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Inactivation of Escherichia coli in Milk by High-HydrostaticPressure Treatment in Combination with Antimicrobial Peptides

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We studied the inactivation in milk of four Escherichia coli strains (MG1655 and three pressure-resistant mutants isolated from MG1655) by high hydrostatic pressure, alone or in combination with the natural antimicrobial peptides lysozyme and nisin and at different temperatures (10 to 50°C). Compared with that of phosphate buffer, the complex physicochemical environment of milk exerted a strong protective effect on E. coli MG1655 against high-hydrostatic-pressure inactivation, reducing inactivation from 7 logs at 400 MPa to only 3 logs at 700 MPa in 15 min at 20 C. An increase in lethality was achieved by addition of high concentrations of lysozyme (400 g/ml) and nisin (400 IU/ml) to the milk before pressure treatment. The additional reduction amounted maximally to 3 logs in skim milk at 550 MPa but was strain dependent and significantly reduced in 1.55% fat and whole milk. An increase of the process temperature to 50°C also enhanced inactivation, particularly for the parental strain, but even in the presence of lysozyme and nisin, a 15-min treatment at 550 MPa and 50°C in skim milk allowed decimal reductions of only 4.5 to 6.9 for the pressure-resistant mutants. A substantial improvement of inactivation efficiency at ambient temperature was achieved by application of consecutive, short pressure treatments interrupted by brief decompressions. Interestingly, this pulsed-pressure treatment enhanced the sensitivity of the cells not only to high pressure but also to the action of lysozyme and nisin.

Document Type: Research Article

Affiliations: Laboratory of Food Microbiology, Katholieke Universiteit Leuven, Kard. Mercierlaan 92, B-3001 Heverlee, Belgium

Publication date: November 1, 1999

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