Inactivation of high pressure resistant Escherichia coli by lysozyme and nisin under high pressure

We studied the inactivation (by high hydrostatic pressure at 20°C) of Escherichia coli MG1655 and the selected pressure-resistant mutants that were derived previously from this strain, and which are the most pressure-resistant vegetative cells described to date. The natural antimicrobial peptides, lysozyme (50 μg/ml) and nisin (100 IU/ml), enhanced considerably the inactivation of the target bacteria under pressure. However, kinetic inactivation experiments in the presence of these compounds revealed pronounced tailing, which limited the level of inactivation that could be achieved under mild conditions of pressure and temperature. Interrupted pressure treatments enhanced the effectiveness of lysozyme and nisin, allowing a reduction by at least 6 logs of all strains at 400 MPa. A hypothetical mechanism of ‘pressure-promoted uptake’ is proposed to explain E. coli outer membrane permeabilization for lipophilic and cationic peptides like lysozyme and nisin under pressure.     

Inactivation of spores of Bacillus cereus in cheese by high hydrostatic pressure with the addition of nisin or lysozyme

The objective of this work was to study high hydrostatic pressure (HHP) inactivation of spores of Bacillus cereus ATCC 9139 inoculated in model cheeses made of raw milk, together with the effects of the addition of nisin or lysozyme. The concentration of spores in model cheeses was approximately 6-log10 cfu/g of cheese. Cheeses were vacuum packed and stored at 8 degrees C. All samples except controls were submitted to a germination cycle of 60 MPa at 30 degrees C for 210 min, to a vegetative cells destruction cycle of 300 or 400 MPa at 30 degrees C for 15 min, or to both treatments. Bacillus cereus counts were measured 24 h and 15 d after HHP treatment. The combination of both cycles improved the efficiency of the whole treatment. When the second pressure-cycle was of 400 MPa, the highest inactivation (2.4 +/- 0.1 log10 cfu/g) was obtained with the presence of nisin (1.56 mg/L of milk), whereas lysozyme (22.4 mg/L of milk) did not increase sensitivity of the spores to HHP. For nisin (0.05 and 1.56 mg/L of milk), no significant differences were found between counts at 24 h and 15 d after treatment. Considering that mesophilic spore counts usually range from 2.6 to 3.0 log10 cfu/ml in raw milk, HHP at mild temperatures with the addition of nisin may be useful for improving safety and preservation of soft curd cheeses made from raw milk.     

High sucrose concentration protects E. coli against high pressure inactivation but not against high pressure sensitization to the lactoperoxidase system

The inactivation of Escherichia coli by high hydrostatic pressure treatment at up to 550 MPa and 20 degrees C was studied in potassium phosphate buffer containing high concentrations of sucrose. E. coli strain MG1655 was pressure-sensitive in the absence of sucrose, but became highly pressure resistant in the presence of 10% to 50% (w/v) sucrose. The pressure resistance of E. coli strain LMM1010, a previously described derivative of MG1655 that is pressure resistant in the absence of sucrose, was further increased in the presence of sucrose, to a similar level as for strain MG1655 in the presence of sucrose. When cell suspensions of either strain were stored after pressure treatment for 24 h at 20 degrees C, a further reduction of the plate counts indicative of pressure induced sublethal injury was observed, that was positively correlated with pressure intensity and negatively with sucrose concentration. Addition of the lactoperoxidase system to the cell suspensions strongly enhanced high pressure inactivation of E. coli at high sucrose concentrations. Using a pressure intensity of only 250 MPa, both E. coli strains were sensitized for the lactoperoxidase system in up to 30% (w/v) sucrose, resulting in at least 10(6)-fold inactivation within 24 h or less after pressure treatment. For comparison, a pressure treatment at 250 MPa in the absence of the lactoperoxidase system did not cause any inactivation of either strain even in the absence of sucrose. At sucrose concentrations above 30% (w/v), no or very little inactivation occurred even in the presence of the lactoperoxidase system.     

