP and DCA

How are new cultivars shaping the adoption and application of 1-MCP and DCA by the North American apple industry? By Anna Mouton.

The use of 1-MCP (1-methylcyclopropene) and DCA (dynamic controlled atmosphere) by the New York State apple industry is changing rapidly, according to Prof. Chris Watkins, postharvest specialist in the Horticulture Section of the School of Integrative Plant Science at Cornell University.

Speaking at the recent Hortgro Science Postharvest Symposium, Watkins described how new cultivars have changed the postharvest landscape due to their unique physiologies and susceptibility to novel storage disorders.

The McIntosh era

Both 1-MCP and DCA became commercially available in the United States in the early 2000s. Many of the traditional cultivars then in production tended to soften rapidly after removal from storage. The improved texture and acidity of 1-MCP-treated fruit were immediately popular.

“The long-term advantages of 1-MCP through the entire marketing chain gave it huge leverage,” said Watkins. “And it was easy to apply. In New York, we have a lot of cold stores that are not super airtight along the lines you need for DCA.”

Watkins illustrated the benefits of 1-MCP with results from storage trials with McIntosh. The apples were either untreated or treated with 1-MCP and stored for either four or eight months in CA (controlled atmosphere) or DCA. Flesh firmness was measured after one and seven days at room temperature following storage.

Apples treated with 1-MCP retained their firmness after seven days. In untreated apples, firmness dropped significantly by seven days, regardless of whether they were stored in CA or DCA.

“DCA did a nice job in storage, but unless you had treated the fruit with 1-MCP, it had a rapid loss of condition once you took it out,” said Watkins.

Furthermore, DPA (diphenylamine) was (and remains) available in the United States, where it is used for controlling superficial scald and carbon dioxide injury. As New York State produces little organic fruit, there wasn’t a strong case for adopting DCA over DPA and 1-MCP.

World-changing Honeycrisp

“Honeycrisp revolutionised the apple industry,” said Watkins. “Our new world is one with Honeycrisp, its progeny, and other firm apples.”

He presented data from storage trials with Honeycrisp and Fuji, using the same experimental design as those for McIntosh. Flesh firmness remained constant between one and seven days at room temperature, whether stored in CA or DCA, or treated with 1-MCP or not.

“The texture of the fruit stays the same,” said Watkins. “However, we know that Honeycrisp treated with 1-MCP is more acidic and will have a better eating quality.”

But the bottom line, as he pointed out, is that flesh softening in these newer cultivars is not a postharvest concern in the same way as for McIntosh.

The sensory laboratory at Cornell had taste panellists rank their overall liking of four cultivars stored for six or nine months in CA or DCA. The CA-stored apples were divided into 1-MCP-treated and untreated subgroups.

Honeycrisp and Fuji achieved the same ranking regardless of whether they were stored in CA or DCA, or treated with 1-MCP or not. Panellists reported significantly greater liking for Golden Delicious and Jonagold apples stored in DCA or treated with 1-MCP and stored in CA compared with the untreated apples stored in CA.

Golden Delicious stored for six or nine months in either CA or DCA softened significantly when not treated with 1-MCP, although untreated apples stored in DCA were firmer than those stored in CA.

Gala and browning

During the reign of Honeycrisp, pack houses had little incentive to adopt DCA. “The big change for our industry in terms of interest in DCA was Gala,” said Watkins. “With increasing Gala volumes, we are seeing a range of physiological disorders.”

Galas have proved susceptible to core, flesh, and stem-end flesh browning. Results of storage trials demonstrated that 1-MCP was ineffective in preventing flesh browning in Gala apples stored at 0.5 °C and 2% oxygen. Storing Gala at 3 °C and 0.5% oxygen controlled browning regardless of whether 1-MCP was applied.

When it comes to browning disorders, Watkins thinks apples fall into three broad categories. Some, like Gala, benefit from low oxygen and therefore DCA, but are not affected by 1-MCP. Some, like NY1, benefit somewhat from DCA and may develop worse browning when treated with 1-MCP.

“Enhanced browning with 1-MCP is a relatively new phenomenon that we’re finding in newer varieties,” said Watkins. “All our early work was done with traditional varieties, and we never saw issues with 1-MCP causing more browning.”

The third category includes cultivars, such as Cripps Pink, where DCA and 1-MCP appear not to affect browning.

“Cripps Pink is a very unusual apple,” reflected Watkins. “Its susceptibility to storage disorders is affected by the growing region. We’ve just started working with it for the first time, so we’re well behind South Africa in terms of understanding this apple.”

New cultivars keep coming

“We have so many club varieties you can’t turn around without finding a new one,” quipped Watkins. “In a way, it’s a little bit out of control.”

Figuring out the response of new cultivars to DCA is complicated by their different cooling requirements, as slower cooling to control browning can result in reduced firmness. Countries also have different cooling protocols.

“In New York, few storage operators use stepwise cooling, or if we do, it’s because our refrigeration capacity isn’t all that good,” said Watkins.

Nonetheless, he believes that DCA adoption is increasing in cultivars where it’s economically justified, although airtight storage rooms remain a limitation. Currently, some pack houses combine ultra-low oxygen and DCA-based technology, with the DCA sensors serving as an insurance against unexpected fruit stress.

Should the United States apple industry lose DPA, DCA would become an important tool for controlling superficial scald. However, DPA also currently controls carbon dioxide injury, so loss of DPA would necessitate major adjustments to the management of fruit in CA and DCA. For example, very low carbon dioxide concentrations would have to be maintained during the first month when fruit are most susceptible to injury.

In addition, while 1-MCP helps prevent superficial scald, it can aggravate carbon dioxide injuries in some cultivars.

Watkins concluded by emphasising that new cultivars will always bring new challenges. Even though researchers and producers have studied apples for a hundred years, they still don’t have all the answers. “Every cultivar is like a new fruit type,” he said.

Watch Watkins’ presentation on the Hortgro YouTube channel.