Inactivation of foodborne pathogens in milk using dynamic high pressure

Improving the microbiological safety of perishable foods is currently a major preoccupation in the food industry. The aim of this study was to investigate the inactivation of three major food pathogens (Listeria monocytogenes [LSD 105-1], Escherichia coli O157:H7 [ATCC 35150], and Salmonella enterica serotype Enteritidis ATCC [13047]) by dynamic high pressure (DHP) in order to evaluate its potential as a new alternative for the cold pasteurization of milk. The effectiveness of DHP treatment against L monocytogenes, E. coli O157H7, and Salmonella Enteritidis was first evaluated in 0.01 M phosphate-buffered saline (PBS) at pH 7.2 as a function of applied pressure (100, 200, and 300 MPa) and of the number of passes (1, 3, and 5) at 25degreesC. A single pass at 100 MPa produced no significant inactivation of the three pathogens, while increasing the pressure up to 300 MPa or the number of passes to five increased inactivation. From an initial count of 8.3 log CFU/ml, complete inactivation of viable L. monocytogenes was achieved after three successive passes at 300 MPa, while 200-MPa treatments with three and five passes completely eliminated viable Salmonella Enteritidis and E. coli O157117, respectively. The effectiveness of DHP for the inactivation of these pathogens was compared to that of hydrostatic high pressure (HHP) using the same pressure (200 MPa, single pass at 25degreesC). In general, two additional log reductions in viable count were obtained with DHP. DHP was less effective against L. monocytogenes and E. coli O157H7 in raw milk than in PBS. After five passes at 200 MPa, an 8.3-log reduction was obtained for E. coli O157117, while a reduction of about 5.8 log CFU/ml was obtained for L. monocytogenes exposed to 300 MPa for five passes. Exposing milk or buffer samples to mild heating (45 to 60degreesC) prior to dynamic pressurization enhanced the lethal effect of DHP. The inactivation of pathogens also depended on the initial bacterial concentration. The highest reduction was obtained when the bacterial load did not exceed 10(5) CFU/ml. In conclusion, DHP was shown to be very effective for the of the tested pathogens. It offers a promising alternative for the cold pasteurization of milk and possibly other liquid foods.