Decontamination of seeds for seed sprout production by high hydrostatic pressure

Garden cress, sesame, radish, and mustard seeds immersed in water were treated with high pressure (250, 300, 350, and 400 MPa) for 15 min at 20 degrees C. After treatment, percentages of seeds germinating on water agar were recorded for up to 11 days. Of the seeds tested, radish seeds were found to be the most pressure sensitive, with seeds treated at 250 MPa reaching 100% germination 9 days later than untreated control seeds did. Garden cress seeds, on the other hand, were the most pressure resistant, with seeds treated at 250 MPa reaching 100% germination 1 day later than untreated control seeds did. Garden cress sprouts from seeds treated at 250 and 300 MPa also took about 1 day longer to reach average sprout length than sprouts from untreated control seeds did, indicating that sprout growth was not retarded once germination had occurred. Garden cress seeds were inoculated with suspensions of seven different bacteria (10(7) CFU/ml) and processed with high pressure. Treatment at 300 MPa (15 min, 20 degrees C) resulted in 6-log reductions of Salmonella Typhimurium, Escherichia coli MG1655, and Listeria innocua, > 4-log reductions of Shigella flexneri and pressure-resistant E. coli LMM1010, and a 2-log reduction of Staphylococcus aureus. Enterococcus faecalis was virtually not inactivated. For suspensions of the gram-positive bacteria, similar levels of inactivation in water in the absence of garden cress seeds were found, but the inactivation of E. coil LMM1010 and S. flexneri in water in the absence of garden cress seeds was significantly less extensive. These data suggest that garden cress seeds contain a component that acts synergistically with high hydrostatic pressure against gram-negative bacteria.     

The lactoperoxidase system increases efficacy of high-pressure inactivation of foodborne bacteria

The inactivation of eight different bacteria comprising Escherichia coli LMM1010 and MG1655, respectively a pressure-resistant strain and the corresponding wild-type, Salmonella Typhimurium, Pseudomonas fluorescens, Staphylococcus aureus, Enterococcus faecalis, Listeria innocua and Lactobacillus plantarum, by high hydrostatic pressure in skim milk supplemented with the lactoperoxidase-hydrogen peroxide-thiocyanate (LP) system at naturally occurring concentration was studied. In the absence of pressure treatment, the LP system had either no effect, i.e. on S. Typhimurium and E. coli LMM1010, a growth inhibiting effect, i.e. on E. coli MG1655, L. innocua, S. aureus, L. plantarum and E. faecalis, or a bactericidal effect, i.e. on P. fluorescens. The presence of the LP system affected inactivation by high pressure in a cell density-dependent manner. At low cell concentration (10(6) cfu/ml), the LP system strongly increased high-pressure inactivation as measured immediately after pressure treatment of all bacteria except the pressure-resistant E. coli. At high cell density (10(9) cfu/ml), only inactivation of L. innocua, E. faecalis and L. plantarum were enhanced. For both E. coli strains, the fate of the bacteria during 24 h following pressure treatment was also studied. It was found that in the presence of the LP system, considerable further inactivation occurred in the first hours after pressure treatment. The potential of the LP system to improve the bactericidal efficiency of high-pressure treatment for food preservation is discussed.     

Pasteurization of Milk Using Pulsed Electrical Field and Antimicrobials

The inactivation of naturally occurring microorganisms in raw skim milk by pulsed electric field (PEF) treatment alone and combined with the antimicrobial agents nisin and lysozyme, added both singly and together, was investigated. A 7.0‐log reduction of microorganisms found in raw skim milk was achieved through a combination of PEF treatment (80 kV/cm, 50 pulses), mild heat (52 °C), and the addition of both the natural antimicrobials nisin (38 IU/mL) and lysozyme (1638 IU/mL). The combination of PEF, mild heat, and antimicrobials resulted in a much higher microbial inactivation than the sum of the individual reductions achieved from each treatment alone, indicating synergy. Varying the pH from 6.7 to 5.0 had no effect on microbial inactivation.     

Inactivation of Escherichia coli O157:H7 in orange juice using a combination of high pressure and mild heat

The effect of high pressure on the survival of a pressure-resistant strain of Escherichia coli O157:H7 (NCTC 12079) in orange juice was investigated over the pH range 3.4 to 5.0. The pH of commercial, sterile orange juice was adjusted to 3.4, 3.6, 3.9, 4.5, or 5.0. The juice was then inoculated with 10(8) CFU ml(-1) of E. coli O157:H7. The inoculated orange juice was subjected to pressure treatments of 400, 500, or 550 MPa at 20 degrees C or 30 degrees C to determine the conditions that would give a 6-log10 inactivation of E. coli O157:H7. A pressure treatment of 550 MPa for 5 min at 20 degrees C produced this level of kill at pH 3.4, 3.6, 3.9, and 4.5 but not at pH 5.0. Combining pressure treatment with mild heat (30 degrees C) did result in a 6-log10 inactivation at pH 5.0. Thus, the processing conditions (temperature and time) must be considered when pressure-treating orange juice to ensure microbiological safety.     

