Improving the microbial inactivation uniformity of pulsed electric field ohmic heating treatments of solid products

The application of Pulsed Electric Fields (PEF) at high frequencies leads to rapid and volumetric ohmic heating. Its application in solid foods could avoid the problem of conventional systems in which the external areas are over-treated in order to apply the desired heat treatment in the center. In this study it was evaluated the heating of technical agar cylinders by applying PEF (2.5 kV/cm, 50 Hz) by warming the electrodes to 25, 32 and 39 degrees C. Results showed that PEF heating without control of the electrode temperature showed temperature differences of 10.5 degrees C after 50 s. However, regulation of the electrodes at 39 degrees C reduced the gradient to 1.7 degrees C achieving 73.4 degrees C at the coldest point after 28 s. To evaluate the impact of the electrode temperature regulation on the uniformity of microbial inactivation, the inactivation of Listeria monocytogenes STCC 5672 (D60 degrees C = 0.63 min, z-value = 5.4 degrees C) was evaluated. Results showed that to inactivate 5-Log10 at the coldest spot of the cylinder, 68 s were necessary when electrode temperatures were not regulated, and 26 s when they were tempered to 39 degrees C, reaching a temperature of 71 degrees C.