Q+A: The Technology Saving Our Salads (And Other Tasty Foods)

romaine lettuce
The complex matrix of leafy greens, like kale, make it a challenge for standard washing and decontamination processes.

A sprinkling of plasma-treated water might be the perfect dressing to keep our salad greens and other vulnerable produce free of contaminants.

It has been more than a decade since U.S. consumers have faced a foodborne E. coli outbreak on par with the current situation traced to romaine lettuce grown in the Yuma region of Arizona. Part of that is due to steady improvement of food handling and processing procedures, as well as advances in the technology used to neutralize diseases and toxins that might find their way into our food.

Greg Fridman, PhD, an assistant research professor in the School of Biomedical Engineering, Science and Health Systems, and researcher in Drexel’s Nyheim Plasma Institute, has been developing one such technology: Cold plasma could prevent food contamination by turning water into an even more formidable adversary of pathogens.

Fridman recently shed light on efforts to improve the array of technology that protects our food.

What are some of the technology and processes that help protect our food from contamination?

There are a number of procedures and treatments currently being used to keep food from contamination. They range from safe-handling procedures from farm to table, to various processes to wash away germs and chemicals, to treatments that actively neutralize organic and chemical contamination. For example, the use of water mixed with sodium hypochlorite (bleach) and peracetic acid to wash leafy greens before packaging them.

One thing that has been driving the evolution of food safety is our understanding of the sources of contamination and impact the disinfection methods have on the environment. New technology strives to reduce water use, remove environmentally damaging chemicals from the treatment process and develop contamination detection methods.

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Leafy greens are frequently the cause of food-borne E.coli outbreaks. What is it about this type of produce that makes it difficult to decontaminate in the treatment process? What other foods have proven to be challenging to treat?

This is true for a number of reasons. Take kale, for example — kale leaves form a complex shape, which is hard to access with disinfectant. To address this, we have attempted to increase the amount of disinfectant. But this can brown or bleach the leaves. Fresh cut produce is also especially vulnerable to contamination as the protective skin of the plant has been breached, so pathogens have easier access.

We are still working on new engineering solutions and technologies to better penetrate into this complex matrix. And safely disinfect cut produce without damaging it.

How did plasma technology emerge as a possible solution for food decontamination? How has it been incorporated into our current food decontamination process?

Non-equilibrium discharges (“cold” plasmas) are part of the non-thermal processing technologies branch of agricultural and food engineering, which also includes water rinsing, high-pressure food processing and pulsed electric fields.

“Cold plasma” is the forth state of mater — other than solid, liquid, and gas — where molecules have been excited by applied electric field and gas becomes ionized. In plasma, a high concentration of active chemical species, such as hydrogen peroxide, is generated. What we’ve found in our research is that these special ions in the plasma can neutralize pathogens found in water or on any surface.

Interestingly, these active species are short-lived in the water (seconds to minutes). Thus, plasma-treated water is strongly antimicrobial only for a short amount of time. This is significant because we want to make sure that anything we use to treat food becomes inert, so it doesn’t affect people or the environment when it is discharged.

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Researchers in the Nyheim Plasma Institute are exploring the use of cold plasma for produce treatment.

To disinfect produce with cold plasma we nebulize water, creating high concentration of droplets smaller than 10 micrometers (a tenth of diameter of a hair), and pass these droplets through a plasma field where they mix with plasma-generated reactive species. This plasma-fog is now ready to be mixed with produce, kept in contact with produce for a few seconds, and then produce can be packaged.

Cold plasma treatment has also shown to be successful in preventing cross-contamination by continuously disinfecting the water used to wash the produce. This is accomplished by generating plasma in the water and continually passing produce washing water through the plasma field.

Our lab tests show this method to be more than 99 percent effective in neutralizing E. coli on baby carrots. We are also testing it on spinach, kale, romaine lettuce and tomatoes. In working closely with the U.S. Department of Agriculture and the National Science Foundation, as well as industry partners, I anticipate that cold plasma technology could be integrated into food safety processing in the next 10 years.

Why does this technology work where others might not be as effective?

Cold plasma water treatment does not need any additional chemicals and operates on electricity with no moving parts that can break. The cold plasma fog is particularly effective for treating complex and high surface-area foods, like leafy greens, because it can penetrate all areas of the produce.

For more information about plasma technology visit: http://drexel.edu/nyheiminstitute/


For media inquiries contact Britt Faulstick, bef29@drexel.edu