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

Abstract

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 at the presence of 0 and 25 mg/L free chlorine (HOCl) and organic matter. Uninoculated tomatoes were introduced into a laboratory-based model flume containing tomatoes inoculated with a cocktail of five rifampicin-resistant Salmonella enterica serovars at 104, 106, or 108 CFU/tomato in water containing 0 or 25 mg/L HOCl and 0 or 300 mg/L chemical oxygen demand (COD). Uninoculated tomatoes were removed from the water at after 5, 30, 60, 120 s and were placed in bags containing tryptic soy broth supplemented with rifampicin and 0.1% sodium thiosulfate. Following incubation, enrichments were plated on tryptic soy agar supplemented with rifampicin 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 (P≤0.05) declined 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 (P≤0.05) significant factors that contributed to increased cross-contamination seen in this study. When HOCl levels were at 25 mg/L, no Salmonella was recovered on non-inoculated tomatoes under all conditions when inoculum levels were at 104 CFU/tomato. When the inoculum was increased to 106 and 108 CFU/tomato, cross-contamination was observed, independent of COD levels. The results from this study show that the currently required sanitizer level (e.g., 100 or 150 mg/L) for flume water may be higher than necessary and warrants re-evaluation.

Evaluating the efficacy of peroxyacetic acid in preventing Salmonella cross-contamination on tomatoes in a model flume system

“…However, this contamination level of 10 8 CFU/tomato represents the worst-case scenario and not one commonly encountered in a commercial packing operation. The findings are in parallel with a similar study demonstrating the efficacy of free chlorine in preventing cross-contamination at much lower concentrations than that recommended for tomato flume tanks [ 5 ]. Bertoldi et al (2022) observed that 25 mg/L free chlorine (measured as hypochlorous acid or HOCl) was effective in preventing cross-contamination when tomatoes were washed for ≥30 s in chlorinated water [ 5 ].…”

“…The findings are in parallel with a similar study demonstrating the efficacy of free chlorine in preventing cross-contamination at much lower concentrations than that recommended for tomato flume tanks [ 5 ]. Bertoldi et al (2022) observed that 25 mg/L free chlorine (measured as hypochlorous acid or HOCl) was effective in preventing cross-contamination when tomatoes were washed for ≥30 s in chlorinated water [ 5 ]. In this study, PAA as low as 10 mg/L showed similar efficacy, suggesting that PAA could be an effective alternative to HOCl.…”

“…Even with recent sanitary advancements, the risk of cross-contamination of pathogens in tomato wash water may occur if sanitizer levels and water quality are poorly maintained [ 1 ]. It has been shown that effective levels of sanitizers are vital in preventing the transfer of human microbial pathogens in tomato wash water [ [2] , [3] , [4] , [5] ]. A major concern surrounding sanitizer efficacy is maintaining water quality during packinghouse operations.…”