A Drexel University research team is developing a cold plasma wash water treatment that could kill harmful pathogens and be used with delicate fresh produce with no adverse quality effects, low cost operations and no added chemicals. This is a potentially huge breakthrough as delicate fresh produce, like romaine lettuce, is difficult to clean and can contain potentially harmful pathogens.

If there is any question about how meaningful this type of washing treatment could be, here is a chart of delicate fresh produce E. coli outbreaks since 1995:

Date Vehicle Etiology Cases States &
July 1995 Lettuce
(leafy green; red; romaine)
E. coli O157:H7 74 1:MT
Sept. 1995 Lettuce (romaine) E. coli O157:H7 20 1:ID
Sept. 1995 Lettuce (iceberg) E. coli O157:H7 30 1:ME
Oct. 1995 Lettuce
(iceberg; unconfirmed)
E. coli O157:H7 11 1:OH
May-June 1996 Lettuce (mesclun; red leaf) E. coli O157:H7 61



May 1998 Salad E. coli O157:H7 2 1:CA
Feb.-Mar. 1999 Lettuce (iceberg) E. coli O157:H7 72 1:NE
Oct. 1999 Salad E. coli O157:H7 92



Oct. 2000 Lettuce E. coli O157:H7 6 1:IN
Nov. 2001 Lettuce E. coli O157:H7 20 1:TX
July-Aug. 2002 Lettuce (romaine) E. coli O157:H7 29 2:WA, ID
Nov. 2002 Lettuce E. coli O157:H7 13 1:Il
Dec. 2002 Lettuce E. coli O157:H7 3 1:MN
Oct. 2003-May 2004 Lettuce (mixed salad) E. coli O157:H7 57 1:CA
Apr. 2004 Spinach E. coli O157:H7 16 1:CA
Nov. 2004 Lettuce E. coli O157:H7 6 1:NJ
Sept. 2005 Lettuce (romaine) E. coli O157:H7 32



Sept. 2006 Spinach (baby) E. coli O157:H7 and other serotypes 205


and Canada

Nov./Dec. 2006 Lettuce E. coli O157:H7 71



Nov./Dec. 2006 Lettuce E. coli O157:H7 81



July 2007 Lettuce E. coli O157:H7 26 1:AL
May 2008 Romaine E. coli O157:H7 9 1:WA
Oct. 2008 Lettuce E. coli O157:H7 59


and Canada

Nov. 2008 Lettuce E. coli O157:H7 130 Canada
Sept. 2009 Lettuce:
Romaine or Iceberg
E. coli O157:H7 29 Multistate
Sept. 2009 Lettuce E. coli O157:H7 10 Multistate
April 2010 Romaine E. coli O145 33

5:MI, NY,


Oct. 2011 Romaine E. coli O157:H7 60 Multistate
April 2012 Romaine E. coli O157:H7 28



June 2012 Romaine E. coli O157:H7 52 Multistate
Sept. 2012 Romaine E. coli O157:H7 9 1:PA
Oct. 2012 Spinach
and Spring Mix Blend
E. coli O157:H7 33 Multistate
Apr. 2013 Leafy Greens E. coli O157:H7 14 Multistate
Aug. 2013 Leafy Greens E. coli O157:H7 15 1:PA
Oct. 2013 Ready-To-Eat Salads E. coli O157:H7 33 Multistate
Apr. 2014 Romaine E. coli O126 4 1:MN
Apr. 2015 Leafy Greens E. coli O145 7



June 2016 Mesclun Mix E. coli O157:H7 11



Nov. 2017 Leafy Greens E. coli O157:H7 67


and Canada

Mar. 2018 Romaine E. coli O157:H7 219


and Canada

Nov. 2018 Romaine E. coli O157:H7 91


and Canada

Sept. 2019 Romaine E. coli O157:H7 23 Multistate
Nov. 2019 Romaine E. coli O157:H7 41


and Canada

The Center for Produce Safety is funding the research team that is working to develop the cold plasma wash water treatment. The team is also partnering with SmartWash Solutions, a large manufacturing partner in the fresh produce processing industry, and Sunterra Produce Traders East representing technology to create this plasma system.

“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,” Alexander Fridmen, Ph.D. and director of the C. and J. Nyheim Plasma Institute at Drexel University said. “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.”

Drexel University had already developed an instrument known as the reverse vortex gliding arc plasmatron, a device that ionizes gas molecules to initiate a chemical reaction. 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.

Key take-aways:

  • Technology is similar to that used in plasma TVs or fluorescent light bulbs.
  • Cold plasma has been used to treat water in other industrial applications. 
  • Research will seek to modify its use for the fresh-cut produce industry. 
  • Cold plasma offers potential as an economical non-chemical, non-thermal disinfection method for wash water.

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

The project is now a year in. They are now validating the technology with a prototype model using a 100-gallon tub into which the plasmatron electrode has been submerged. The research team will simulate increased organic loading seen in a fresh-cut processing facility.

Plasma-treated water will 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. The final step will be for project collaborator SmartWash Solution to install the pilot system in a commercial-scale fresh-cut wash system.

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