Reduction in Levels of Escherichia coli O157:H7 in Apple Cider by Pulsed Electric Fields

Abstract

Many studies have demonstrated that high voltage pulsed electric field (PEF) treatment has lethal effects on microorganisms including Escherichia coli O157:H7; however, the survival of this pathogen through the PEF treatment is not fully understood. Fresh apple cider samples inoculated with E. coli O157:H7 strain EC920026 were treated with 10, 20, and 30 instant charge reversal pulses at electric field strengths of 60, 70, and 80 kV/cm, at 20, 30, and 42°C. To accurately evaluate the lethality of apple cider processing steps, counts were determined on tryptic soy agar (TSA) and sorbitol MacConkey agar (SMA) to estimate the number of injured and uninjured E. coli O157:H7 cells after PEF treatment. Cell death increased significantly with increased temperatures and electric field strengths. A maximum of 5.35-log10 CFU/ml (P < 0.05) reduction in cell population was achieved in samples treated with 30 pulses and 80 kV/cm at 42°C. Cell injury measured by the difference between TSA and SMA counts was found to be insignificant (P > 0.05). Under extreme conditions, a 5.91-log10 CFU/ml reduction in cell population was accomplished when treating samples with 10 pulses and 90 kV/cm at 42°C. PEF treatment, when combined with the addition of cinnamon or nisin, triggered cell death, resulting in a reduction in E. coli O157:H7 count of 6 to 8 log10 CFU/ml. Overall, the combination of PEF and heat treatment was demonstrated to be an effective pasteurization technique by sufficiently reducing the number of viable E. coli O157:H7 cells in fresh apple cider to meet U.S. Federal Drug Administration recommendations.