Use of Hydrostatic Pressure for Inactivation of Microbial Contaminants in Cheese

The objective of this study was to determine the effect of high pressure (HP) on the inactivation of microbial contaminants in Cheddar cheese (Escherichia coli K-12, Staphylococcus aureus ATCC 6538, and Penicillium roqueforti IMI 297987). Initially, cheese slurries inoculated with E. coli, S. aureus, and P. roqueforti were used as a convenient means to define the effects of a range of pressures and temperatures on the viability of these microorganisms. Cheese slurries were subjected to pressures of 50 to 800 MPa for 20 min at temperatures of 10, 20, and 30°C. At 400 MPa, the viability of P. roqueforti in cheese slurry decreased by >2-log-unit cycles at 10°C and by 6-log-unit cycles at temperatures of 20 and 30°C. S. aureus and E. coli were not detected after HP treatments in cheese slurry of >600 MPa at 20°C and >400 MPa at 30°C, respectively. In addition to cell death, the presence of sublethally injured cells in HP-treated slurries was demonstrated by differential plating using nonselective agar incorporating salt or glucose. Kinetic experiments of HP inactivation demonstrated that increasing the pressure from 300 to 400 MPa resulted in a higher degree of inactivation than increasing the pressurization time from 0 to 60 min, indicating a greater antimicrobial impact of pressure. Selected conditions were subsequently tested on Cheddar cheese by adding the isolates to cheese milk and pressure treating the resultant cheeses at 100 to 500 MPa for 20 min at 20°C. The relative sensitivities of the isolates to HP in Cheddar cheese were similar to those observed in the cheese slurry, i.e., P. roqueforti was more sensitive than E. coli, which was more sensitive than S. aureus. The organisms were more sensitive to pressure in cheese than slurry, especially with E. coli. On comparison of the sensitivities of the microorganisms in a pH 5.3 phosphate buffer, cheese slurry, and Cheddar cheese, greatest sensitivity to HP was shown in the pH 5.3 phosphate buffer by S. aureus and P. roqueforti while greatest sensitivity to HP by E. coli was exhibited in Cheddar cheese. Therefore, the medium in which the microorganisms are treated is an important determinant of the level of inactivation observed.     

Combined effect of high-pressure treatments and bacteriocin-producing lactic acid bacteria on inactivation of Escherichia coli O157:H7 in raw-milk cheese

The effect of high-pressure (HP) treatments combined with bacteriocins of lactic acid bacteria (LAB) produced in situ on the survival of Escherichia coli O157:H7 in cheese was investigated. Cheeses were manufactured from raw milk inoculated with E. coli O157:H7 at approximately 10(5) CFU/ml. Seven different bacteriocin-producing LAB were added at approximately 10(6) CFU/ml as adjuncts to the starter. Cheeses were pressurized on day 2 or 50 at 300 MPa for 10 min or 500 MPa for 5 min, at 10 degrees C in both cases. After 60 days, E. coli O157:H7 counts in cheeses manufactured without bacteriocin-producing LAB and not pressurized were 5.1 log CFU/g. A higher inactivation of E. coli O157:H7 was achieved in cheeses without bacteriocin-producing LAB when 300 MPa was applied on day 50 (3.8-log-unit reduction) than if applied on day 2 (1.3-log-unit reduction). Application of 500 MPa eliminated E. coli O157:H7 in 60-day-old cheeses. Cheeses made with bacteriocin-producing LAB and not pressurized showed a slight reduction of the pathogen. Pressurization at 300 MPa on day 2 and addition of lacticin 481-, nisin A-, bacteriocin TAB 57-, or enterocin AS-48-producing LAB were synergistic and reduced E. coli O157:H7 counts to levels below 2 log units in 60-day-old cheeses. Pressurization at 300 MPa on day 50 and addition of nisin A-, bacteriocin TAB 57-, enterocin I-, or enterocin AS-48-producing LAB completely inactivated E. coli O157:H7 in 60-day-old cheeses. The application of reduced pressures combined with bacteriocin-producing LAB is a feasible procedure to improve cheese safety.     

