Moderate Temperatures Affect Escherichia coli Inactivation by High-Pressure Homogenization Only through Fluid Viscosity

The inactivation of suspensions of Escherichia coli MG1655 by high-pressure homogenization was studied over a wide range of pressures (100-300 MPa) and initial temperatures of the samples (5-50 °C). Bacterial inactivation was positively correlated with the applied pressure and with the initial temperature. When samples were adjusted to different concentrations of poly(ethylene glycol) to have the same viscosity at different temperatures below 45 °C and then homogenized at these temperatures, no difference in inactivation was observed. These observations strongly suggest, for the first time, that the influence of temperature on bacterial inactivation by high-pressure homogenization is only through its effect on fluid viscosity. At initial temperatures >=45 °C, corresponding to an outlet sample temperature >65 °C, the level of inactivation was higher than what would be predicted on the basis of the reduced viscosity at these temperatures, suggesting that under these conditions heat starts to contribute to cellular inactivation in addition to the mechanical effects that are predominant at lower temperatures. Second-order polynomial models were proposed to describe the impact of a high-pressure homogenization treatment of E. coli MG1655 as a function of pressure and temperature or as a function of pressure and viscosity. The pressure-viscosity inactivation model provided a better quality of fit of the experimental data and furthermore is more comprehensive and versatile than the pressure-temperature model because in addition to viscosity it implicitly incorporates temperature as a variable.