The effect of cold plasma on inactivation of chlorine adapted Salmonella in fresh onion

Abstract

Chlorine-based disinfectants have been used extensively in leafy vegetable production. One of the main drawbacks of chlorine disinfectants is the emergence of chlorine adapted (CA) or resistant microbial cells. This research aimed to investigate the effect of chlorine adaptation on resistance of Salmonella enterica upon atmospheric cold plasma (ACP) application at different voltages (6, 8, and 11 kV) and times (5, 10, and 15 min). Cold plasma can be considered an alternative to thermal-based technologies due to the potential ability for eliminating microbial contamination of fresh-cut vegetables and preserving quality characteristics.

Method and material: Broth microdilution method was used for MIC determination in a 96-well sterile microtiter plate. Stress adaptation was done from exposing Salmonella enterica to progressive concentration of sodium hypochlorite (from 1/20 MIC). ACP applied to CA and non-CA Salmonella enterica cells in broth media and those inoculated on spring onion leaves. Inactivation and injury induction of Salmonella enterica cells calculated after ACP application in different voltages and times. The effect of ACP on biofilm formation ability and antibiotic resistance pattern of CA and non-CA S. enterica cells were examined. The effect of ACP on the preservation of nutritional (vitamin C and antioxidant activity) and sensory attributes during storage at 4˚C were evaluated. Results: A higher lethality effect was observed from a higher voltage and longest times (11 kV-15 min) on CA and non-CA S. enterica. Still, it induced higher percentages of injured cells in CA (58.77%) than non-CA (0.61%) (p<0.05). Inactivation data on indigenous natural flora of onion leaves indicated the highest ACP effect at the lowest voltage (p<0.05). More than 3 log CFU/g reduction was observed at 6 kV after 5 and 10 min. ACP reduced CA and non-CA S. enterica cells on onion leave surface to a lower extent than pure treated cells in broth media. Nevertheless similar to broth media, a high percentage of injury (61.03%) was induced on CA cells at higher voltage (11 kV-10 min). Biofilm results revealed ACP application reduced average ODs compared to CA and non-CA cells without ACP treatment (control). Chlorine adaptation and ACP treatment influenced the antibiotic resistance pattern according to applied voltage, time, and antibiotic type. The green onion leaves treated with ACP (8 kV-10 min) showed a significant increase and decrease in vitamin C and DPPH free radical scavenging activity after ACP application (day 0), respectively. However, vitamin C and DPPH free radical scavenging activity diminished and increased during storage at 4˚C, respectively. ACP improved the color, odor, and acceptance of green onion leaves during storage at 4˚C. The finding showed despite highest lethality of high voltages, given the high percentages of injured cells, lower voltages may offer acceptable inactivation of pathogenic bacteria with lower injury induction. Conclusion: In conclusion, ACP has the potential ability to eliminate CA cells of S. enterica, which is predominant in fresh-cut vegetable outbreaks. However, it is necessary to optimize the conditions of practical application before use. Keywords: , Chlorine adaptation, Atmospheric cold plasma, Onion leaves