Inactivation of Escherichia coli by high-pressure homogenisation is influenced by fluid viscosity but not by water activity and product composition

The inactivation of Escherichia coli MG1655 by high-pressure homogenisation (HPH) at pressures ranging from 100 to 300 MPa was studied in buffered suspensions adjusted to different relative viscosities (1.0, 1.3, 1.7, 2.7 and 4.9) with polyethylene glycol 6000 (PEG 6000). The water activity of these suspensions was not significantly affected by this high molecular weight solute. Bacterial inactivation was found to decrease with increasing viscosity of the suspensions, an effect that was more pronounced at higher pressures. To study the effect of water activity, series of E. coli suspensions having a different water activity (0.953-1.000) but the same relative viscosity (1.3, 1.7, 2.7 and 4.9) were made using PEG of different molecular weights (400, 600, 1000 and 6000), and subjected to HPH treatment. The results indicated that water activity does not influence inactivation. Finally, inactivation of E. coli MG1655 by HPH in skim milk, soy milk and strawberry-raspberry milk drink was found to be the same as in PEG containing buffer of the corresponding viscosity. These results identify fluid viscosity as a major environmental parameter affecting bacterial inactivation by HPH, as opposed to water activity and product composition, and should contribute to the development of HPH applications for the purpose of bacterial inactivation.     

High-Pressure Processing of Orange Juice: Kinetics of Pectinmethylesterase Inactivation

ABSTRACT: A kinetic study of pectinmethylesterase (PME) inactivation in orange juice was conducted. Juice samples were subjected to combinations of high pressure (400, 500, 600 MPa) and thermal (25, 37.5, 50 °C) treatments for various time periods. PME inactivation followed a first-order kinetic model with a residual activity of pressure-resistant enzyme remaining. Calculated D-values ranged from 4.6 min to 117.5 min at 600 MPa/50 °C and 400 MPa/25 °C, respectively. Pressures in excess of 500 MPa resulted in sufficiently fast inactivation rates for economic viability of the process.     

超高压协同乳酸链球菌素抑制低温火腿中的耐压腐败菌

为探寻低温肉制品中耐压腐败菌的抑制手段,揭示超高压和乳酸链球菌素之间的协同抑菌效应。以肉制品中典型的耐压腐败菌绿色魏斯菌和肠膜明串珠菌为供试材料,评价二者在1～500μg/mL的乳酸链球菌素存在条件下的生长情况,并结合100～600MPa(5min,20℃)的超高压处理研究不同压力条件与乳酸链球菌素之间的协同增效作用。结果表明:随着乳酸链球菌素添加量的增加,两种菌的数量急剧降低,当添加量达到200μg/mL时,对二者的抑制程度达到最大;单独超高压处理时,绿色魏斯菌和肠膜明串珠菌分别在400MPa和300MPa的压力下受到显著抑制。而超高压结合200μg/mL乳酸链球菌素处理时,二者受到显著抑制的压力均降低到100MPa;500MPa,5min,20℃的超高压条件结合200μg/mL乳酸链球菌素处理能够抑制绿色魏斯菌数达9lg(cfu/mL)。超高压和乳酸链球菌素之间存在显著的协同作用,乳酸链球菌素的添加将会有效降低工业化生产中超高压的压力水平。     

超高压结合温热处理对脱脂乳透光率和粒径及蛋白溶解性的影响

目的：研究高压结合温热（≤50 ℃）处理对脱脂乳粒径、透光率及蛋白溶解性的影响。方法：采用不同温度（常温、30、40、50 ℃）和压力（0.1～700 MPa）分别处理脱脂乳10～30 min,利用激光纳米粒度仪检测脱脂乳粒径变化,分光光度法检测透光率变化,考马斯亮蓝法测定可溶性蛋白质量浓度变化。结果表明,脱脂乳透光率在压力不高于100 MPa范围内不受温度、压力和处理时间的影响;在200～700 MPa范围内,常温条件下处理的脱脂乳透光率随压力的升高和处理时间的延长而增大,在700 MPa下处理20 min时透光率最大,增幅为1 011%;在30～50 ℃范围内,透光率随压力增大（200～700 MPa）呈现先升高后降低的趋势,在40 ℃、500 MPa下处理10 min时透光率增幅最大（537%）,透光率受温度和保压时间的影响,且超高压结合温热处理脱脂乳透光率均高于未处理脱脂乳。在常温、0.1～400 MPa范围内,脱脂乳的中位径（Dx(50)）随压力的增大而总体降低,在400～700 MPa范围内变化趋势平稳,但均小于未处理脱脂乳的Dx(50);30、40、50 ℃下,脱脂乳Dx(50)随压力增大呈现先升高后降低的趋势,分别在400～700、300～500、200～400 MPa范围内变化趋势平稳,且受时间影响较小。经高压处理的脱脂乳中可溶性乳蛋白（soluble protein,S-Pro）质量浓度总体呈增加趋势,且受压力、时间和温度的影响,在30 ℃、500 MPa下处理30 min,S-Pro质量浓度增幅最大（83.55%）。pH 4.6下可溶性蛋白（S-Pro-pH 4.6）质量浓度在压力不高于100 MPa时不受温度、压力和时间的影响;在200～700 MPa范围内,不同温度下,随压力升高和处理时间的延长,S-Pro-pH 4.6质量浓度呈现下降趋势。对各指标间相关性进行分析发现,透光率与Dx(50)间的相关性随温度升高减弱;透光率与S-Pro质量浓度呈正相关,与S-Pro-pH 4.6质量浓度呈负相关。S-Pro与S-Pro-pH 4.6之间呈负相关。结论：经超高压结合温热处理,能够引发脱脂乳透光率、粒径及蛋白溶解性的变化,且这些变化存在一定的相关性。     

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 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.     

