Reduction of counts of Listeria monocytogenes in cheese by means of high hydrostatic pressure

Inactivation of Listeria monocytogenes (strains NCTC 11994 and Scott A) was evaluated in model cheeses submitted to 10 min HHP treatments of 300, 400 or 500 MPa at 5 or 20 degrees C. Counts were measured immediately after high hydrostatic pressure (HHP) treatment (day 1) and after 2, 15 and 30 days of storage at 8 degrees C. Both strains behaved significantly different after 400 and 500 MPa, being NCTC 11994 more sensitive. Scarce differences were found among final values at both HHP treatment temperatures. Initial reductions (log cfu/g) for 400 MPa at 20 degrees C were 2.9 +/- 0.2 for strain NCTC 11994 and 1.5 +/- 0.2 for Scott A. They reached after 30-day storage 5.3 +/- 0.2 and 4.6 +/- 0.4 log cfu/g for NCTC 11994 and Scott A, respectively. For 500 MPa treatments, day-1 reductions of both strains were around 5-log cfu/g, and counts fell below quantification limit after 30 days. Injured cells (around 0.8-log cfu/g) were mostly observed in 400 MPa treated samples on days 1 and 2. Starter cells suffered higher inactivation and injury. For 20 degrees C treatments, its final counts (log cfu/g) at 300, 400 and 500 MPa were: 8.5 +/- 0.2, 5.4 +/- 0.3 and 2.5 +/- 0.1, respectively. These figures evidence the HHP potential to improve safety of cheese products.     

Effect of small, acid-soluble proteins on spore resistance and germination under a combination of pressure and heat treatment

To find the range of pressure required for effective high-pressure inactivation of bacterial spores and to investigate the role of alpha/beta-type small, acid-soluble proteins (SASP) in spores under pressure treatment, mild heat was combined with pressure (room temperature to 65 degrees C and 100 to 500 MPa) and applied to wild-type and SASP-alpha-/beta- Bacillus subtilis spores. On the one hand, more than 4 log units of wild-type spores were reduced after pressurization at 100 to 500 MPa and 65 degrees C. On the other hand, the number of surviving mutant spores decreased by 2 log units at 100 MPa and by more than 5 log units at 500 MPa. At 500 MPa and 65 degrees C, both wild-type and mutant spore survivor counts were reduced by 5 log units. Interestingly, pressures of 100, 200, and 300 MPa at 65 degrees C inactivated wild-type SASP-alpha+/beta+ spores more than mutant SASP-alpha-/beta- spores, and this was attributed to less pressure-induced germination in SASP-alpha-/beta- spores than in wild-type SASP-alpha+/beta+ spores. However, there was no difference in the pressure resistance between SASP-alpha+/beta+ and SASP-alpha-/beta- spores at 100 MPa and ambient temperature (approximately 22 degrees C) for 30 min. A combination of high pressure and high temperature is very effective for inducing spore germination, and then inactivation of the germinated spore occurs because of the heat treatment. This study showed that alpha/beta-type SASP play a role in spore inactivation by increasing spore germination under 100 to 300 MPa at high temperature.     

High-pressure processing applied to cooked sausages: bacterial populations during chilled storage

Vacuum-packaged cooked sausages were pressurized at 500 MPa for 5 or 15 min at mild temperature (65 degrees C) and later stored at 2 and 8 degrees C for 18 weeks. Counts of aerobic mesophiles and psychrotrophs, lactic acid bacteria, enterobacteria, Baird-Parker microbiota, and Listeria spp. were determined 1 day and 3, 6, 9, 12, 15, and 18 weeks after treatment and compared with those of cooked sausages treated at 80 to 85 degrees C for 40 min. Pressurization generated reductions of about 4 log CFU/g in psychrotrophs and lactic acid bacteria. Enterobacteria and Listeria proved the most pressure sensitive; insignificant or no growth was detected throughout the study. Heat treatment inactivated psychrotrophs and enterobacteria similarly to pressure treatment. Listeria monocytogenes and enterotoxigenic Staphylococcus aureus were not found in treated samples. In general, there was no significant difference in counts of any bacterial populations either among treatments or between storage temperatures. High-pressure processing at mild temperature is an effective preservation method that can replace heat pasteurization applied to some cooked meat and poultry products after packaging.     

