Study: Overhead cooling does not appear to impact the survival of E. coli on apples
Geraldine Warner, Shannon Dininny // March 10, 2017
Gala apples are sprayed with overhead sprinklers near Prosser, Washington, in the heat of the summer. (TJ Mullinax/Good Fruit Grower)
As growers prepare to implement new food safety requirements in their orchards, several lingering questions remain.
Among them: Can water they spray to cool their crop impact the survival of contaminants?
In Washington, many growers spray water from overhead sprinklers and misters during summer’s hottest months to create evaporative cooling to control sunburn. They typically begin cooling in July and continue through harvest for Gala and Golden Delicious apples.
Under the Food Safety Modernization Act, growers who apply untreated surface water to their crop can use water that exceeds generic E. coli standards as long as they wait for a certain percentage of the microbes to die off before harvesting the crop.
But what is the die-off rate of E. coli on apples, with and without evaporative cooling, and how could that affect harvest times? Turning off water even for a day close to harvest could lead to vastly increased rates of sunburn and cause significant economic losses to growers.
Researchers in Washington aimed to answer that question to determine just how big a concern it should be for the industry.
“Of course, we knew from the beginning, if we’re doing field studies, we are dealing with a very complex system. We have to consider an abundance of variables,” said Ines Hanrahan, project manager for the Washington Tree Fruit Research Commission.
They include variation between apple varieties, the developmental stage of the fruit, orchard location, weather, the location of each piece of fruit within the tree, the type of cooling system used and the amount of inoculum applied.
“And of course, there’s lots of year-to-year variability,” Hanrahan said.
Overall, the study showed that growers don’t need to worry: Overhead cooling does not appear to impact the survival of E. coli on apples, though some of those variables warrant additional review. More data crunching remains.
Control and inoculation
Hanrahan and researchers Meijun Zhu of Washington State University and Karen Killinger, formerly of WSU and now with the Food and Drug Administration, played roles in part or all of the study, which first required an examination of harvested apples to determine the background level of pathogens appearing naturally on fruit.
They examined a total of 465 apples, representing the untreated control fruit, for total coliforms, generic E. coli as well as pathogenic E. coli and salmonella over the three years of the study.
None of the apples tested positive for pathogenic E. coli; 37 apples, or 8 percent, had detectable levels of generic E. coli, and 174 apples, or 37 percent, had detectable levels of total coliforms.
Two apples tested positive for salmonella, both Fujis from WSU’s Roza research orchard in Prosser, Washington.
(Hanrahan noted that water for overhead cooling is stored in a pond there, which can occasionally pose problems because the orchard sits at the end of the irrigation line.)
In order to study die-off rates, though, the researchers had to develop methods of inoculating fruit with nonpathogenic, surrogate organisms that are similar to pathogenic E. coli.
These methods were developed in collaboration with University of California, Davis scientists.
Then, to ensure that die-off rates could be accurately measured and studied, much higher populations of organisms were applied to the fruit — mature Gala, nearly mature Golden Delicious and immature fruit of both varieties for each of the three years, and mature and immature Fuji for the latter two years of the study — than would normally be found.
They inoculated fruit in research orchards, not commercial orchards, and in both cooled and uncooled plots to find out whether water promoted growth of pathogens or helped with die-off.
In the cooled plots, water was applied at a rate of 35 gallons per minute per acre on a 15-minutes on, 15-minutes off schedule between noon and 6 p.m. on days when the temperature exceeded 90 degrees F.
The scientists set up the worst-case scenario for potential contamination, applying overhead cooling until sunset and picking fruit early the next morning.
Sample times differed slightly between years, but generally, fruit were sampled for up to one week after inoculation at the following times: after two, 10, 18, 34, 42, 58, 82, 106 and 154 hours.
For Galas and Golden Delicious, the trial showed that applying overhead cooling after inoculating the fruit with generic (nonpathogenic) E. coli made no significant difference to the die-off rate.
Generally, the greatest reduction rate of generic E. coli on apples occurred within the first eight to 10 hours after inoculation — between 99 and 99.9 percent of the microorganisms died off — with additional reductions at a slower rate over the next four days.
However, in general, the reduction in generic E. coli on Fuji apples during the first 10 hours occurred at a slower rate than the other two varieties.
This finding is important because there may be differences in generic E. coli survival on apples harvested later in the year, Hanrahan said, which is relevant for growers harvesting late season varieties — particularly if they use any overhead cooling or irrigation.
Hanrahan said although most of the organisms die off as soon as the apple is dry, die-off continues over the course of a week; in slow drying conditions, die-off takes longer.
The Food and Drug Administration is proposing a requirement of 0.5 log reduction in colony forming units per day, for a maximum of four days, to determine the number of days a grower must wait to harvest the crop.
(On the logarithmic scale, a 1-log reduction would be a decrease in bacteria from 100,000 to 10,000. A 0.5-log reduction would decrease from 100,000 to 32,000.)
“Evaporative cooling did not affect the rate of die-off, and I’m very happy to report that for Gala and Golden Delicious, we had a rate that was greater than 0.5 log per day, as proposed by FDA, so that’s very good,” she said during the Washington Tree Fruit Commission’s Apple Horticulture and Postharvest Research Review in January.
The research also showed that the type of overhead cooling system — spray or mist — and the inoculum level did not have a difference in the die-off rate. There also was no difference in the rate between mature and immature fruit.
In terms of the variables that could play a role in die-off rates, the study showed that the type of canopy and fruit position within the canopy have the potential to impact microbial survival, warranting further investigation.
Funding for the study follows: $294,433 from the Washington Tree Fruit Research Commission, $80,768 from the Western Center for Food Safety and $45,304 from a Washington Specialty Crop Block Grant. •
Shannon Dininny is the managing editor of Good Fruit Grower. She writes articles for the print magazine and website and plans and prepares editorial content. -- Follow the author: Office (509) 853-3522 Cell: (509) 834-5321 -- email