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theses

Plasma activated water (PAW) has been shown to be a promising surface decontamination technique. Antimicrobial effects of PAW have been attributed to reactive oxygen and reactive nitrogen species, which act as oxidizing agents and also contribute to the acidifying effect, causing the pH of water to drop. To isolate the effect of low pH on microbial inactivation, a buffer with the same pH (3.1) as that of PAW was evaluated. Plasma Activated Buffer (PAB) was generated to study the interactive effects of low pH and plasma activated species. Previously, substrate properties (roughness) have been shown to affect the antimicrobial efficacy of plasma. The objectives of this research were: (1) To isolate the effect of pH in PAW using a buffer solution, (2) Characterize plasma, PAW, and PAB, and (3) To evaluate the effect of surface roughness on microbial inactivation using PAW and PAB. PAW and PAB were generated by exposing sterilized distilled water and citrate-phosphate buffer (pH = 3.1), respectively, to atmospheric pressure air plasma jet. Efficacy of distilled water, PAW, buffer, and PAB for inactivation of Enterobacter aerogenes was evaluated in a planktonic system, and for different surfaces with increasing roughness. Surface roughness (Pq) values for four sample surfaces (glass slide, grape tomatoes, limes, spiny gourd) were obtained using Confocal Laser Scanning Microscopy (CLSM). Optical Emission Spectroscopy (OES) was used to obtain an emission spectra for plasma. Electrical conductivity and Oxidation Reduction Potential (ORP) were measured for PAW and PAB. In the planktonic system for treatment time of 10 min, a (1.92 ± 0.70) log CFU/ml reduction using PAW (pH = 3.1) was achieved, however, no reduction was observed using only the buffer at the same pH. This confirmed that the inactivation was due to the reactive species in PAW, and not due to the low pH. A (5.11 ± 0.63) log CFU/ml reduction was observed using PAB in the same system, suggesting interactive effects of plasma generated species and low pH in the buffer system. In studies with glass slide, grape tomatoes, limes, and spiny gourd, it was found that as the surface roughness (Pq) value increased, the inactivation due to PAB treatment decreased. Highest reduction of (6.32 ± 0.43) log CFU/surface was achieved for glass slide (Pq = 0.28 ± 0.02 μm), followed by (5.31 ± 0.14) log CFU/surface for grape tomatoes (Pq = 5.17 ± 0.53 μm), and (3.80 ± 0.63) log CFU/surface for limes (Pq = 18.76 ± 3.00 μm). The least reduction of (2.52 ± 0.46) log CFU/surface was observed for spiny gourd, which had the highest roughness (Pq = 101.50 ± 10.95 μm). For PAW treatment, lower inactivation for each surface was observed. Moreover, no significant difference in microbial inactivation between the samples of different roughness values, was observed when treated with PAW. The ORP and electrical conductivity values of PAW and PAB showed a positive correlation with microbial inactivation in the planktonic system. Thus, PAW and PAB can potentially be used for fresh produce decontamination. However, further research is needed to confirm the suitability of PAW as an industrial sanitizer. In addition, the effectiveness of plasma activated organic acids also should be explored.

ETD_7919

M.S.

Includes bibliographical references

by Isha Joshi

doi:10.7282/T3X92F5Z

ETD graduate

The author owns the copyright to this work.