Reduction of indicator and pathogenic microorganisms by psychrophilic anaerobic digestion in swine slurries

The objective of this study was to evaluate the efficiency of a low temperature anaerobic treatment to reduce viable populations of indicator microorganisms (total coliforms, Escherichia coli) and the presence of selected pathogens (Salmonella, Yersinia enterocolitica, Cryptosporidium and Giardia) in swine slurries from different sources. Experiments were carried out in 40 l Sequencing Batch Reactors (SBRs). Experimental results indicated that anaerobic digestion of swine manure slurry at 20 degrees C for 20 days in an intermittently fed SBR: (1) reduced indigenous populations of total coliforms by 97.94-100%; (2) reduced indigenous populations of E. coli by 99.67-100%; (3) resulted in undetectable levels of indigenous strains of Salmonella, Cryptosporidium, and Giardia. It can be considered as a promising method for reducing indigenous indicator and pathogenic microorganisms populations in liquid swine manure slurries.     

Survival and growth of Escherichia coli O157:H7 in ground, roasted beef as affected by pH, acidulants, and temperature.

A study was undertaken to determine the fate of Escherichia coli O157:H7 in ground, roasted beef as influenced by the combined effects of pH, acidulants, temperature, and time. There was essentially no change in the viable population of E. coli O157:H7 when beef salads (pH 5.40 to 6.07) containing up to 40% mayonnaise were incubated at 5 degrees C for up to 72 h. At 21 and 30 degrees C, significant (P < or = 0.05) increases in populations of the organism occurred in salads containing 16 to 32% mayonnaise (pH 5.94 to 5.55) between 10 and 24 h of incubation. Death was more rapid as the pH of acidified beef slurries incubated at 5 degrees C was decreased from 5.98 to 4.70. E. coli O157:H7 grew in control slurries (pH 5.98) and in slurries containing citric and lactic acids (pHs 5.00 and 5.40) incubated at 21 degrees C for 24 h; decreases occurred in slurries acidified to pHs 4.70, 5.00, and 5.40 with acetic acid or pH 4.70 with citric or lactic acid. At 30 degrees C, populations decreased in slurries acidified to pHs 4.70 and 5.00 with acetic acid. Citric and lactic acids failed to prevent significant increases in populations in slurries at pH 4.70 to 5.40 between 10 and 24 h of incubation. The order of effectiveness of acidulants in inhibiting growth was acetic acid > lactic acid > or = citric acid. The same order was observed for inactivation of E. coli O157:H7 in acidified (pH 5.00) beef slurry heated at 54 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inactivation of Escherichia coli and Listeria innocua in milk by combined treatment with high hydrostatic pressure and the lactoperoxidase system

We have studied inactivation of four strains each of Escherichia coli and Listeria innocua in milk by the combined use of high hydrostatic pressure and the lactoperoxidase-thiocyanate-hydrogen peroxide system as a potential mild food preservation method. The lactoperoxidase system alone exerted a bacteriostatic effect on both species for at least 24 h at room temperature, but none of the strains was inactivated. Upon high-pressure treatment in the presence of the lactoperoxidase system, different results were obtained for E. coli and L. innocua. For none of the E. coli strains did the lactoperoxidase system increase the inactivation compared to a treatment with high pressure alone. However, a strong synergistic interaction of both treatments was observed for L. innocua. Inactivation exceeding 7 decades was achieved for all strains with a mild treatment (400 MPa, 15 min, 20 degrees C), which in the absence of the lactoperoxidase system caused only 2 to 5 decades of inactivation depending on the strain. Milk as a substrate was found to have a considerable effect protecting E. coli and L. innocua against pressure inactivation and reducing the effectiveness of the lactoperoxidase system under pressure on L. innocua. Time course experiments showed that L. innocua counts continued to decrease in the first hours after pressure treatment in the presence of the lactoperoxidase system. E. coli counts remained constant for at least 24 h, except after treatment at the highest pressure level (600 MPa, 15 min, 20 degrees C), in which case, in the presence of the lactoperoxidase system, a transient decrease was observed, indicating sublethal injury rather than true inactivation.     

Combined Effect of High-Pressure Treatments and Bacteriocin-Producing Lactic Acid Bacteria on Inactivation of Escherichia coli O157:H7 in Raw-Milk Cheese

