GERMINATION AND SUBSEQUENT INACTIVATION OFBACILLUS SUBTILISSPORES BY PULSED ELECTRIC FIELD TREATMENT

Abstract

ABSTRACT The use of high-intensity pulsed electric field (PEF) as a nonthermal method for triggering spore germination was investigated. Spores suspended in 0.01% NaCl solution (ca. 108cells/mL) were treated with an exponential-decay pulse (20–50 kV/cm) at selected temperatures. The inactivation resulting from the direct effect of PEF treatment was less than 0.5 log cycles, even with a relatively high electric field intensity of 40 kV/cm. Most of the surviving PEF-treated spores germinated after inoculation into selected germination media (0.85% NaCl solution, nutrient broth [NB], or tryptic soy broth [TSB]) and subsequent incubation at 37C for 40 min. The extent of germination increased with the treatment temperature and electric field intensity. More than 98% of surviving spores in TSB germinated during incubation after PEF treatment with an electric field strength of 40 kV/cm for 1,000 µs at 50C. The rate of germination in nutrient-rich TSB medium was higher than that in NB. PRACTICAL APPLICATIONS High-intensity pulsed electric field (PEF) treatments have been studied recently as an alternative method of conventional thermal sterilization. High-intensity PEF inactivation of microorganisms can be carried out at ambient temperature with a short treatment time of seconds or less without inducing a significant temperature increase. Considerable research has been conducted on the inactivation of microbial cells using PEF. However, a major obstacle to the application of PEF as a technology for the preservation of foods and pharmaceuticals is that it is inefficient at inactivating bacterial spores. Bacterial spores are also resistant to inactivation by adverse physical and chemical agents. Methods to overcome this problem have been studied extensively with the aim of establishing reliable sterilization procedures for the food and pharmaceutical industries.