Effect of pulsed electric fields pretreatments on S. cerevisiae inactivation by high pressure carbon dioxide

The effect of coupling pulsed electric fields (PEF) as pretreatment to enhance the microbial inactivation of the high pressure carbon dioxide (HPCD) treatment was investigated. McIlvaine buffer solutions inoculated with S. cerevisiae (CBS 1171) cells were pretreated with PEF at different field strength (E=6-12 kV/cm) and energy input (WT=10-40 J/mL) and then processed with HPCD at pressures of 8.0, 11.0 and 14.0 MPa and contact times ranging from 3 to 30 min. Results showed that treating microbial suspensions only with PEF (12 kV/cm, 20 J/ml, 25°C) the maximum inactivation level achieved was 0.35 Log-cycles. On the other hand, when yeast cells were treated only with HPCD, the inactivation level was almost independent on the processing conditions and never greater than 2 Log-cycles. The exposure of yeast cells to an external electric field prior to the application of HPCD treatment resulted in a marked increase of the inactivation level. Total inactivation was achieved when PEF (12 kV/cm, 20 J/mL, 25°C) was coupling as pretreatment to the HPCD processing (8.0 MPa, 25°C, 30 min). A mathematical model based on the Weibull distribution adequately described the inactivation kinetics of S. cerevisiae cells treated with the combined process at different PEF treatment conditions and at fixed CO2 pressure and temperature (8.0 MPa, 25°C)