Conditions for a 5-log reduction of Vibrio vulnificus in oysters through high hydrostatic pressure treatment

Vibrio vulnificus is frequently associated with oysters, and since oysters are typically consumed raw on a half shell, they can pose a threat to public health due to ingestion of this pathogenic marine microorganism. Oysters should be processed to reduce the number of this pathogen. High pressure processing is gaining more and more acceptance among oyster processors due to its ability to shuck oysters while keeping the fresh-like characteristics of oysters. Nine strains of V. vulnificus were tested for their sensitivities to high pressure. The most pressure-resistant strain of V. vulnificus, MLT 403, was selected and used in the subsequent experiments to represent a worst case scenario for evaluation of the processing parameters for inactivation of V. vulnificus in oysters. To evaluate the effect of temperature on pressure inactivation of V. vulnificus, oyster meats were inoculated with V. vulnificus MLT 403 and incubated at room temperature for 24 h. Oyster meats were then blended and treated at 150 MPa for 4 min, and 200 MPa for 1 min. Pressure treatments were carried out at -2, 1, 5, 10, 20, 30, 40, and 45 degrees C. Cold temperatures (<20 degrees C) and slightly elevated temperatures (>30 degrees C) substantially increased pressure inactivation of V. vulnificus. For example, a 4-min treatment of 150 MPa at -2 and 40 degrees C reduced the counts of V. vulnificus by 4.7 and 2.8 log, respectively, while at 20 degrees C the same treatment only reduced counts by 0.5 log. Temperatures of -2 and 1 degrees C were used to determine the effect of pressure level, temperature, and treatment time on the inactivation of V. vulnificus infected to live oysters through feeding. To achieve a >5-log reduction in the counts of V. vulnificus in a relatively short treatment time (or=250 MPa at -2 or 1 degrees C.     

Effects of High‐Pressure Processing on Inactivation of Salmonella Typhimurium,Eating Quality,and Microstructure of Raw Chicken Breast Fillets

Abstract: High‐pressure inactivation of Salmonella Typhimurium DMST 28913, eating quality, and microstructure of pressurized raw chicken breast meat was determined. The inoculated samples (approximately 7 log CFU/g initial load) were processed at 300 and 400 MPa, using pressurized medium of 25 to 35 °C during pressurization. Weibull model was well fitted to the survival curves with tailing. Least severe conditions with acceptable inactivation levels were 300 MPa, 35 °C, 1 min (approximately 2 log reduction) and 400 MPa, 30 °C, 1 min (approximately 4 log reduction). Based on these 2 conditions, the 400 MPa treatment yielded the raw chicken meat with higher L* value, greater cooking loss, and lower water holding capacity. Cooked chicken breast meat prepared from the pressurized samples had firmer texture than the control. Scanning electron microscopic images showed that higher pressure resulted in increasing extent of protein coagulation and the contraction of the muscle bundles. Practical Application: For raw chicken breast fillet, 300 MPa, 35 °C, 1 min was the condition that reasonably reduced the load of Salmonella Typhimurium. However, the pressurized samples had greater cooking loss. Marination with brine containing sodium chloride and phosphate prior to pressurization might help improve this eating quality.     

Inactivation of Bacillus cereus spores in milk by mild pressure and heat treatments

