pH and solute concentration of suspension media affect the outcome of high hydrostatic pressure treatment of Listeria monocytogenes

The effect of pH and solute concentration of suspension media on high hydrostatic pressure (HHP) induced inactivation of Listeria monocytogenes (approximate 108 CFU/ml) was investigated by the using treatment between 300 MPa and 600 MPa at 25 °C for 10 min. The suspension media used in this study represented different concentrations (0.1% to 10%) of buffered peptone water (BPW) with an adjusted pH of 4 to 7. An increase in the concentration of BPW resulted in a decreased HHP-induced inactivation of L. monocytogenes that was dependent on the pH of the medium. HHP-treatment at 300 MPa showed no bactericidal effect at neutral pH regardless of the BPW concentration. When the pH of BPW (0.1% to 5%) was reduced to 4, L. monocytogenes was completely inactivated (more than an 8 log cycle reduction) with a HHP-treatment of at least 300 MPa. HHP-treatment above 400 MPa completely inactivated L. monocytogenes in a relatively dilute BPW (0.1% and 1%) with an adjusted pH below 6. While only a 2 log cycle reduction was observed in 10% BPW at the pH ranging from 5 to 7 after treatment with 600 MPa, L. monocytogenes in 10% BPW at pH 4 was completely inactivated. Even though a significant bactericidal effect of HHP-treatment was not observed when applied with a low pressure such as 300 MPa or suspended in higher BPW at neutral pH, a reduction of the pH greatly affected the HHP-induced inactivation of L. monocytogenes. These results indicated that information concerning the pH of food or media would greatly assist an optimization of HHP-treatment for the inactivation of bacteria.