Abstract
The effect of pulsed electric field (PEF) pretreatments on the inactivation of microbial populations by high pressure carbon dioxide (HPCD) processing was studied. Saccharomyces cerevisiae cells suspended in McIlvaine buffer solutions were used in three different tests: a single PEF treatment, a single HPCD treatment and a combined PEF and HPCD treatment. Experiments were carried out testing the effects of different electric field strengths (6, 9 and 12 kV/cm) and energy inputs (10, 20 and 40 J/ml) on the microbial inactivation during the PEF pretreatment. Pressures of 8.0, 11.0 and 14.0 MPa and treatment times ranging from 3 to 30 min were applied during the HPCD treatment following the PEF pretreatment stage, with the aim of investigating the synergistic effect of the combined treatment. For comparison, inactivation curves of the microorganism during the single PEF and HPCD treatments were determined. Results showed that treating the cell suspensions only with PEF, the maximum inactivation level that could be achieved was 0.35 log-cycles after a treatment at 12 kV/cm and 20 J/ml. On the other hand, when yeast cells were treated only with HPCD, the maximum inactivation level detected was 1.79 log-cycles treating the sample at a pressure of 8.0 MPa for 3 min. With increasing the pressure up to 14.0 MPa and the treatment time up to 30 min no further increase of cells inactivation was detected. A completely different behavior was observed utilizing the combination of PEF and HPCD treatments. In this case, with increasing the field strength and the electrical energy input in the selected range, a remarkable increase of the inactivation after the combined treatment was detected. At a certain intensity of the PEF pretreatment, the use of more severe CO pressures and HPCD processing time did not determine any further reduction of the microbial population. The inactivation kinetics obtained after the combined treatment were fitted with the Peleg equation.