The objective of this work was to study the germination and subsequent inactivation of Bacillus cereus spores in milk by mild hydrostatic pressure treatment. In an introductory experiment with strain LMG6910 treated at 40 degrees C for 30 min at 0, 100, 300 and 600 MPa, germination levels were 1.5 to 3 logs higher in milk than in 100 mM potassium phosphate buffer (pH 6.7). The effects of pressure and germination-inducing components present in the milk on spore germination were synergistic. More detailed experiments were conducted in milk at a range of pressures between 100 and 600 MPa at temperatures between 30 and 60 degrees C to identify treatments that allow a 6 log inactivation of B. cereus spores. The mildest treatment resulting in a 6 log germination was 30 min at 200 MPa/40 degrees C. Lower treatment pressures or temperatures resulted in considerably less germination, and higher pressures and temperatures further increased germination, but a small fraction of spores always remained ungerminated. Further, not all germinated spores were inactivated by the pressure treatment, even under the most severe conditions (600 MPa/60 degrees C). Two possible approaches to achieve a 6 log spore inactivation were identified, and validated in three additional B. cereus strains. The first is a single step treatment at 500 MPa/60 degrees C for 30 min, the second is a two-step treatment consisting of pressure treatment for 30 min at 200 MPa/45 degrees C to induce spore germination, followed by mild heat treatment at 60 degrees C for 10 min to kill the germinated spores. Reduction of the pressurization time to 15 min still allows a 5 log inactivation. These results illustrate the potential of high-pressure treatment to inactivate bacterial spores in minimally processed foods.     

Temperature and treatment time influence high hydrostatic pressure inactivation of feline calicivirus, a norovirus surrogate

Interest in high hydrostatic pressure processing as a nonthermal pasteurization process for foods continues to increase. Feline calicivirus (FCV), a propagable virus that is genetically related to the nonpropagable human noroviruses, was used for detailed evaluation of the high pressure processing parameters necessary for virus inactivation. Pressure inactivation curves of FCV strain KCD in Dulbecco's modified Eagle medium with 10% fetal bovine serum were obtained at 200 and 250 MPa as a function of time at room temperature. Pressure inactivation curves at 200 and 250 MPa also were determined as a function of temperature ranging from --10 to 50 degrees C at treatment times of 4 and 2 min, respectively. Tailing was observed for inactivation as a function of treatment time, indicating that the linear model was not adequate for describing these curves. The two nonlinear models, the log logistic and Weibull functions, consistently produced better fit to inactivation curves than did the linear model. The mean square errors were 0.381 for the log logistic model, 0.425 for the Weibull model, and 1.546 for the linear model. For inactivation as a function of temperature, FCV was most resistant to pressure at 20 degrees C. Temperatures above and below 20 degrees C significantly increased pressure inactivation of FCV. A 4-min treatment of 200 MPa at --10 and 50 degrees C reduced the titer of FCV by 5.0 and 4.0 log units, respectively; whereas at 20 degrees C the same treatment only reduced the titer by 0.3 log units. These novel results point to the potential for using temperatures above and particularly below room temperature to lower the pressure needed to cause the desired level of virus inactivation.     

Use of mild-heat treatment following high-pressure processing to prevent recovery of pressure-injured Listeria monocytogenes in milk

This study examined the inactivation of Listeria monocytogenes in milk by high-pressure processing (HPP) and bacterial recovery during storage after HPP. We developed a technique to inhibit the bacterial recovery during storage after HPP (550 MPa for 5 min) using a mild-heat treatment (30-50 degrees C). Various mild-heat treatments were conducted following HPP to investigate the condition on which the bacterial recovery was prevented. Immediately after HPP of 550 MPa at 25 degrees C for 5 min, no L. monocytogenes cells were detected in milk regardless of the inoculum levels (3, 5, and 7 log(10)CFU/ml). However, the number of L. monocytogenes cells increased by >8 log(10)CFU/ml regardless of the inoculum levels after 28 days of storage at 4 degrees C. Significant recovery was observed during storage at 25 degrees C; the bacterial number increased by >8 log(10)CFU/ml after 3 days of storage in the case of an initial inoculum level of 7 and 5 log(10)CFU/ml. Even in the case of an initial inoculum level of 3 log(10)CFU/ml, the bacterial number reached the level of 8 log(10)CFU/ml after 7 days of storage. No bacterial recovery was observed with storage at 37 degrees C for 28 days. Milk samples were treated by various mild-heat treatments (30-50 degrees C for 5-240 min) following HPP of 550 MPa at 25 degrees C for 5 min, and then stored at 25 degrees C for 70 days. The mild-heat treatment (e.g., 37 degrees C for 240 min or 50 degrees C for 10 min) inhibited the recovery of L. monocytogenes in milk after HPP. No recovery of L. monocytogenes in milk was observed during 70-day storage at 25 degrees C in samples that received mild-heat treatments such as mentioned above following HPP (550 MPa for 5 min). Moreover, the mild-heat treatment conditions (temperature and holding time) required to inhibit the recovery of L. monocytogenes in milk was modelled using a logistic regression procedure. The predicted interface of recovery/no recovery can be used to calculate the mild-heat treatment condition to control bacterial recovery during storage at 25 degrees C after HPP (550 MPa for 5 min). The results in this study would contribute to enhance the safety of high-pressure-processed milk.     