The effect of high-pressure (HP) treatments combined with bacteriocins of lactic acid bacteria (LAB) produced in situ on the survival of Escherichia coli O157:H7 in cheese was investigated. Cheeses were manufactured from raw milk inoculated with E. coli O157:H7 at approximately 10⁵ CFU/ml. Seven different bacteriocin-producing LAB were added at approximately 10⁶ CFU/ml as adjuncts to the starter. Cheeses were pressurized on day 2 or 50 at 300 MPa for 10 min or 500 MPa for 5 min, at 10°C in both cases. After 60 days, E. coli O157:H7 counts in cheeses manufactured without bacteriocin-producing LAB and not pressurized were 5.1 log CFU/g. A higher inactivation of E. coli O157:H7 was achieved in cheeses without bacteriocin-producing LAB when 300 MPa was applied on day 50 (3.8-log-unit reduction) than if applied on day 2 (1.3-log-unit reduction). Application of 500 MPa eliminated E. coli O157:H7 in 60-day-old cheeses. Cheeses made with bacteriocin-producing LAB and not pressurized showed a slight reduction of the pathogen. Pressurization at 300 MPa on day 2 and addition of lacticin 481-, nisin A-, bacteriocin TAB 57-, or enterocin AS-48-producing LAB were synergistic and reduced E. coli O157:H7 counts to levels below 2 log units in 60-day-old cheeses. Pressurization at 300 MPa on day 50 and addition of nisin A-, bacteriocin TAB 57-, enterocin I-, or enterocin AS-48-producing LAB completely inactivated E. coli O157:H7 in 60-day-old cheeses. The application of reduced pressures combined with bacteriocin-producing LAB is a feasible procedure to improve cheese safety.     

Biphasic inactivation kinetics of Escherichia coli in liquid whole egg by high hydrostatic pressure treatments.

Kinetic studies on the isothermal high hydrostatic pressure (HHP) inactivation of Escherichia coli in liquid whole egg (LWE) were performed at 5 and 25 degrees C in the pressure range of 250-400 MPa. The characteristic tailing inactivation curves were described by a first-order biphasic model. As compared to a previous rheological study, it is suggested that the phase change of LWE during pressure treatment affects the inactivation rate of E. coli. Within the processing criteria where the rheological properties of LWE were still comparable to those of fresh LWE, HHP treatments at 5 degrees C induced more E. coli inactivations than those at 25 degrees C. From the results of approximately 3 log reductions of E. coli and over 5 log reductions of Pseudomonas and Paenibacillus, HHP treatment of LWE at 5 degrees C is regarded to be as effective as conventional thermal pasteurization. However, no post-process contamination and the consistency of temperature during preparation, HHP treatment, and storage provide clear processing advantages.     

Inactivation of Staphylococcus aureus in raw milk cheese by combinations of high-pressure treatments and bacteriocin-producing lactic acid bacteria

AIMS: To investigate the combined effect of high-pressure treatments (HPT) and milk inoculation with bacteriocin-producing lactic acid bacteria (BP-LAB) on the survival of Staphylococcus aureus during ripening of raw milk cheese. METHODS AND RESULTS: Cheeses were manufactured from raw milk artificially contaminated with S. aureus at ca 5 log CFU ml(-1), a commercial starter culture and one of seven strains of BP-LAB, added as adjuncts at 0.1%. HPT of cheeses were performed on days 2 or 50 at 300 MPa (10 degrees C, 10 min) or 500 MPa (10 degrees C, 5 min). On day 3, S. aureus counts were 6.46 log CFU g(-1) in control cheese. Milk inoculation with different BP-LAB lowered S. aureus counts on day 3 when compared with control cheese by up to 0.46 log CFU g(-1), HPT at 300 MPa on day 2 by 0.45 log CFU g(-1) and HPT at 500 MPa on day 2 by 2.43 log CFU g(-1). Combinations of BP-LAB with HPT at 300 and 500 MPa on day 2 lowered S. aureus counts on day 3 by up to 1.02 and 4.00 log CFU g(-1) respectively. CONCLUSIONS: The combined effect of milk inoculation with some of the BP-LAB tested and HPT of cheese on S. aureus inactivation was synergistic. SIGNIFICANCE AND IMPACT OF THE STUDY: The combination of HPT at lower pressures with BP-LAB inoculation is a feasible system to improve cheese safety in case of deleterious effects on cheese quality caused by HPT at higher pressures.     

