Purpose: The aim of this study was to evaluate the efficacy of aqueous chlorine dioxide (ClO2) on the reduction of supplemented, non-pathogenic Escherichia coli in the washing water and on processed “Lollo rossa” lettuce at the pilot scale.
Methods: A 3.5 m3 washing line was continuously supplied with ClO2, until approximately 5 ppm, before and during 800 kg of lettuce washing (~90 minutes). Then, the lettuce input and the ClO2 supply halted. A non-pathogenic, overnight cultured (37°C), Escherichia coli nontypable strain, which was isolated from surface water, was added to the tank resulting in an approximate final concentration of 106CFU/ml. Wash water and produce samples for microbiological and chemical analyses were taken before and after the input and supply halted.
Results: No detectable levels of E. coli were determined within 1 minute after supplemented E. coli entered the ClO2 containing washing water. A log 2 reduction was achieved on leaf samples, compared to control experiments without aqueous ClO2. ClO2 concentrations quickly decreased after introduction of the strain, yet a residual concentration (>2 ppm) remained present in the washing water.
Significance: Results demonstrated that ClO2 application at the pilot scale was able to reduce the load that entered the process wash water. However, this treatment was not able to prevent attachment to the lettuce. Nevertheless, the use of ClO2 decreased the possibility for cross-contamination between batches in comparison to no disinfectant. Overall, cross-contamination prevention via the washing water remains critical and application at the industrial scale is attainable.