Effect of electric field frequency on inactivation of the Bacillus subtilis spore and its mutants during ohmic heating

Abstract

Ohmic heating (OH) achieves superior spore inactivation compared to conventional heating (CH) under identical temperature conditions. This study examined the impact of different frequencies (60 Hz, 1 kHz, and 5 kHz) and temperatures (95°C, 105°C, and 115°C) at various applied field strengths (30 V/cm, 40 V/cm, and 50 V/cm) during OH on spores of Bacillus subtilis. To gain more insight on which spore components might interact with the field, we tested spore strains 533 (wild type), 578 (SASP-deficient), and 2318 (RecA-deficient), in comparison with CH. A specialized apparatus enabled matching of temperature profiles between OH and CH, with all experiments conducted while holding electric field strengths constant while allowing temperature to rise linearly to a final set point without a holding time. Spore inactivation was independent of frequency under conditions of lowest (low field strength-temperature combinations) and highest (high field strength-temperature combinations) lethality; however differentiation between frequencies became apparent at intermediate levels of frequency. Where such differences could be observed, 1 kHz yielded the highest inactivation. Removal of SASP or RecA components did not alter this general trend. Differences between frequencies could neither be correlated to power inputs nor to calculated molecular displacements. More rigorous modeling approaches will be needed to more accurately delineate frequency effects.