Inactivation kinetics of Saccharomyces cerevisiae by pulsed electric fields in a batch treatment chamber: The effect of electric field unevenness and initial cell concentration

Microbial inactivation of Saccharomyces cerevisiae inoculated in liquid and solid model foods and treated with PEF in a batch chamber with parallel plate electrodes was studied. Electric field strength, cumulated treatment time and initial microbial concentration were the main parameters investigated. Results obtained without promoting mixing of the liquid sample showed that, at any electric field strength, the inactivation kinetics of S. cerevisiae was represented by a non-linear relationship when the log10 of the survival fraction was plotted against treatment time. The maximum level of inactivation achieved was 4.51 log-cycles at an electric field intensity of 30.9 kV/cm and after a total process time of 1600 s. The intrinsic heterogeneity of PEF treatment was confirmed and found to be mainly due to a non-uniform distribution of the electric field in the treatment region. Simulating the agitation of liquid samples (Trizma buffer pH 7.2, k = 2 mS/cm) in the treatment chamber during pulse processing enhanced the effectiveness of microbial inactivation process. Experiments carried out immobilizing the yeast cells in a solid model food (Potato dextrose agar, pH 5.3 and k = 1.61 mS/cm at 25 °C) allowed to confirm the existence of a dead space inside the treatment chamber due to field fringing effects. Tests carried out to study the effect of the initial concentration of S. cerevisiae cells in the range 103–108 CFU/ml on PEF processing effectiveness showed that the level of inactivation achieved increases with decreasing the initial concentration of microorganisms