Among food-borne pathogens, some strains could be resistant to hydrostatic pressure treatment. This information is necessary to establish processing parameters to ensure safety of pressure-pasteurized foods (N. Kalchayanand, A. Sikes, C. P. Dunne, and B. Ray, J. Food Prot. 61:425–431, 1998). We studied variation in pressure resistance among strains of Listeria monocytogenes, Staphylococcus aureus,Escherichia coli O157:H7, and Salmonellaspecies at two temperatures of pressurization. Early-stationary-phase cells in 1% peptone solution were pressurized at 345 MPa either for 5 min at 25°C or for 5, 10, or 15 min at 50°C. The viability loss (in log cycles) following pressurization at 25°C ranged from 0.9 to 3.5 among nine L. monocytogenes strains, 0.7 to 7.8 among sevenS. aureus strains, 2.8 to 5.6 among six E. coliO157:H7 strains, and 5.5 to 8.3 among six Salmonellastrains. The results show that at 25°C some strains of each species are more resistant to pressure than the others. However, when one resistant and one sensitive strain from each species were pressurized at 345 MPa and 50°C, the population of all except the resistantS. aureus strain was reduced by more than 8 log cycles within 5 min. Viability loss of the resistant S. aureusstrain was 6.3 log cycles even after 15 min of pressurization. This shows that strains of food-borne pathogens differ in resistance to hydrostatic pressure (345 MPa) at 25°C, but this difference is greatly reduced at 50°C. Pressurization at 50°C, in place of 25°C, will ensure greater safety of foods.

High hydrostatic pressure, because it can kill microorganisms, is being investigated for potential use as a nonthermal food preservation method. The objective of this study was to determine the hydrostatic pressurization parameters, pressure, time, and temperature, and a bacteriocin that in combination would destroy 7 to 8 log cycles of pathogenic and spoilage bacterial populations. We suspended cells of Staphylococcus aureus, Listeria monocytogenes, Salmonella typhimurium, Escherichia coli O157:H7, Lactobacillus sake, Leuconostoc mesenteroides, Serratia liquefaciens, and Pseudomonas fluorescens in peptone solution and exposed them to the combination of treatments. The combined parameters used were hydrostatic pressure (138 to 345 MPa), time (5 to 15 min), temperature (25 to 50 degrees C), and pediocin AcH (3,000 AU/ml, final concentration). In general, cell death increased as the pressure, time, or temperature increased; however, the cells developed proportionately greater sensitivity as the pressure increased to 276 MPa and higher and the temperature increased above 35 degrees C. Pressurization for longer than 5 min, especially at lower pressure and temperature ranges, had very little added benefit. Among the four gram-negative species, E. coli O157:H7 was the most resistant to pressurization while among the four gram-positive species, L. sake and L. mesenteroides had greater resistance. The death rate at high pressure (345 MPa) and high temperature (50 degrees C) in combination followed first-order kinetics; at lower pressure and temperature combination it showed a late tailing effect. Estimated D value data indicated that even at 345 MPa and 50 degrees C an 8-log-cycle viability loss could not be achieved within 5 min for all eight species. However, when pediocin AcH was included during pressurization this loss was achieved.

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Sucrose laurates, sucrose palmitate, sucrose stearates, and monolaurin (Lauricidin) were evaluated for inhibitory effects against spores of Bacillus sp., Clostridium sporogenes PA3679, and Alicyclobacillus sp. in a model agar system. The combined treatment of sucrose laurate, high hydrostatic pressure, and mild heat was evaluated on spores of Bacillus and Alicyclobacillus in foods. The minimum inhibitory concentrations of the sucrose esters were higher than that of Lauricidin for all spores tested in the model agar system, but Lauricidin was not the most readily suspended in the test media. The sucrose laurates and sucrose palmitate were more effective and more readily suspended than the sucrose stearates. A combined treatment of sucrose laurate (<1.0%), 392 megaPascals (MPa) at 45 degrees C for 10 to 15 min provided 3- to 5.5-log10 CFU/ml reductions from initial populations of 10(6) CFU/ml for Bacillus subtilis 168 in milk, Bacillus cereus 14579 in beef, Bacillus coagulans 7050 in tomato juice (pH 4.5), Alicyclobacillus sp. N1089 in tomato juice (pH 4.5), and Alicyclobacillus sp. N1098 in apple juice. The most notable change in the appearance of the products was temporary foaming during mixing of the sucrose laurate in the foods. The effect of sucrose laurate appeared to be inhibitory rather than lethal to the spores. The inhibitory effects observed on Bacillus and Alicyclobacillus spores by the combined treatment of pressure, mild heat, and sucrose laurate appear promising for food applications where alternatives to high heat processing are desired.

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