Predictive model for inactivation of Campylobacter spp. by heat and high hydrostatic pressure

Campylobacter represents one of the leading causes of foodborne enteritis. Poultry and its products frequently transmit the pathogen. The objective of the present study was to model predictively the short-term inactivation of Campylobacter in a ready-to-eat poultry product to develop an economic high-pressure treatment. We inactivated baroresistant strains of Campylobacter jejuni and Campylobacter coli, grown to stationary phase on nutrient agar and inoculated in poultry meat slurry, by heat and high hydrostatic pressure. Incubation at ambient pressure at 70 degrees C for 1 min and at 450 MPa at 15 degrees C for 30 s inactivated more than 6 log CFU of this foodborne pathogen per ml of poultry meat slurry. Thermal and pressure inactivation kinetics of C. coli and C. jejuni in poultry meat slurry were accurately described by a first-order kinetic model. A mathematical model was developed from 10 to 65 degrees C and from ambient to 500 MPa that predicts the reduction in numbers of Campylobacter in response to the combination of temperature, pressure, and treatment time. We suggest the high-pressure treatment of foods to avoid health risks caused by Campylobacter. The nonthermal short-term treatment of the examined food model system represents a successful step to an economic high-pressure procedure.     

Listeria innocua and aerobic mesophiles during chill storage of inoculated mechanically recovered poultry meat treated with high hydrostatic pressure

Mechanically recovered poultry meat (MRPM) was inoculated with Listeria innocua 910 CECT at a level of approximately 10⁸ CFU g⁻¹. Vacuum-packaged samples were treated by combinations of pressure (350, 400, 450 and 500 MPa), time (5, 10, 15 and 30 min) and temperature (2, 10 and 20°C) and later stored at 2°C for 2 months. Counts of L. innocua and aerobic mesophilic bacteria were determined 1, 4, 7, 15, 30 and 60 days after pressurisation. For mesophiles, in most treatments, pressurization at 2°C gave the significantly best results. High pressure caused a marked bactericidal effect on L. innocua: reductions higher than 7.5 log units were achieved in several cases. Some cells were just sublethally injured by pressure. Samples treated at 500 MPa for 30 min at 2°C had counts of only 2.3 log units after 60 days of chill storage. Noninoculated pressurised MRPM did not show Listeria growth throughout storage. These results suggest that high pressure processing can enhance the microbiological quality of MRPM.     

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)相似文献   

Inactivation of hepatitis A virus by high-pressure processing: the role of temperature and pressure oscillation

Inactivation of hepatitis A virus (HAV) in Dulbecco's modified Eagle medium with 10% fetal bovine serum was studied at pressures of 300, 350, and 400 MPa and initial sample temperatures of -10, 0, 5, 10, 20, 30, 40, and 50 degrees C. Sample temperature during pressure application strongly influenced the efficiency of HAV inactivation. Elevated temperature (> 30 degrees C) enhanced pressure inactivation of HAV, while lower temperatures resulted in less inactivation. For example, 1-min treatments of 400 MPa at -10, 20, and 50 degrees C reduced titers of HAV by 1.0, 2.5, and 4.7 log PFU/ml, respectively. Pressure inactivation curves of HAV were obtained at 400 MPa and three temperatures (-10, 20, and 50 degrees C). With increasing treatment time, all three temperatures showed a rapid initial drop in virus titer with a diminishing inactivation rate (or tailing effect). Analysis of inactivation data indicated that the Weibull model more adequately fitted the inactivation curves than the linear model. Oscillatory high-pressure processing for 2, 4, 6, and 8 cycles at 400 MPa and temperatures of 20 and 50 degrees C did not considerably enhance pressure inactivation of HAV as compared with continuous high-pressure application. These results indicate that HAV exhibits, unlike other viruses examined to date, a reduced sensitivity to high pressure observed at cooler treatment temperatures. This work suggested that slightly elevated temperatures are advantageous for pressure inactivation of HAV within foods.     

