In this study, we investigated the antimicrobial effects of plasma activated fine droplet (PAD) produced from arc discharge plasma on planktonic Listeria monocytogenes and Escherichia coli O157:H7. NaCl (0.9%, w/v) was used as the feeding solution for the plasma discharge. The inactivation mechanism of the PAD treatment was also investigated. PAD mainly contains H2O2 and OCl−, which play a significant role in the inactivation process against L. monocytogenes and E. coli O157:H7. The population of L. monocytogenes and E. coli O157:H7 was significantly reduced by approximately 3 and 4 log units, respectively, within 5 min of exposure to PAD. However, the bactericidal effects of PAD against L. monocytogenes and E. coli O157:H7 showed different trends by showing 0.58 and 4.13 log reductions, respectively, after 1 min of PAD exposure time. The change of membrane integrity was evaluated using two DNA-binding fluorescence dyes, SYTO 9 and propidium iodide (PI). The breakage of the cell wall and membrane of both microorganisms was evidenced by the uptake of PI by cells after 5 min of exposure to PAD, but the effect was less in L. monocytogenes compared to E. coli O157:H7 after 1 min of PAD exposure time. The transmission electron microscopy results clearly showed morphological changes in both microorganisms, including denaturation or leakage of intracellular materials as a consequence of PAD treatment. These findings suggest that PAD-induced chemical species can eventually affect the intracellular materials of bacterial cells by passing through or attacking the cell envelope. In addition, L. monocytogenes are less susceptible to PAD compared with E. coli O157:H7.

Time-delayed transition of normal-to-abnormal glow was investigated in discharge between spoke-like pins and ultrapure water by applying AC-driven power at a frequency of 14.3 kHz at atmospheric pressure. The normal-to-abnormal transition can be recognized from the slope changes of current density, gas temperature, electrode temperature, and OH density. The slope changes took place in tens of minutes rather than just after discharge, in other words, the transition was delayed. The time-delay of the transition was caused by the interaction between the plasma and water. The plasma affected water properties, and then the water affected plasma properties.

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The demand for fresh-cut vegetables is increasing due to the convenience of their use in restaurants and households. As the demand for fresh-cut foods increases, the most important issues are safety, freshness, and healthfulness. Fresh-cut vegetables are defined as those that have been physically altered in form by peeling, slicing, chopping, shredding, coring, or trimming (with or without washing); and which remain fresh (Schnabel et al., 2019). The production of potatoes in 2017 was the fifth largest in the global market, reaching 388 Mt (FAO, 2020). Potatoes are good sources of minerals, protein, vitamins, and phytochemicals such as acids, anthocyanins, and carotenoids (Tian et al., 2016). Some of the issues related to the use of fresh-cut potatoes include such as microbiological hazards, the browning reaction, loss of firmness, and loss of weight. Such factors are influenced by the washing agents and storage conditions used (