Aug 4, 2020
Project taps plasma technology to disinfect wash water

A Drexel University research team is working to harness technology similar to that used in plasma televisions and fluorescent lights to develop an effective, non-chemical, low-cost wash water disinfection system.

Cold plasma is created when an electromagnetic field is used to excite electrons in a gas without raising its overall temperature. In a fluorescent light bulb, for example, the electrons are excited to the point where they emit visible light while the gas within the bulb remains at room temperature.

If you possibly have listeria or E. coli or salmonella in the wash water, you want to get rid of it, so you have to add chemicals that you may not want to,” said Alexander Fridman, Ph.D. and director of the C. & J. Nyheim Plasma Institute at Drexel University. “Cold plasma not only uses no chemicals but no thermal sterilization. And it’s more than that. It’s significantly less sensitive to organic loads.”

The project, titled “Post-harvest fresh produce wash water disinfection by submerged cold plasma non-chemical continuous treatment system,” seeks to adapt some of Drexel’s previous plasma-based water-treatment technologies to the fresh-cut produce industry. The goal is to reduce the potential for cross-contamination via wash water using a cost-effective plasma-based system.

What Fridman and his team have to demonstrate is that the technology’s use in a fresh-cut produce setting does not damage the fragile vegetables and yields an end product safe for human consumption.

“If we can produce fresh produce that’s safe without chemicals, it’s a big deal,” he said. “That, I think, will be the biggest impact.”

Joining Fridman as co-principle investigators from Drexel are Jasreen Sekhon, Ph.D., Department of Food and Hospitality Management; Christopher Sales, Ph.D., Department of Civil, Architectural and Environmental Engineering; and Alexander Rabinovich, Ph.D., an associate director of the C. & J. Nyheim Plasma Institute. Also involved are a handful of Drexel students.

As part of previous work, Drexel had developed an instrument known as the reverse vortex gliding arc plasmatron. When water is injected through the plasma stream, the ionized gas molecules initiate chemical reactions in the water that produce disinfectant compounds, such as ozone. 

These reactions are very short lived and the compounds quickly break down into harmless products, such as water and oxygen. But during that split second, the compounds deactivate pathogens in the water.

“We’re able to safely apply plasma to the water, and we’re able to kill microorganisms without any chemicals,” he said. “We’re able to generate very energetic plasma very safely.”

The plasmatron they’re using only requires about 3 kilowatts of electricity – slightly more than a refrigerator uses – making it economical for industry use, Fridman said. And unlike many wash water systems that rely on chemical disinfectants, such as chlorine, cold plasma is not significantly affected by organic loading or water pH.

A year into the project, the researchers have already conducted a proof of concept in the laboratory. They are now validating the technology with a prototype model using a 100-gallon (400-liter) tub into which the plasmatron electrode has been submerged. The system has been modified and carefully grounded for safety.

As part of the project, they will simulate increased organic loading seen in a fresh-cut processing facility. Plasma-treated water will also be used to wash both produce inoculated with a microbial cocktail of E. coli strains and non-inoculated items. 

Afterward, the produce and wash water will be tested for pathogens to determine the rate of inactivation. The washed produce also will be inspected and monitored for quality changes.

After the Drexel team optimizes the technology, Fridman said, the next step will be for project collaborator SmartWash Solutions LLC to install the pilot system in a commercial-scale fresh-cut wash system.

Center for Produce Safety

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