High-pressure inactivation of Saccharomyces cerevisiae and Lactobacillus plantarum at subzero temperatures

High hydrostatic pressure is a new technology in the food processing industry, and is used for cold pasteurization of food products. However, the pressure inactivation of food-borne microorganisms requires very high pressures (generally more than 400 MPa) and long pressure holding times (5 min or more). Carrying out pressure processing at low temperatures without freezing can reduce these parameters, which presently limit the application of this technology, in keeping the quality of fresh raw product. The yeast, Saccharomyces cerevisiae and the bacterium, Lactobacillus plantarum were pressurized for 10 min at temperatures between -20 and 25 degrees C and pressure between 100 and 350 MPa. Pressurization at subzero temperatures without freezing significantly enhanced the effect of pressure. For example, at a pressure of 150 MPa, the decrease in temperature from ambient to -20 degrees C allowed an increase in the pressure-induced inactivation from less than 1 log up to 7-8 log for each microorganism studied. However, for comparable inactivation levels, the kinetics of microorganism inactivation did not differ, which suggests identical inactivation mechanisms. Implications of water thermodynamical properties like compression, protein denaturation, as well as membrane phase transitions, are discussed.     

Induction of Shiga toxin-converting prophage in Escherichia coli by high hydrostatic pressure

Since high hydrostatic pressure is becoming increasingly important in modern food preservation, its potential effects on microorganisms need to be thoroughly investigated. In this context, mild pressures (<200 MPa) have recently been shown to induce an SOS response in Escherichia coli MG1655. Due to this response, we observed a RecA- and LexA-dependent induction of lambda prophage upon treating E. coli lysogens with sublethal pressures. In this report, we extend this observation to lambdoid Shiga toxin (Stx)-converting bacteriophages in MG1655, which constitute an important virulence trait in Stx-producing E. coli strains (STEC). The window of pressures capable of inducing Stx phages correlated well with the window of bacterial survival. When pressure treatments were conducted in whole milk, which is known to promote bacterial survival, Stx phage induction could be observed at up to 250 MPa in E. coli MG1655 and at up to 300 MPa in a pressure-resistant mutant of this strain. In addition, we found that the intrinsic pressure resistance of two types of Stx phages was very different, with one type surviving relatively well treatments of up to 400 MPa for 15 min at 20 degrees C. Interestingly, and in contrast to UV irradiation or mitomycin C treatment, pressure was not able to induce Stx prophage or an SOS response in several natural Stx-producing STEC isolates.     

Synergistic and antagonistic effects of combined subzero temperature and high pressure on inactivation of Escherichia coli

The combined effects of subzero temperature and high pressure on the inactivation of Escherichia coli K12TG1 were investigated. Cells of this bacterial strain were exposed to high pressure (50 to 450 MPa, 10-min holding time) at two temperatures (-20 degrees C without freezing and 25 degrees C) and three water activity levels (a(w)) (0.850, 0.992, and ca. 1.000) achieved with the addition of glycerol. There was a synergistic interaction between subzero temperature and high pressure in their effects on microbial inactivation. Indeed, to achieve the same inactivation rate, the pressures required at -20 degrees C (in the liquid state) were more than 100 MPa less than those required at 25 degrees C, at pressures in the range of 100 to 300 MPa with an a(w) of 0.992. However, at pressures greater than 300 MPa, this trend was reversed, and subzero temperature counteracted the inactivation effect of pressure. When the amount of water in the bacterial suspension was increased, the synergistic effect was enhanced. Conversely, when the a(w) was decreased by the addition of solute to the bacterial suspension, the baroprotective effect of subzero temperature increased sharply. These results support the argument that water compression is involved in the antimicrobial effect of high pressure. From a thermodynamic point of view, the mechanical energy transferred to the cell during the pressure treatment can be characterized by the change in volume of the system. The amount of mechanical energy transferred to the cell system is strongly related to cell compressibility, which depends on the water quantity in the cytoplasm.     

The protective effect of some food ingredients on Staphylococcus aureus MF31

The upper limiting temperature of growth of Staphylococcus aureus MF31 in heart infusion broth (HI) was about 44 degrees C but addition of monosodium glutamate (MSG) and soy sauce permitted the organism to grow above this temperature. This effect is similar to that of NaCl. Tomato ketchup, Worcestershire and HP sauces added to HI did not allow growth at the non-permissive temperature of 46 degrees C but death was delayed. Staphylococcus aureus died in unsupplemented chicken meat slurry at 46 degrees C but grew at 48 degrees C in slurry supplemented with 5.8% NaCl and survived incubation for 18 h at 50 degrees C in slurry supplemented with 5.8% NaCl and 5% MSG. Cultures grown at 37 degrees C had a D60 value of 2 min in 50 mmol/l Tris (pH 7.2) buffer. Cultures grown at 46 degrees C in HI containing 5.8% NaCl had a D60 value of 8 min in Tris buffer. Addition of 5.8% NaCl plus 5% MSG to the buffer increased the D60 by a factor of about 7 for both cultures. In storage experiments at room temperature, the culture grown at 37 degrees C and at 46 degrees C plus 5.8% NaCl died at about the same rate in salami. In milk powder, however, the count of 37 degrees C culture decreased from 10% g to 10(6)/g in 5 weeks while the count of 46 degrees C culture remained unchanged. In cottage cheese, freeze-dried rice and macaroni, the 37 degrees C cultures also died more rapidly. It is suggested that cultures grown at 46 degrees C plus 5.8% NaCl may be suitable for experiments with artificially contaminated foods.     

