Effect of Combined Ozone and Organic Acid Treatment for Control of Escherichia coli O157:H7 and Listeria monocytogenes on Lettuce

This study was conducted to determine the effects of ozonated water (1, 3, and 5 ppm) alone with different exposure times (0.5,1,3, or5min), and combinations of 3 ppm ozone with 1% organic acids (acetic, citric, or lactic acids) during 1‐min exposure for inactivating Escherichia coli O157:H7 and Listeria monocytogenes on lettuce and to observe the regrowth of these pathogenic bacteria on treated lettuce during storage for 10 d at 15°C. Results showed that 5 ppm ozone treatment for 5 min gave 1.09‐log and 0.94‐log reductions of E. coli O157:H7 and L. monocytogenes , respectively, indicating insignificant reductions compared with 3 ppm ozone treatment for 5 min. Treatment with 3 ppm ozone combined with 1 % citric acid for 1 min immersing resulted in 2.31 ‐ and 1.84‐log reductions ( P < 0.05), respectively. During storage at 15°C for 10 d after combined treatment and packaging, populations of E. coli O157:H7 and L. monocytogenes increased to approximately 9.0‐log colony forming unit (CFU) /g, indicating that this treatment did not have a residual antimicrobial effect during storage. Although the storage study did not show control of these pathogens, the combined ozone‐organic acid treatment was more effective in reducing population levels of these pathogens on lettuce than individual treatments.