Cross-protective effects of temperature, pH, and osmotic and starvation stresses in Escherichia coli O157:H7 subjected to pulsed electric fields in milk

The effect of exposure of Escherichia coli O157:H7 to prior stress on the effectiveness of pulsed electric field (PEF) treatment of milk was studied. Cells were exposed to a variety of temperatures (7–45 °C), pH (4.0–7.0), osmotic conditions (0–10% NaCl), starvation (100 h at 25 °C), and cold shock (5 °C for 250 h). Stress responses were explained by the Gompertz and Weibull models with similar goodness-of-fit. Using these models, conditions that induced adequate stress were identified and used to pre-condition E. coli O157:H7 cells. The inactivation of the bacterial cells in milk by PEF was characterized by downward concavity with differences in mean time of resistance between cells exposed to temperature, acid or osmotic stress. Application of the Weibullian–Log-Logistic model indicated that, at higher field strengths, inactivation occurred at lower onset temperatures, and this was related to an increase in heat dissipation.