Plasma discharge for food sterilization

Nonequilibrium plasmas, generated at atmospheric pressure through electric discharge, are in wide commercial use, especially for cleaning surfaces and improving surface adhesion. By producing a range of reactive chemical species, both charged and neutral, they can also inactivate or kill bacteria and other pathogens. When bacteria colonize the surface of fresh fruits and leafy vegetables, they encase themselves in a protective layer known as a biofilm. Plasmas’ ability to breach biofilms has caught the attention of the food industry. Yet for plasmas to be deployed as food decontamination tools, their ability to do the job must be understood, controllable, and reproducible. In one step toward that goal, Dawei (David) Liu, Xinpei Lu, and colleagues at China’s Huazhong University of Science and Technology and at Shanghai Jiao Tong University have now used a two-dimensional discharge model to analyze the interactions between the plasma and a biofilm on the surface of an apple. In the simulations, which spanned a region 3 mm wide by 5 mm high, the apple served as a floating electrode, and a series of nanosecond pulses of −25 kV applied to a full-width electrode 4 mm above it triggered narrow plasma filaments known as streamers. To calculate the details of the streamers’ propagation and interactions with the 400-µm-wide, 100-µm-thick biofilm, shown here in white, the researchers self-consistently accounted for 115 reactions involving 18 different chemical species, photoionization, and biofilm structure and conductivity. They found that localized ionization occurs above the biofilm, and the plasma spreads into the biofilm recesses. Although the biofilm’s irregular shape may shield some nooks from reactive chemical species, diffusion between the plasma pulses evens out the distribution to a level that could satisfy decontamination thresholds. (H. Cheng et al., Phys. Plasmas 23, 073517, 2016, doi:10.1063/1.4955323 .)