For more than a decade, Washington State University geneticist Amit Dhingra has been trying to figure out the underlying genetics of pear ripening in order to design postharvest products that could deliver consistent fruit quality.
A series of research projects that began with trying to understand the seemingly unpredictable impacts of 1-MCP (1-methylcyclopropene, which blocks ethylene receptors) on pears and find postharvest products that would deliver consistent fruit quality are now starting to fit together like the pieces of a puzzle, Dhingra said, with new tools to spur ripening and a genetic understanding of how and why they work.
A new Specialty Crop Block Grant from the Washington State Department of Agriculture will help Dhingra and a commercial partner, Crunch Pak of Cashmere, to commercialize the use of the ripening tools on 1-MCP-treated fruit in a sliced pear application that Dhingra believes could create a new pear market.
“This is a unique niche where 1-MCP-treated fruit can be put into this category,” Dhingra said. “Before we replant the entire acreage of pears, we need to bring money into the industry. A toolkit of 1-MCP plus the ripening compound can make that possible.”
A new ripening 101
On the research front, Dhingra calls pears “a great teacher” for studying which genes control fruit ripening.
“We wanted to understand what is happening in pears during conditioning, and most of the previous studies looked at ethylene only,” Dhingra said. Ripening fruit produce ethylene, which then drives most of the ripening process in climacteric fruit — fruit that continues to ripen after harvest — such as pears. Many Anjou pears are now ethylene-treated to promote more uniform ripening just before delivery to customers. But, as Dhingra’s team found, ethylene doesn’t tell the whole story for European pears.
“When we started looking at a whole suite of genes beyond ethylene, to our surprise we found there is a metabolic pathway that gets activated, preclimateric, in pears,” he said. Preclimateric refers to the fruit before the surge in ethylene that occurs during ripening.
Looking at dozens of genes in freshly picked and ripe pears, to see which played a role in ripening, Dhingra and his graduate students, Seanna Hewitt and Christopher Hendrickson, zoomed in on some unexpected gene activity — in what’s known as the alternative oxidase pathway — that started in both Bartlett and Anjou pears during cold conditioning.
“Pears, when they are harvested, they don’t have the entire energy currency they need to drive the starch conversion to sugar,” Dhingra said. “This alternative respiratory pathway produces the energy to kick-start the ripening pathway.”
The cold temperatures that pear packers have long known are key to successful ripening appear to kick-start this AOX pathway, which then activates the better understood ethylene-driven ripening process. That’s different than most other climacteric fruits, where the AOX pathway exists but occurs after ethylene surges.
However, cold alone doesn’t spur the AOX into action. Once they understood the pathway involved, the researchers went searching for chemical compounds that might also trigger it, Dhingra said. They found one called glyoxylic acid, about five years ago.
Since then, they’ve tested it, patented it and published several scientific papers proposing the underlying mechanisms. That genetic understanding could someday be used to inform breeding or genetic engineering to target pear ripening.
But the road from promising research to commercial use is long.
Longtime pear research advisor Bob Gix said he’s excited about Dhingra’s findings but also cautious, considering the challenges of proving new tools in a commercial setting.
“These new fundamental understandings represent a fantastic building block for ripening in pear fruit with the goal of delivering a more consistent high-quality product to the consumer,” Gix said. “The greatest endorsement of these discoveries will be common industry use. I do not believe we are there yet.”
Packaging the potential
On the commercial front, the most promising application so far seems to be in sliced pears, where exposed flesh treated with glyoxylic acid consistently overcomes the ripening pause created by 1-MCP. Ray Schmitten, a grower serving on an industry research advisory committee, suggested the idea.
“Everybody would like to see (sliced pears), but it’s not as easy as we would like it to be,” Schmitten said, reflecting on many years of industry-funded research into the feasibility of sliced pears. The warehouse he works for, Blue Star Growers, provides fruit to Dhingra’s research trials.
“It’s something obvious in hindsight, but much to our surprise, we saw a huge jump in ethylene when we applied (glyoxylic acid) to sliced 1-MCP fruit,” Dhingra said. It would allow packers to treat and store pears for subsequent slicing and ripening so the consumers get the convenient fruit at peak quality.
With funding from the Fresh Pear Committee, Dhingra partnered with Crunch Pak several years ago to test out the viability of the sliced pear application, which was well received by consumers in taste tests, he said. Crunch Pak, which primarily markets sliced apple products, did not return requests for comment.
But the team also discovered that packaging plays a significant role in the quality of the final product, Dhingra said. Modified atmosphere plastics regulate gas exchange to keep produce fresh longer.
“In the modified atmosphere bag world, you have to find a bag that corresponds to the respiration rate of the fruit in question,” Dhingra said. He found that in one bag, fruit spoiled too quickly, and in another, the fruit moved toward ripening too slowly.
The new grant will enable testing of four different modified atmosphere bags to find the “Goldilocks fit” for sliced Anjous, he said, as well as dialing in the correct amount of the ripening compounds to apply for consistent, high-quality results.
Part of the challenge in commercializing the glyoxylic acid is that for whole pears, the amount needed depends on the amount of 1-MCP the fruit was treated with. That’s a surprisingly hard thing to measure, as warehouses treat pears with 1-MCP in parts per billion, as opposed to the parts-per-million rates common in apples.
“It’s a technical challenge. When you are looking at a parts-per-billion application changing physiology, any small variation can have a significant effect,” he said.
The use of 1-MCP has become controversial in pears in recent years, with some packing houses swearing off it and others working with longtime manufacturer AgroFresh to dial in the dosing to account for growing conditions, harvest timing and storage goals to optimize performance, commercial development manager Fernando Edagi said. “1-MCP is not a one-size-fits-all solution.”
Schmitten, who took a strong stand against overuse of 1-MCP, now says the product “has its place” in long-term storage, and applications such as pairing with this novel ripening compound could be another beneficial use.
Dhingra hopes his new compound can help packers take advantage of the benefits of 1-MCP and reduce the drawbacks, saying it basically exploits “this back door that nature showed us to use these metabolic compounds to overcome the limitations of 1-MCP.” •
—by Kate Prengaman