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Much is still unknown about contamination risks from pathogens in fruit packing warehouses. Researchers across the country are working to learn more about the survivability of specific pathogens on different surfaces and how warehouses can improve processes to address the problem.

Case in point: No one really knows how easily Listeria monocytogenes, a highly deadly soilborne pathogen, survives on fresh apples in storage and whether die off changes by type of storage — refrigerated atmosphere, controlled atmosphere (CA), or CA with ozone.

Ozone, in particular, raises interesting questions, because it already has been shown to reduce microbes on surfaces in general, and there’s been exponential growth of ozone use in storage warehouses industrywide over the past decade to alleviate fruit rot and pests.

Researchers at Washington State University, in cooperation with the Washington State Tree Fruit Commission, aim to answer the Listeria die-off question for two apple varieties, Fuji and Granny Smith, in different storage situations.

The goal is to provide information for apple producers about the fate of Listeria on apples during cold storage in commercial facilities and to develop an intervention method to control Listeria contamination on apples during cold storage.

Initial findings were presented at the Washington State Tree Fruit Association annual meeting held in Wenatchee, Washington, in December.

The study

Mei-Jun Zhu

Mei-Jun Zhu

Led by Mei-Jun Zhu, WSU associate professor in the School of Food Science, the study examined inoculated apples in both laboratory and warehouse settings, using Listeria inoccua, a nonpathogenic substitute for L. monocytogenes.

In 2015, Zhu inoculated 560 Fuji apples with L. innocua and transported them to a Washington packing house, then she examined the apples for 18 weeks in three different types of storage: RA, CA and CA with ozone at 70 to 80 parts per billion.

In all three storage situations, the apples saw a significant reduction in Listeria in the first six weeks and only a gradual reduction over the next 12 weeks. However, the biggest drop in those first six weeks came in the CA room with ozone, where apples saw an almost 99.99 percent reduction in bacteria levels — one step short of what would be considered a “kill step” in the industry.

The RA storage and CA storage without ozone saw reduction of only about 99 percent.

Zhu stressed that these are early results and that more testing is needed before researchers can draw any conclusions.

Said Ines Hanrahan, project manager for the Washington Tree Fruit Research Commission: “This is the first study of its kind, and we are very pleased with the initial results. We are hoping to be able to repeat this study while including more orchards and apple varieties of concern, such as Granny Smith.”

In 2016, Zhu inoculated 750 Granny Smith apples with Listeria monocytogenes (aka the “real” pathogenic strain) in a WSU laboratory in Pullman, Washington, then stored the apples at different temperatures:

—1 degree Celsius (33 degrees Fahrenheit, a typical cold storage temperature).
—4 degrees C (36 to 38 degrees F, often employed for Honeycrisp).
—10 degrees C (50 degrees F, often used to temperature condition Honeycrisp in preparation for storage).

The study showed there was very little die off of bacteria in the first eight weeks, regardless of whether a low dose or high dose of bacteria was applied and regardless of temperature.

Zhu said the researchers were unable to establish a stable ozone level in the first two to three weeks of this study; conducting experiments in commercial facilities means there will always be some unexpected variables.

Overall, it’s too early to draw conclusions, Hanrahan said. Fuji and Granny Smith apples from the 2016 harvest are in storage for another year of study this year. •

Bacterial levels: The ‘log’ view

To understand bacterial contamination, it helps to know how researchers talk about an increase or reduction in bacterial levels.

Ines Hanrahan, project manager for the Washington Tree Fruit Research Commission, offered this guidance:
Say you have one bacterium on one apple and it increases to 10 bacteria. That would be considered a 1-log increase (in science-speak). An increase to 100 bacteria would be a 2-log increase, and so on, increasing by factors of 10 for each log increase.

Now for a reduction. Say you have 10,000 bacteria on an apple, and you reduce the bacteria levels by 90 percent.

That would be a 1-log reduction. A 99 percent reduction would be a 2-log reduction, continuing on to 99.9 percent, 99.99 percent and 99.999 percent, a five-log reduction that would, in the science world, effectively be considered a “kill step.”

Or to put it another way, say you have 10,000 apples. They’re all contaminated. If you clean the apples, but 1,000 are still contaminated, you’ve had a 1-log reduction.

A 2-log reduction would mean 100 apples are still contaminated, and a 3-log reduction would mean 10 apples are still contaminated. A 4-log reduction: Only one bad apple left in the bunch, but still short of a kill step.

– by Shannon Dininny