The effect of low pressure-homogenized lactic cultures on the development of proteolysis in cheese slurry

Summary The aim of this study was to determine the effect of low pressure-homogenization of lactic acid bacteria (LAB) on the development of proteolysis in the slurry medium. For the slurry, the milk was pasteurized at 65 °C for 30 min, cooled to 32 °C and coagulated. The curd obtained was blended; the dry matter was adjusted to 30% by adding distilled water, placed into the flasks and autoclaved. The LAB Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, and Lactobacillus helveticus were used in cheese slurry. Homogenization was performed at 30 MPa and 40 °C. The cheese slurries were incubated with and without homogenized cultures at 9 and 30 °C for up to 72 h. During incubation, the changes in trichloroacetic acid-soluble nitrogen (TCA-SN) and phosphotungstic acid-soluble nitrogen (PTA-SN) as well as pH were monitored. The results showed that pH development was slower in the slurries to which homogenized culture was added. Higher TCA-SN and PTA-SN values were obtained from the slurries incubated at 30 °C. Moreover, higher TCA-SN and PTA-SN values were found in the slurries incubated with homogenized mesophilic culture and Lb. helveticus (P<0.05). The results suggested that homogenization of the cultures was a promising method for the acceleration of cheese ripening.     

Fate of Escherichia coli O157:H7 during the manufacture of Mozzarella cheese

AIMS: The fate of Escherichia coli O157:H7 was investigated during the manufacture of Mozzarella cheese. METHODS AND RESULTS: The Mozzarella cheese was made from unpasteurized milk which was inoculated to contain ca 10(5) cfu ml(-1)E. coli O157:H7. Two different heating temperatures (70 and 80 degrees C), commonly used during curd stretching, were investigated to determine their effects on the viability of E. coli O157:H7 in Mozzarella cheese. Stretching at 80 degrees C for 5 min resulted in the loss of culturability of E. coli O157:H7 strains, whereas stretching at 70 degrees C reduced the number of culturable E. coli O157:H7 by a factor of 10. CONCLUSIONS: The results show that stretching curd at 80 degrees C for 5 min is effective in controlling E. coli O157:H7 during the production of Mozzarella cheese. Brining and storage at 4 degrees C for 12 h was less effective than the stretching. Significance and Impact of the Study: Mozzarella cheese should be free of E. coli O157:H7 only if temperatures higher than or equal to 80 degrees C are used during milk processing.     

Combined effects of high hydrostatic pressure and temperature for inactivation of Bacillus anthracis spores

Spores of Bacillus anthracis are known to be extremely resistant to heat treatment, irradiation, desiccation, and disinfectants. To determine inactivation kinetics of spores by high pressure, B. anthracis spores of a Sterne strain-derived mutant deficient in the production of the toxin components (strain RP42) were exposed to pressures ranging from 280 to 500 MPa for 10 min to 6 h, combined with temperatures ranging from 20 to 75 degrees C. The combination of heat and pressure resulted in complete destruction of B. anthracis spores, with a D value (exposure time for 90% inactivation of the spore population) of approximately 4 min after pressurization at 500 MPa and 75 degrees C, compared to 160 min at 500 MPa and 20 degrees C and 348 min at atmospheric pressure (0.1 MPa) and 75 degrees C. The use of high pressure for spore inactivation represents a considerable improvement over other available methods of spore inactivation and could be of interest for antigenic spore preparation.     

