Researchers examine the effectiveness of disinfectants in the drain tanks of apple packing lines

A new research project is examining the effectiveness of sanitizers used in apple processing discharge tanks.

During processing, the apples are soaked in dump tanks and canal water systems. The water is generally reused for several days of treatment. According to the researchers, there is little data on the potential risks of discharge tanks from apple packing plants. Meijun Zhu, Ph.D., of Washington State University, is leading a project that she hopes will fill knowledge gaps with scientific data that apple packers can use to improve food safety plans or risk assessments.

The research project is titled “Validation of Washwater Sanitizer Disinfection in Drain Tank Operation of Apple Packing Lines” and is funded by the Center for Produce Safety (CPS).

“You can never totally eliminate or prevent cross-contamination, but we are trying to look at additional options to see if we can improve (disinfectant) efficacy,” Zhu said.

The researchers focused on Listeria monocytogenes, which has been linked to previous outbreaks associated with apples.

First results
The researchers completed bench-scale research in the laboratory, where they examined the effectiveness of three concentrations and three contact times of two standard disinfectants used in landfill tanks – chlorine and peroxyacetic acid ( AAP).

They inoculated water with three different concentrations of a cocktail of three Listeria strains and measured the number of pathogens recovered at three contact times.

To mimic conditions in packing plant discharge tanks, the researchers tested three types of water ranging from high organic matter to plain water. The highest biological rate was designed to simulate the worst-case scenario and was created using a recipe that involved soil from an apple orchard in Yakima, WA, and fluids from freshly picked apples and rotten apples.

In laboratory tests, chlorine at 25 ppm had limited effectiveness against Listeria in high organic content waste water. At 100 ppm, chlorine effectiveness was not affected by low levels of organic matter but was reduced by high organic loads.

The efficacy of PAA, on the other hand, increased with concentration and contact time and was only marginally influenced by organic loads. PAA at 40 ppm was more effective than 100 ppm chlorine.

“Both PAA and chlorine can reduce Listeria compared to no disinfectant,” Zhu said. “It certainly helps even if we couldn’t totally eliminate the pathogen, and it depends on the concentration.”

The next step
Next, the researchers will test the effectiveness of the disinfectant in a pilot landfill tank at Washington State University. The researchers will use the same recipe to simulate water from a landfill reservoir.

“These tests will be essential before moving to commercial packing plants,” Zhu said. “This [tank system] will be much smaller than a commercial operation but much larger than our bench scale tests. If the system is just water, this will be an easy assessment. Once you put in some fruit, it gets complicated.

The researchers plan to validate their findings at laboratory and pilot plant scale in three to four cooperating packing plants before the end of 2022.

About the SCP
The Center for Produce Safety is a 501(c)(3) non-profit organization. CPS is a collaborative partnership that leverages the combined expertise of industry, government, and the scientific and academic communities to focus on the research needed to continually improve food safety. This level of collaboration allows CPS to fill gaps in food safety knowledge and respond to both research priorities and immediate industry needs.

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