Inactivation kinetics of inoculated Escherichia coli O157:H7 and Salmonella enterica on lettuce by chlorine dioxide gas

The purpose of this investigation was to study inactivation kinetics of inoculated Escherichia coli O157:H7 and Salmonella enterica on lettuce leaves by ClO2 gas at different concentrations (0.5, 1.0, 1.5, 3.0, and 5.0 mg l−1) for 10 min and to determine the effect of ClO2 gas on the quality and shelf life of lettuce during storage at 4 °C for 7 days. One hundred microliters of each targeted organism was separately spot-inoculated onto the surface (5 cm2) of lettuce (approximately 8–9 log CFU ml−1), air-dried, and treated with ClO2 gas at 22 °C and 90–95% relative humidity for 10 min. Surviving bacterial populations on lettuce were determined using a membrane transferring method, which included a non-selective medium followed by a selective medium. The inactivation kinetics of E. coli O157:H7 and S. enterica was determined using first-order kinetics to establish D-values and z-values. The D-values of E. coli and S. enterica were 2.9±0.1 and 3.8±0.5 min, respectively, at 5.0 mg l−1 ClO2 gas. The z-values of E. coli and S. enterica were 16.2±2.4 and 21.4±0.5 mg l−1, respectively. A 5 log CFU reduction (recommended by the United States Food and Drug Administration) for E. coli and S. enterica could be achieved with 5.0 mg l−1 ClO2 gas for 14.5 and 19.0 min, respectively. Treatment with ClO2 gas significantly reduced inherent microflora on lettuce and microbial counts remained significantly (p<0.05) lower than the uninoculated control during storage at 4 °C for 7 days. However, treatment with ClO2 gas had a significantly (p<0.05) negative impact on visual leaf quality. These results showed that treatment with ClO2 gas significantly reduced selected pathogens and inherent microorganisms on lettuce; however, the processing conditions would likely need to be altered for consumer acceptance.