The combined effect of high hydrostatic pressure, heat and bacteriocins oninactivation of foodborne pathogens in milk and orange juice

The objective of this study was to combine pressure (345 MPa) with heat (50 degrees C), and bacteriocins (5000 AU/ml sample) for a short time (5 min) for the inactivation of relatively pressure-resistant strains of four foodb orne pathogens: Staphylococcus aureus, Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella in pasteurized milk and orange juice. Without b acteriocin addition, 5.5 log-cycle reduction was obtained for S. aureus 485 in milk whereas more than 8 log-cycle reduction was achieved for all the o ther strains studied. After storage of samples for 24 h at 4 degrees C, S. aureus 765 also gave positive results on selective media, where no growth w as observed for all the other micro-organisms assayed. Incubation of the sa me pressurized samples at 37 degrees C for 48 h showed growth of L. monocyt ogenes strains in addition to S. aureus strains, where still no growth was observed for E. coli O157:H7 and Salmonella strains in their respective sel ective media. For orange juice samples, more than 8 log-cycle reduction was achieved for all the bacterial species studied. No growth was seen for the se species on their respective selective media agar plates after storage at 4 degrees C for 24 h and at 37 degrees C for 48 h. When a bacteriocin-base d biopreservative (BP1) was combined with pressurization, more than 8 log-c ycle reduction in cell population of the resistant strains of S. aureus and L. monocytogenes were achieved in milk after pressurization. Milk samples were stored at 25 degrees C up to 30 days to test the effect of treatment a nd samples showed no growth whereas all the controls were positive.