Characteristics of Listeria monocytogenes important for pulsed electric field process optimization.

摘要

Processing-resistant Listeria monocytogenes strains appear sporadically in ready-to-eat food, and constitute a considerable challenge to food processors. Pulsed electric field (PEF) is an alternative to thermal processing and it inactivates pathogens in liquid food. The high-intensity electric pulses kill rapidly microbial contaminants. Since L. monocytogenes is among the most PEF-resistant non-sporing foodborne pathogens, this study focused on identifying characteristics of Listeria that may be useful for PEF process optimization.; Resistance to PEF treatment, at 25 kV/cm, varied among L. monocytogenes strains suspended in 0.1% NaCl (pH 7.0) or 50% acid whey (pH 4.2). Listeria monocytogenes OSY-8578 was identified as a potential target strain for process optimization, based on its high PEF-resistance. Resistance to PEF could not be associated with genotypic differences, as determined by pulsed field gel electrophoresis or arbitrarily primed-polymerase chain reaction method. However, the target strain, OSY-8578, had a unique genotype, which is potentially useful for its identification.; Sublethal PEF process (15 kV/cm for 29 mus) did not induce general stress response, a physiological state with over-expression of major molecular chaperones. On the contrary, expression of the chaperones, GroEL, GroES and DnaJ decreased within 5 to 20 min after PEF treatment. The decrease in expression was more marked for Scott A (PEF-sensitive) than OSY-8578 (PEF-resistant). Interestingly, the chaperone expression profile, after PEF treatment, paralleled an increased sensitivity of the pathogen to mild heat. This sensitization was not observed when mild heating was applied prior to PEF.; Lactobacillus plantarum ATCC 8014, but not L. innocua ATCC 33090, was identified as potential surrogate of the target L. monocytogenes, OSY-8578, when these bacteria were suspended in a neutral simple medium (e.g., 0.1% NaCl solution) and treated at 25 kV/cm for 144 mus. Inactivation of Listeria by PEF was greater in acid or acidified whey (pH 4.2) than in sweet whey (pH 6.8) or in 0.1% NaCl (pH 7.0). In acid or acidified cheese whey, Lb. plantarum was more resistant to PEF than was L. monocytogenes OSY-8578; the former bacterium, therefore, may serve as a surrogate of the latter in such media.