Effect of Sanitizers and Ozone Combinations against Salmonella enterica Typhimurium on Green Onions

Introduction: Salmonella is one of the most causative microorganisms for foodborne illnesses in United States and many other countries. Four deaths and 1028 illnesses in United States have been attributed to three microbial foodborne outbreaks associated with green onions.

Purpose: The objective of our study was to investigate the efficacy of generally regarded as safe (GRAS) antimicrobial washing treatments including hydrogen peroxide (H₂O₂), SDS, organic acids (citric acid, acetic acid and malic acid), thymol, bubbling ozone as well as their combinations for inhibiting Salmonella enterica Typhimurium on green onions without affecting the produce quality.

Methods: Salmonella were surface inoculated on green onions, and various washing treatments including acetic acid, malic acid, citric acid (1-2 mg/ml), hydrogen peroxide (300-800 ppm), SDS (1-4%), thymol (0.2-0.4 mg/ml), bubbling ozone (6.25 ppm) as well as their binary combinations were investigated to inactivate Salmonella on green onions. The Salmonella population on green onions was enumerated by plating method. The antimicrobial effects of the test treatments were compared to 200 ppm chlorine, no wash treatment and water wash. Quality of green onion including color, texture and overall visual appearance was also evaluated.

Results: Most of our wash treatments reduced Salmonella enterica Typhimurium significantly compared with “no wash” control group. Among single antimicrobial treatments, 0.4 mg/ml thymol was the most effective with 4.85-log reduction and it had similar efficiency as compared to 200 ppm chlorine (4.97-log reduction). Other treatments were less effective, 2 mg/mL acetic acid, malic acid or citric acid and 6.25 ppm bubbling ozone had approximately 2-log reduction. However, all of them showed significantly stronger Salmonella inactivation capacity than water washing (1.07-log reduction). Combinations such as 0.2 mg/ml Thymol+2 mg/ml acetic acid (4.32-log reduction) and 300 ppm H₂O₂+4% SDS (2.02-log reduction) also provided effective activities to mitigate Salmonella. Ozone-SDS or ozone-hydrogen peroxide combinations significantly increased Salmonella log reduction. In quality tests, none of the treatments aforementioned had significant negative effect on green onions, although increase of green color has been observed in some groups.

Significance: Our work suggests a new environmentally friendly washing solution might be developed without use of chlorine to inactivate Salmonella on green onions.