Purpose: In this study, inactivation of Salmonella, E. coli O157:H7 and Non-O157 STEC by CW (hypochlorite solutions) were evaluated for a range of free chlorine (FC) concentrations, contact times, and organic loads.
Methods: Two experiments were conducted. First, four strains of Salmonella, E. coli O157:H7 or Non-O157 STEC cells were separately inoculated into 12-well microplate containing FC (0-2 mg/l). Second, a multi-strain cocktail of pathogens was inoculated into 12-well microplates with fresh tomato extract (0-2.5%) or lettuce extract (0-2%), followed by CW (initial: 7-8 mg/l). In both experiments, after exposure times of 5, 10, 15, 30, 45, 60, 90, or 120 sec, 1-ml aliquots/per well were transferred into 96-well microplates with 2X tryptic soy broth plus 0.1% sodium pyruvate and dechlorination reagent. Pathogen survival was enumerated by spread plating onto XLT-4 or sorbitol MacConkey agar. Water quality, including pH, turbidity, chemical oxygen demand (COD) and free chlorine concentration, was tested. Statistical analysis of data was performed using PROC Mixed procedure of SAS.
Results : Water quality is highly impacted by the addition of tomato or lettuce juice extract as indicated by the rapid increase in water turbidity, COD, and decrease in residual free chlorine with the increase in extract concentration. This, in turn, significantly affects the survival of pathogens. In general, no survival of pathogens was detected in the solutions when the residual FC >0.5 mg/l for >30-sec contact time, or residual FC >1.0 mg/l and exposure time >5 sec. When the FC concentration and contact time were less than the above conditions, the survival of pathogens was strain dependant with the survival ranked as: Salmonella > E. coli O157:H7 > Non-O157 STEC.
Significance: These results show high organic loads severely and rapidly limit CW inactivation of foodborne pathogens.