The effect of produce washing using electrolyzed water on the induction of the viable but non-culturable (VBNC) state in Listeria monocytogenes and Escherichia coli O157:H7

Abstract

The effects of pathogen inoculum concentration, pH and organic load of electrolyzed oxidizing (EO) water treatment on the culturability and viability of Listeria monocytogenes and Escherichia coli O157:H7 was examined. Inoculum concentrations (8, 6, and 4 log CFU/mL), EO water at pH between 2.7 and 8.5 and two lettuce-to-EO water ratios (1:20 and 1:10 w/v) were tested by exposing pathogens to EO water for 5 min, followed by culturing on nonselective media and viability assessment through flow cytometry and resuscitation attempts. Treating pathogens at high initial populations with EO water led to maintenance of culturability, while lower populations lost culturability at free chlorine concentrations (FCC) above 1 mg/L. The pH of EO water significantly affected the reduction of viable populations of both pathogens (p < 0.05). The ultraviolet absorbance at 254 nm (UV254) was used as the organic load indicator and formation of VBNC cells was observed at 6 and 7.5 mg/L FCC of EO water samples with UV254 readings of 0.059 and 0.102, respectively. Increased organic load of EO water raised the VBNC-inducing FCC levels. However, keeping residual chlorine of EO water above 9 mg/L prevented the formation of VBNC cells and hence helped prevent cross-contamination during fresh produce washing treatment.