Survival and growth of Yersinia enterocolitica strains inoculated in skimmed milk treated with high hydrostatic pressure

Four human pathogenic strains of Yersinia enterocolitica (serotypes O:1, O:3, O:8, and O:9) were inoculated (7-8 log CFU/ml) in UHT skimmed milk and treated at 300, 400, and 500 MPa for 10 min at 20 degrees C, and then kept at 8 degrees C to assess their evolution for 15 days. Treatments at 400 and 500 MPa caused the highest lethality, generally reaching counts below detection level (1 CFU/ml) in the culture media. At 300 MPa, the most baroresistant serotypes were O:3 and O:8. After 15 days of storage at 8 degrees C, Y. enterocolitica showed growth over 8 log (CFU/ml) in all treatments. Kinetic study of microbial inactivation in skimmed milk was performed with serotype O:8 at 300 MPa, showing a tailing after 35 min of pressure treatment.     

High pressure inactivation of Escherichia coli, Campylobacter jejuni, and spoilage microbiota on poultry meat

This study evaluated the high pressure inactivation of Campylobacter jejuni, Escherichia coli, and poultry meat spoilage organisms. All treatments were performed in aseptically prepared minced poultry meat. Treatment of 19 strains of C. jejuni at 300 MPa and 30°C revealed a large variation of pressure resistance. The recovery of pressure-induced sublethally injured C. jejuni depended on the availability of iron. The addition of iron content to enumeration media was required for resuscitation of sublethally injured cells. Survival of C. jejuni during storage of refrigerated poultry meat was analyzed in fresh and pressuretreated poultry meat, and in the presence or absence of spoilage microbiota. The presence of spoilage microbiota did not significantly influence the survival of C. jejuni. Pressure treatment at 400 MPa and 40°C reduced cell counts of Brochothrix thermosphacta, Carnobacterium divergens, C. jejuni, and Pseudomonas fluorescens to levels below the detection limit. Cell counts of E. coli AW1.7, however, were reduced by only 3.5 log (CFU/g) and remained stable during subsequent refrigerated storage. The resistance to treatment at 600 MPa and 40°C of E. coli AW1.7 was compared with Salmonella enterica, Shiga toxin-producing E. coli and nonpathogenic E. coli strains, and Staphylococcus spp. Cell counts of all organisms except E. coli AW 1.7 were reduced by more than 6 log CFU/g. Cell counts of E. coli AW1.7 were reduced by 4.5 log CFU/g only. Moreover, the ability of E. coli AW1.7 to resist pressure was comparable to the pressure-resistant mutant E. coli LMM1030. Our results indicate that preservation of fresh meat requires a combination of high pressure with high temperature (40 to 60°C) or other antimicrobial hurdles.     

Effect of cheese water activity and carbohydrate content on the barotolerance of Listeria monocytogenes scott A

High-pressure processing is an appropriate technique for improving the microbiological safety of packaged ready-to-eat foods. The effect of high-pressure treatment on Listeria monocytogenes Scott A inoculated into fresh Hispánico-type cheese and ripe Mahón cheese was investigated. A 3.8-log reduction in the counts of L. monocytogenes Scott A in fresh cheese was recorded after 3 min at 400 MPa and 12 degrees C, whereas 18 min under the same conditions was required to obtain a 1-log reduction in ripe cheese. Dry matter values were 48.96% for fresh cheese and 58.79% for ripe cheese, and water activity (aw) values were 0.983 and 0.922, respectively. In dehydrated fresh cheese (58.20% dry matter) in which 5% NaCl was added to achieve a 0.904 aw value, L. monocytogenes Scott A counts were lowered by only 0.4 log after treatment for 10 min at 400 MPa. On the other hand, in a 60:40 mixture of ripe cheese:distilled water with a 0.976 aw value, the reduction under the same conditions was 3.9 log. Within the aw range of 0.945 to 0.965, L. monocytogenes Scott A barotolerance was significantly higher in fresh cheese than in ripe cheese for equivalent aw values. Carbohydrate content was higher in fresh cheese than in ripe cheese. The addition of lactose at a concentration of 5 mg/g to an 85:15 mixture of ripe cheese:distilled water did not influence L. monocytogenes Scott A barotolerance during treatment for 10 min at 400 MPa. Galactose at a concentration of 5 mg/g had a protective effect during high-pressure treatment, and glucose at a concentration of 5 mg/g favored L. monocytogenes Scott A survival during refrigerated storage of pressurized samples at 8 degrees C for 5 days.     

