Purpose: This study measured the transfer of Salmonella from inoculated to non-inoculated portions of iceberg lettuce during washing in water containing chlorine at different concentrations, simulating different production scenarios.
Methods: The experiments were carried out with portions of iceberg lettuce (100 g) inoculated with Salmonella at ~6 log CFU/g. The Salmonella strain used in the study was modified to express Green Fluorescent Protein to facilitate recovery. Two scenarios were investigated: i) washing of one inoculated lettuce portion in non-chlorinated water followed by washing of several non-inoculated portions in the same water and ii) washing of one inoculated lettuce portion in chlorinated water (5, 10, 20, 30, 40, 50 and 250 mg/l) followed by washing of several non-inoculated portions in the same water.
Results: After washing the inoculated lettuce portion in non-chlorinated wash water, the population of Salmonella in the water was ~4.8±0.1 log CFU/ml. Washing of non-inoculated lettuce portions in this contaminated non-chlorinated wash water resulted in counts of Salmonella around 4.1±0.1 log CFU/g. Chlorinated water reduced Salmonella concentration by 0.5 to 2.0 log CFU/g and was able to prevent Salmonella transfer when at least 10 mg/l of chlorine was used. When the concentration was 5 mg/l, Salmonella in non-inoculated lettuce was below quantification (<100 CFU/g), but was detected by enrichment (e.g., at least 1 CFU/25g).
Significance: Our findings show the importance of using chlorine at proper concentrations in the wash water for production of minimally processed lettuce. To prevent cross-contamination with Salmonella, the concentration of chlorine should be above 10 mg/l.
Acknowledgements: FAPESP (2012/03471-1; 2014/17017-6) and CNPq (400477/2012-2).