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 & Provinces |
| 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 |
3:CT, IL, NY |
| 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 |
3:OR, PA, OH |
| 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 |
3:MN, WI, OR |
| Sept. 2006 | Spinach (baby) | E. coli O157:H7 and other serotypes | 205 |
Multistate and Canada |
| Nov./Dec. 2006 | Lettuce | E. coli O157:H7 | 71 |
4:NY, NJ, PA, DE |
| Nov./Dec. 2006 | Lettuce | E. coli O157:H7 | 81 |
3:IA, MN, WI |
| 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 |
Multistate 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, OH, PA, TN |
| Oct. 2011 | Romaine | E. coli O157:H7 | 60 | Multistate |
| April 2012 | Romaine | E. coli O157:H7 | 28 |
1:CA Canada |
| 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 |
3:MD, SC, VA |
| June 2016 | Mesclun Mix | E. coli O157:H7 | 11 |
3:IL, MI, WI |
| Nov. 2017 | Leafy Greens | E. coli O157:H7 | 67 |
Multistate and Canada |
| Mar. 2018 | Romaine | E. coli O157:H7 | 219 |
Multistate and Canada |
| Nov. 2018 | Romaine | E. coli O157:H7 | 91 |
Multistate and Canada |
| Sept. 2019 | Romaine | E. coli O157:H7 | 23 | Multistate |
| Nov. 2019 | Romaine | E. coli O157:H7 | 41 |
Multistate 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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