Comparison of ozone inactivation, in flowing water, of hepatitis A virus, poliovirus 1, and indicator organisms

In steadily flowing water at 20 degrees C and pH 7, five organisms had the following order of resistance to ozone (at constant levels of ozone): poliovirus 1 (PV1) less than Escherichia coli less than hepatitis A virus (HAV) less than Legionella pneumophila serogroup 6 less than Bacillus subtilis spores. The tests were repeated at 10 degrees C with HAV, PV1, and E. coli. Ozone inactivation of HAV and E. coli was faster at 10 degrees C than at 20 degrees C. At 20 degrees C, 0.25 to 0.38 mg of O3 per liter was required for complete inactivation of HAV but only 0.13 mg of O3 per liter was required for complete inactivation of PV1.     

Comparison of ozone inactivation, in flowing water, of hepatitis A virus, poliovirus 1, and indicator organisms.

In steadily flowing water at 20 degrees C and pH 7, five organisms had the following order of resistance to ozone (at constant levels of ozone): poliovirus 1 (PV1) less than Escherichia coli less than hepatitis A virus (HAV) less than Legionella pneumophila serogroup 6 less than Bacillus subtilis spores. The tests were repeated at 10 degrees C with HAV, PV1, and E. coli. Ozone inactivation of HAV and E. coli was faster at 10 degrees C than at 20 degrees C. At 20 degrees C, 0.25 to 0.38 mg of O3 per liter was required for complete inactivation of HAV but only 0.13 mg of O3 per liter was required for complete inactivation of PV1.     

A staphylococcal plasmid that replicates and expresses ampicillin, gentamicin and amikacin resistance in Escherichia coli

Plasmid pPG1 from Staphylococcus aureus coding for ampicillin (Apr), gentamicin (Gmr) and amikacin (Akr) resistance was transformed into Escherichia coli. Transformation efficiency was about 2 x 10(3) transformants/micrograms of plasmid DNA. The plasmids present in the E. coli transformants were identical to pPG1 according to their restriction patterns. The copy number of pPG1 was estimated to be at least 20-times less in E. coli than in S. aureus. The minimal inhibitory concentrations (MICs) for Ap and Gm were lower in E. coli than in S. aureus. However, the MIC for Ak was higher in E. coli transformants than in S. aureus. pPG1 was maintained in the E. coli transformants for at least 80 generations at 37 degrees C without antibiotic selection pressure.     

Kinetics of pressure inactivation at subzero and elevated temperature of lipoxygenase in crude green bean (Phaseolus vulgaris L.) extract

Lipoxygenase (LOX) in crude green bean extract was irreversibly inactivated by pressure treatments combined with subzero or elevated temperature. LOX inactivation was described accurately assuming a first-order reaction. In the entire pressure-temperature domain studied (200 to 700 MPa and -10 to 60 degrees C), an increase in pressure at constant temperature enhanced the LOX inactivation rate, whereas at constant pressure, an increase in reaction rate was obtained by either increasing or decreasing temperature at 20 degrees C. At elevated pressure, LOX exhibited the greatest stability around 20 degrees C. Also the pressure dependence of the inactivation rate constants for LOX was the highest around 20 degrees C. On the basis of the estimated LOX inactivation rate constants, an iso-rate contour diagram as a function of pressure and temperature was constructed, and an empirical mathematical model describing the combined pressure-temperature dependence of the LOX inactivation rate constants was formulated.     

Moderate temperatures affect Escherichia coli inactivation by high-pressure homogenization only through fluid viscosity

The inactivation of suspensions of Escherichia coli MG1655 by high-pressure homogenization was studied over a wide range of pressures (100-300 MPa) and initial temperatures of the samples (5-50 degrees C). Bacterial inactivation was positively correlated with the applied pressure and with the initial temperature. When samples were adjusted to different concentrations of poly(ethylene glycol) to have the same viscosity at different temperatures below 45 degrees C and then homogenized at these temperatures, no difference in inactivation was observed. These observations strongly suggest, for the first time, that the influence of temperature on bacterial inactivation by high-pressure homogenization is only through its effect on fluid viscosity. At initial temperatures > or =45 degrees C, corresponding to an outlet sample temperature >65 degrees C, the level of inactivation was higher than what would be predicted on the basis of the reduced viscosity at these temperatures, suggesting that under these conditions heat starts to contribute to cellular inactivation in addition to the mechanical effects that are predominant at lower temperatures. Second-order polynomial models were proposed to describe the impact of a high-pressure homogenization treatment of E. coli MG1655 as a function of pressure and temperature or as a function of pressure and viscosity. The pressure-viscosity inactivation model provided a better quality of fit of the experimental data and furthermore is more comprehensive and versatile than the pressure-temperature model because in addition to viscosity it implicitly incorporates temperature as a variable.