Effect of Non-Thermal Atmospheric Plasma on Food-Borne Bacterial Pathogens on Ready-to Eat Foods: Morphological and Physico-Chemical Changes Occurring on the Cellular Envelopes

Abstract

Currently, there is a need for new technological interventions to guarantee the microbiological safety of ready-to-eat (RTE) foods. Non-thermal atmospheric plasma (NTAP) has emerged as a promising strategy for inactivating microorganisms on thermo-sensitive foods, and the elucidation of its mechanisms of action will aid the rational optimization and industrial implementation of this technology for potential applications in the food industry. In this study, the effectiveness of NTAP for inactivating strains of Salmonella Enteritidis, Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes contaminating the surface of different sliced RTE foods (“chorizo”, salami, bacon, smoked salmon, tofu and apple) was investigated. In addition, to further assess the bacterial inactivation mechanisms of NTAP, the morphological and physico-chemical damages in bacterial cells were analyzed. NTAP was effective for the surface decontamination of all products tested and, especially, of cut apple, where the microbial populations were reduced between 1.3 and 1.8 log units for the two Salmonella strains and E. coli O157: H7, respectively, after 15 min of exposure. In the rest of foods, no significant differences in the lethality obtained for the E. coli O157:H7 strain were observed, with inactivation rates of between 0.6 and 0.9 log cycles after a 15-min treatment. On the other hand, the strains from the rest of pathogenic microorganisms studied were extremely resistant on tofu, where barely 0.2–0.5 log units of inactivation were achieved after 15 min of plasma exposure. S. Enteritidis cells treated for 10 min exhibited noticeable morphological and structural changes, as observed by transmission electron microscopy, which were accompanied by a loss in membrane integrity, with an increased leakage of intracellular components and uptake of propidium iodide and marked changes in regions of their FTIR spectra indicating major alterations of the cell wall components. Overall, this indicates that loss of viability was likely caused for this microorganism by a significant damage in the cellular envelopes. However, the plasma-treated cells of L. monocytogenes did not show such obvious changes in morphology, and exhibited less marked effects on the integrity of their cytoplasmic membrane, what suggests that the death of this pathogenic microorganism upon NTAP exposure is more likely to occur as a consequence of damages in other cellular targets.

Effect of cold atmospheric plasma torch distance on the microbial inactivation and sensorial properties of ready-to-eat olivier salad

“…In this study, three factors were efficient in the inactivation of microorganisms; the type of the microorganism, time of plasma exposure, and the distance of the exerted plasma from the nozzle to the product surface (Calvo et al, 2020;Silvetti et al, 2021). As the results indicated, by reducing the distance between the nozzle and Olivier salad surface at equal times, microbial reduction was observed in all cases.…”

“…Despite all benefits, plants are cultivated in the soil with a naturally vast microbial flora, which might be caused by irrigation water, excessive use of pesticide or fertilizer, human activities, or even inherently. Therefore, RTE foods (such as salads) are growing as a proper transferor for spreading of some contamination, attributed to gram-negative and positive bacteria (Chajęcka-Wierzchowska et al, 2020; European Food Safety Authority and European Centre for Disease Prevention and Control, 2018; Makinde et al, 2020; Wang et al, 2020).…”

“…Although some studies have been accomplished on the effect of cold plasma disinfection on RTE foods (Calvo et al, 2020;Lee et al, 2020Lee et al, , 2021Lin et al, 2020;Risa Vaka et al, 2019;Silvetti et al, 2021;Yadav et al, 2020), no previous studies have reported the effects of cold plasma treatment on the quality of the Olivier salad. The objective of this study was to investigate the effect of cold atmospheric plasma torch (CAPT) with different nozzle distances on microbial inactivation and sensorial properties of the Olivier salad.…”

Advances in the Application of Cold Plasma Technology in Foods

“…However, most of these foods are consumed without any culinary preparation to eliminate microbial loads, which could be a source of foodborne outbreaks, as other authors have reported. To deal with this problem, Calvo et al [ 4 ] evaluated the effectiveness of CP for the inactivation of different bacteria inoculated on the surface of different slices of RTE foods (“chorizo”, salami, bacon, smoked salmon, tofu and apple). They concluded CP is a successful non-thermal technique for the disinfection of RTE products, with different antimicrobial mechanisms for Gram-positive and Gram-negative bacteria.…”