Influence of Free Chlorine and Contact Time on the Reduction of Salmonella Cross-Contamination of Tomatoes in a Model Flume System

The process of washing tomatoes in dump (flume) tanks has been identified as a potential source of cross-contamination. This study's objective was to assess the potential for Salmonella enterica cross-contamination at various inoculation levels in the presence of free chlorine (HOCl) and organic matter. Uninoculated tomatoes were introduced into a laboratory-based model flume containing tomatoes inoculated with a cocktail of five rifampin-resistant S. enterica serovars at 104, 106, or 108 CFU per tomato in water containing 0 or 25 mg/L HOCl and 0 or 300 mg/L chemical oxygen demand (COD). Uninoculated tomatoes exposed to the inoculated tomatoes were removed from the water after 5, 30, 60, and 120 s and placed in bags containing tryptic soy broth supplemented with rifampin and 0.1% sodium thiosulfate. Following incubation, enrichment cultures were plated on tryptic soy agar supplemented with rifampin and xylose lysine deoxycholate agar to determine the presence of Salmonella. HOCl and pH were measured before and after each trial. The HOCl in water containing 300 mg/L COD significantly declined (P < 0.05) by the end of each 120-s trial, most likely due to the increased demand for the oxidant. Higher inoculum levels and lower HOCl concentrations were significant factors (P < 0.05) that contributed to increased cross-contamination. At 25 mg/L HOCl, no Salmonella was recovered under all conditions from uninoculated tomatoes exposed to tomatoes inoculated at 104 CFU per tomato. When the inoculum was increased to 106 and 108 CFU per tomato, cross-contamination was observed, independent of COD levels. The results from this study indicate that the currently required sanitizer concentration (e.g., 100 or 150 mg/L) for flume water may be higher than necessary and warrants reevaluation.