Inactivation of Escherichia coli inoculated into cloudy apple juice exposed to dense phase carbon dioxide

The inactivation of Escherichia coli in cloudy apple juice by dense phase carbon dioxide (DPCD) was investigated. With CO2 at 20 MPa and 37 degrees C or at 30 MPa and 42 degrees C, the inactivation of E. coli significantly increased (p<0.05) when increasing the exposure time, which conformed to a fast-to-slow two-stage kinetics. The two stages were well fitted to first-order reactions. Higher temperature or pressure significantly enhanced the bactericidal effect of DPCD (p<0.05), the maximum reduction was 7.66 log CFU at 45 MPa and 52 degrees C for 30 min. The survival curves against temperature or pressure were fitted using a linear equation with high regression coefficients (R2>0.94). The temperature inactivation rate (kT) and pressure inactivation rate (kP) were obtained. Higher kT or kP indicated higher susceptibility of E. coli to temperature or pressure. Moreover, there was good linear correlation of kT with pressure (R2=1.00). Also, kP increased with increasing temperature except for 37 degrees C. Greater inactivation of E. coli was obtained with 99.9% CO2 than with 99.5% CO2 or with the initial number of 10(5) CFU/mL than with that of 10(8) CFU/mL at 20 MPa and 37 degrees C.     

Conditions for high pressure inactivation of Vibrio parahaemolyticus in oysters

The objective of this study was to identify the high pressure processing conditions (pressure level, time, and temperature) needed to achieve a 5-log reduction of Vibrio parahaemolyticus in live oysters (Crassostrea virginica). Ten strains of V. parahaemolyticus were separately tested for their resistances to high pressure. The two most pressure-resistant strains were then used as a cocktail to represent baro-tolerant environmental strains. To evaluate the effect of temperature on pressure inactivation of V. parahaemolyticus, Vibrio-free oyster meats were inoculated with the cocktail of V. parahaemolyticus and incubated at room temperature (approximately 21 degrees C) for 24 h. Oyster meats were then blended and treated at 250 MPa for 5 min, 300 MPa for 2 min, and 350 MPa for 1 min. Pressure treatments were carried out at -2, 1, 5, 10, 20, 30, 40, and 45 degrees C. Temperatures >/=30 degrees C enhanced pressure inactivation of V. parahaemolyticus. To achieve a 5-log reduction of V. parahaemolyticus in live oysters, pressure treatment needed to be >/=350 MPa for 2 min at temperatures between 1 and 35 degrees C and >/=300 MPa for 2 min at 40 degrees C.     

Sensitivity of Staphylococcus aureus and Lactobacillus helveticus in ovine milk subjected to high hydrostatic pressure.

Ovine milk, standardized to 6% fat, was inoculated with Staphylococcus aureus CECT 534 and Lactobacillus helveticus CECT 414 at a concentration of 10(7) cfu/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). Staphylococcus aureus was highly resistant to pressure; only pressurizations at 50 degrees C of 500 MPa for 15 min achieved reductions of > or = 7.3 log units. For L. helveticus, the number of surviving cells was reduced considerably at pressures of 400 MPa or more (up to 4.5 log units at 50 degrees C for 15 min), and pressure was more effective at low (2 and 10 degrees C) and moderately high (50 degrees C) temperatures than at room temperature (25 degrees C). Both species showed first-order kinetics of destruction in the range 0 to 60 min. The D values for S. aureus were 20 min (2 degrees C at 450 MPa) and 16.7 min (25 degrees C at 450 MPa), and D values for L. helveticus were 7.1 min (2 degrees C at 450 MPa) and 9.1 min (25 degrees C at 450 MPa). Lactobacillus helveticus showed higher rates of survival of pressure than those reported in previous studies for other Lactobacillus spp.     

