Use of linear, Weibull, and log-logistic functions to model pressure inactivation of seven foodborne pathogens in milk.

Survival curves of six foodborne pathogens suspended in ultra high-temperature (UHT) whole milk and exposed to high hydrostatic pressure at 21.5 degrees C were obtained. Vibrio parahaemolyticus was treated at 300 MPa and other pathogens, Listeria monocytogenes, Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Salmonella enterica serovar Typhimurium, and Staphylococcus aureus were treated at 600 MPa. All the survival curves showed a rapid initial drop in bacterial counts followed by tailing caused by a diminishing inactivation rate. A linear model and two nonlinear models were fitted to these data and the performances of these models were compared using mean square error (MSE) values. The log-logistic and Weibull models consistently produced better fits to the inactivation data than the linear model. The mean MSE value of the linear model was 6.1, while the mean MSE values were 0.7 for the Weibull model and 0.3 for the log-logistic model. There was no correlation between pressure resistance and the taxonomic group the bacteria belong to. The order, most to least pressure-sensitive, of the single strains tested was: V. parahaemolyticus (gram negative)<L. monocytogenes (gram positive)<Salmonella Typhimurium (gram negative) approximately = Salmonella Enteritidis (gram negative)<E. coli O157:H7 approximately = Staphylocollus aureus (gram positive)<Shigella flexneri (gram negative). The most pressure-resistant gram-negative bacterium, Shigella flexneri, and most pressure resistant gram-positive bacterium, Staphylocollus aureus, were pressurized at 50 degrees C. Staphylocollus aureus was treated at 500 MPa and Shigella flexneri at 600 MPa. Elevated temperature considerably enhanced pressure inactivation of these two pathogens, but did not affect the overall shape of the survival curves. Pressure level (250 MPa) and substrate (1% peptone water plus 3% NaCl) in which V. parahaemolyticus was suspended affected the shape of survival curves of V. parahaemolyticus.