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A genetic discovery in the United States (US) has led to a new compound that could kick-start pear ripening and shows potential in sliced pears. For more than a decade, Washington State University geneticist, Amit Dhingra, has been trying to figure out the underlying genetics of pear ripening to design post-harvest products that could deliver consistent fruit quality.

 A series of research projects were launched in an attempt to understand the seemingly unpredictable effects of 1-methylcyclopropene (1-MCP), which blocks ethylene receptors. A new speciality crop block grant from the Washington State Department of Agriculture will help Dhingra and a commercial partner, Crunch Pak of Cashmere, to commercialise 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. We wanted to understand what happens in pears during conditioning, and most of the previous studies only looked at ethylene,” 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 does not tell the whole story for European pears.

Understanding genes and pear ripening

“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, pre-climacteric, in pears,” he said. Pre-climacteric 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 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.

“When pears 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 and appear to kick-start this AOX pathway, activating the better-understood ethylene-driven ripening process. It is different from most other climacteric fruits, where the AOX pathway exists but occurs after ethylene surges.

However, cold alone does not spur the AOX into action. Once they understood the pathway involved, the researchers searched for chemical compounds that might also trigger it, Dhingra said. They found one called glyoxylic acid that has been tested and patented. That genetic understanding could someday be used to inform breeding or genetic engineering to target pear ripening.

Potential for sliced pears

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.

“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 sliced pear application’s viability, which was well received by consumers in taste tests, he said. Crunch Pak primarily markets sliced apple products.

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 for 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 fruit spoiled too quickly in one bag, 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 dialling in the correct amount of the ripening compounds to apply for consistent, high-quality results. – Good Fruit Grower