Good Fruit Grower / October 2008 / Soft fruit killing zone

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Soft fruit killing zone

Cold storage chilling injury is a cause of consumer dissatisfaction with stone fruits, a postharvest researcher says.

Cold-storage temperatures are not always kind to peaches and nectarines. Scientists identified several years ago that certain temperatures, dubbed "killing zone" temperatures, can result in internal fruit browning and mealiness. While growers and shippers are taking steps in the short term to minimize chilling injuries, genomic tools may someday offer long-term solutions.

Consumers can be dissatisfied with their stone fruit purchases of peaches, plums, and nectarines for a number of reasons, said Dr. Carlos Crisosto, postharvest physiologist at the University of California's Plant Sciences Department, Davis, located at the Kearney Agricultural Research Center in Parlier. He cited a survey of European consumers in which 30 percent said that stone fruit had little flavor, 21 percent said fruit were too hard, with 14 and 15 percent mentioning that fruit never ripen or are mealy.

The European survey is significant because Europeans eat three times more peaches than U.S. consumers, but peach consumption in Europe has declined since the 1990s.

Crisosto believes that much of the decline in consumption is linked to cold-storage chilling in­juries that can occur in sensitive varieties exposed to temperatures between 36 and 50°F. "We know that temperatures are part of the problem," Crisosto said, adding that varieties susceptible or sensitive to low temperatures show internal browning, mealiness, bleeding, and lack of juiciness and flavor when they are held in killing-zone temperatures during storage.

"From 36° to about 50°F is the killing temperature range," he said. "Temperatures in that range reduce the storage life of sensitive varieties down to only two weeks. But if you keep the fruit at temperatures close to 32°F (0°C), you'll have a longer shelf life."

He worries that with stone fruit now exported around the world, the length of time it takes to deliver fruit to consumers has expanded. "It used to be that our market window was between 14 and 25 days," he said. "But now, with some locations, it can take 35 to 40 days to get fruit into distribution channels. We're pushing the market life of stone fruit varieties, especially peaches, to the extreme."

Susceptibility

Through years of study, Crisosto found that not all cultivars exposed to killing-zone temperatures exhibited the same chilling injury symptoms. In his screening of varieties for sensitivity to chilling injury, Crisosto found that cultivars could be grouped into three categories:

• Type A cultivars did not develop chilling injury at either storage ­temperatures of 32°F or 41°F;
  
• Type B varieties showed no symptoms at 32°F but did develop chilling injury when stored at 41°F; and,
  
• Type C cultivars developed symptoms when exposed to both 32° and 41°F.

Crisosto has screened older and newer varieties, with older varieties dating back to the 1970s and more modern varieties released in the early 2000s. Though most of the older varieties were sensitive to killing-zone temperatures and internal browning during storage, the newer varieties were more resistant. Among the ­cultivars evaluated for chilling injury in the mid-1990s, about 25 percent were classified as Type A and did not develop chilling-injury symptoms at either 32° or 41°F, and 62 percent were classified as Type C (sensitive at both temperatures).

Evaluations of cultivars from the early 2000s showed that 30 percent fell into the Type A category, while only 42 percent were Type C.

It is important to point out that recent peach releases from California breeding programs are less prone to chilling injury, he said. "What's happening is that nurseries are looking at this storage quality issue. Today, more than 65 percent of the varieties are in the right category. We do have genetic material now available that is more appealing to consumers. We are moving forward."

Solutions

Crisosto, who discussed his research during the International Fruit Tree Association's annual conference in Visalia, California, said that short-term solutions to chilling injury include choosing cultivars that are resistant to the killing-zone temperatures, following postharvest temperature and handling guidelines to avoid the killing zone, and implementing ripening and preconditioning protocols.

He added that research has helped packing houses and warehouses better manage cold storage temperatures. In the last eight years, California shippers have begun to preripen fruit for retailers, picking fruit at specific pressures and holding it at designated temperatures to deliver riper fruit to consumers. However, when fruit moves fast through the distribution channel, retailers don't always have time to follow the ripening protocols, which require holding fruit for a few days in ­specially designed ripening centers.

Experiments with the use of the ethylene inhibitor MCP (1-methylcyclopropene) showed no improvement of peach and nectarine quality, Crisosto said. "In fact, there is damage on 1-MCP–treated peaches after cold storage, as peaches will not ripen properly. In plums, there is a very limited benefit of improving quality during storage,"

Moreover, MCP was very cultivar specific, he said. Each variety responded differently, and responses were very erratic.

Studies using controlled atmosphere to improve stone fruit quality during storage have also shown limited success.

"Currently, the best treatment to protect stone fruit quality during storage is the proper application of a preconditioning treatment [holding fruit for 48 hours at 68°F] immediately after harvesting," Crisosto said.

Long-term solutions to chilling injuries will come from genomics and breeding varieties that are free from chilling ­problems, Crisosto stated.

UC molecular genetics specialist Dr. Ebenezer Ogundiwin is looking for genetic markers that can be used early in the breeding of new varieties to help identify which cultivars are free of chilling temperature susceptibility and other postharvest quality problems.

Ogundiwin is part of a research team that has constructed a partial genetic linkage map for peaches, locating genetic factors that control fruit chilling injuries, such as mealiness, browning, and bleeding. Their goal is to develop molecular tools that can be used for efficient marker-assisted breeding of peaches.