Evaluation of chemical and physical (high-pressure and temperature) treatments to improve the safety of minimally processed mung bean sprouts during refrigerated storage

The objective of this study was to compare the effects of combined high hydrostatic pressure and temperature treatments with different chemical sanitation treatments (water, sodium hypochlorite, and hydrogen peroxide) on the microbiological properties of mung bean sprouts. In a first study, the raw product was subjected to several combined high-pressure and temperature treatments for calculating a mathematical model by a response surface methodology. The number of pressure-temperature (150 to 400 MPa; 20 to 40 degrees C) combinations was limited to 10. In addition, a model system consisting of mung bean sprout juice was inoculated with Listeria monocytogenes (CECT 4032). Microbial inactivation with this model system was also investigated by a response surface methodology. The highest aerobic mesophilic bacteria and L. monocytogenes inactivation was achieved at maximum pressure and temperature (5.5 and 1.8 log cycles, respectively). In a second study, the effect of five different processing lines on the microbial load reduction of minimally processed mung bean sprouts during refrigerated storage was studied. All treatments reduced the initial population of aerobic mesophilic bacteria and fecal coliforms, with the physical treatment of 400 MPa and 40 degrees C being the most effective, showing initial reductions of 5.8 and 7.8 log CFU/ g, respectively. Recovery of bacteria from sprouts treated under these conditions was not observed during storage. However, the sprouts that received washing treatments with water, sodium hypochlorite, and hydrogen peroxide exhibited increases in aerobic mesophilic and fecal coliform counts after 3 days of storage at 4 degrees C.     

Water activity of bacterial suspension media unable to account for the baroprotective effect of solute concentration on the inactivation of Listeria monocytogenes by high hydrostatic pressure

Inactivation of Listeria monocytogenes (10(8) CFU/ml) by high hydrostatic pressure (HHP) from 400 to 600 MPa at 25 degrees C for 10 min was investigated with various concentrations of sodium chloride, sucrose, and sodium phosphate buffer solutions. Sodium chloride significantly inhibited HHP-induced inactivation of L. monocytogenes at concentrations higher than 2.6 M. A low concentration of sodium chloride within 1.7 M had no effect on HHP-induced inactivation. Almost complete inactivation at relatively low sodium chloride concentration solution was observed with treatments above 500 MPa. Sucrose also significantly inhibited HHP-induced inactivation of L. monocytogenes when greater than 1.2 M sucrose solutions were used. HHP-treatment at 400 MPa reduced the number of L. monocytogenes in 1.2 M, 1.5 M, and 1.8 M sucrose solutions by 4.8, 2.0, and 0.7 log cycles, respectively. Higher pressure did not yield significant reductions. Sodium phosphate buffer significantly inhibited HHP-induced inactivation of L. monocytogenes. In particular, 1 M phosphate buffer completely inhibited HHP-induced inactivation even at 600 MPa. HHP-treatment at 400 MPa reduced the number of L. monocytogenes in 0.1 M, 0.25 M, and 0.5 M phosphate buffer solutions by 5.6, 4.1, and 3.2 log cycles, respectively. The effect of HHP-induced inactivation of L. monocytogenes in the three kinds of solution was evaluated by adjusting water activity (a(w)). However, the baroprotective effect differed depending on the kind of solute even at the same a(w). This result showed no consistent correlation between a(w) and solute concentration in terms of the baroprotective effect. As an alternative approach, saturation of suspension solution was used for evaluating the effect of HHP-induced inactivation of L. monocytogenes. As the saturation of suspension media increased, the effect of HHP-induced inactivation of L. monocytogenes decreased regardless of the kinds of solute. The saturation of solution would be an alternative parameter of inhibition in terms of HHP-induced inactivation of bacteria.