The 10-state Salmonella outbreak announced this week by the U.S. Centers for Disease Control and Prevention (CDC) is an example of foodborne illnesses that the CDC says sicken tens of millions of people and kill several thousand each year. But what if it was possible to easily kill potentially harmful bacteria on fruits and vegetables, without harming the food or the environment?
University of Maryland researchers are working on a plasma-based technology that could provide consumers, restaurants and food processors with a low-temperature microwave-like machine to kill pathogens, while bypassing significant problems associated with existing washing or treatment methods. Such problems include massive water use, inadvertent spread of contamination, creation of antimicrobial resistance, and chemical residues left on food.
In a paper published in Plasma Processes and Polymers, researchers from the A. James Clark School of Engineering and the College of Agriculture and Natural Resources reported that 99 percent of E.coli on the surface of fresh produce were killed by one minute of a process called “etching and surface modification.”
Their method uses what is essentially electrified air to damage the outer membrane of bacteria on food, killing them. But the process, which doesn’t involve heat, has no known impact on produce itself.
“We can use electrical energy to produce this [plasma] state from air, and the reactive species generated have very strong impacts on pathogens where they can etch part of their outer membranes and change them biochemically,” said co-author Gottlieb Oehrlein, professor of materials science and engineering with a joint appointment with the Institute for Research in Electronics and Applied Physics.
Plasma is already used in the healthcare industry to sanitize surgical tools, and clinical trials in dermatology have also been performed for the treatment of chronic skin diseases. The plasma is concentrated—almost like a tiny blowtorch—but cold to the touch.
“Microscopically the bacteria surface is bombarded by these exotic plasma species. This leads to material removal and surface modification,” said Pingshan Luan, Ph.D. ’18, lead author on the paper. “Once the composition is changed, the bacteria cell wall loses its functional and structural integrity.”
The technology could have great advantages over other means to kill bacteria, said co-author Rohan Tikekar, an assistant professor of nutrition and food science, eliminating risks associated with antimicrobials and even washing.
“The washing process is a double-edged sword,” he said. “It makes produce look appealing and removes dirt, but if it is not done properly, water becomes a carrier for this small amount of bacteria to spread to a larger batch of produce. You may start out with, say, 10 lettuce heads that are contaminated, and with improper washing, you might end up with 10 tons of lettuce that is contaminated.”
Their plasma food sanitation process potentially could be used at an industrial scale, in restaurants and dining halls, or even at the level of individual consumers.
The researchers are planning further testing, such as how the device affects the nutritional value of food. However, because it only works on a thin layer at the surface, they expect little impact, and consumers could one day have an effective safety measure that Oehrlein said would be as easy as “flipping a light switch on and off.”