Lethal and sublethal injury and kinetics of Escherichia coli, Listeria monocytogenes and Staphylococcus aureus in milk by pulsed electric fields

Abstract

Lethal and sublethal injury of two Gram-positive (Staphylococcus aureus and Listeria monocytogenes) and one Gram-negative (Escherichia coli) bacteria in milk by pulsed electric fields (PEF) were determined using non-selective and selective media. PEF inactivation kinetics including lethal and sublethal injury fractions was also studied. The proportion of the sublethally injured microbial cells depended on the microorganisms, electric field strength and treatment time. The proportion of sublethally injured microbial cells reached maximum after a specific PEF treatment, and it kept constant or progressively decreased at greater electric field strengths and with longer PEF treatments. For the strain of L. monocytogenes, the proportion of sublethally injured cells increased from 18.98% to 43.64% with the increasing electric field strength from15 to 30 kV/cm. While for the strains of E. coli and S. aureus, the proportion of sublethally injured cells achieved the maximum (40.74% and 36.51%, respectively) at 25 kV/cm and then decreased. The proportion of the sublethally injured microbial cells reached maximum at 400 μs (S. aureus and L. monocytogenes) or 500 μs (E. coli), and decreased at longer treatments at 30 kV/cm. The PEF inactivation kinetics including lethal and sublethally injured fractions was analyzed by the Hülsheger model, and the model parameters (EC, tC, kE, bt) for lethal and sublethal injury were also calculated.