Sensitivities of foodborne pathogens to pressure changes

Eight foodborne pathogens were suspended in ultrahigh-temperature whole milk and treated at pressure levels of 0.1 to 690 MPa at 21.5 degrees C for 10 min. There was no clear trend in pressure resistance between gram-negative and gram-positive organisms. The order of the single strains tested, from most to least pressure sensitive, was Vibrio parahaemolyticus < Yersinia enterocolitica < Listeria monocytogenes < Salmonella enterica serovar Typhimurium < S. enterica serovar Enteritidis < Escherichia coli O157:H7 approximately equal to Staphylococcus aureus < Shigella flexneri. For each organism there existed a pressure range in which log(number of survivors) had a near linear relationship when plotted versus treatment pressure level. In this study, a decimal reduction pressure (Dp) value was defined and used to measure the sensitivity of these pathogens to pressure changes. L. monocytogenes and V. parahaemolyticus were most sensitive to pressure changes, and S. flexneri was most resistant. The D(P) values were 16.3 MPa for L. monocytogenes, 21.7 MPa for V. parahaemolyticus, and 127.0 MPa for S. flexneri. The most pressure-resistant gram-negative bacterium, S. flexneri, and most pressure-resistant gram-positive bacterium, S. aureus, were treated at 50 degrees C and pressures of 0.1 to 650 MPa for 10 min. High temperature considerably enhanced pressure inactivation of these two organisms and affected their sensitivities to pressure changes. The effect of treatment time on the D(P) values of L. monocytogenes and V. parahaemolyticus was also determined, and it was found that it did not significantly affect their D(P) values.     

Effect of high hydrostatic pressure on Listeria innocua 910 CECT inoculated into Ewe's milk.

Ewe's milk standardized to 6% fat was inoculated with Listeria innocua 910 CECT at a concentration of 10(7)CFU/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 350, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). To determine numbers of L. innocua, listeria selective agar base with listeria selective supplement and plate count agar was used. Low-temperature (2 degrees C) pressurizations produced higher L. innocua inactivation than treatments at room temperatures (25 degrees C). Pressures between 450 and 500 MPa for 10 to 15 min were needed to achieve reductions of 7 to 8 log units. The kinetics of destruction of L. innocua were first order with D-values of 3.12 min at 2 degrees C and 400 MPa and 4 min at 25 degrees C and 400 MPa. A baroprotective effect of ewe's milk (6% fat) on L. innocua was observed in comparison with other studies using different media and similar pressurization conditions.     

Pressure death and tailing behavior of Listeria monocytogenes strains having different barotolerances

The objectives of this study were to investigate the variability among Listeria monocytogenes strains in response to high-pressure processing, identify the most resistant strain as a potential target of pressure processing, and compare the inactivation kinetics of pressure-resistant and pressure-sensitive strains under a wide range (350 to 800 MPa) of pressure treatments. The pressure resistance of Listeria innocua and nine strains of L. monocytogenes was compared at 400 or 500 MPa and 30 degrees C. Significant variability among strains was observed. The decrease in log CFU/ml during the pressure treatment was from 1.4 to 4.3 at 400 MPa and from 3.9 to >8 at 500 MPa. L. monocytogenes OSY-8578 exhibited the greatest pressure resistance, Scott A showed the greatest pressure sensitivity, and L. innocua had intermediate resistance. On the basis of these findings, L. monocytogenes OSY-8578 is a potential target strain for high-pressure processing efficacy studies. The death kinetics of L. monocytogenes Scott A and OSY-8578 were investigated at 350 and 800 MPa. Survivors at 350 MPa were enumerated by direct plating, and survivors at 800 MPa were enumerated by the most-probable-number technique. Both pressure-resistant and pressure-sensitive strains exhibited non-first-order death behavior, and excessive pressure treatment did not eliminate the tailing phenomenon.     

Inactivation of Salmonella typhimurium DT 104 in UHT whole milk by high hydrostatic pressure

Cell suspensions of Salmonella typhimurium DT 104 in ultra-high temperature (UHT) whole milk were exposed to high hydrostatic pressure at 350, 400, 450, 500, 550, and 600 MPa at ambient temperature (ca. 21 degrees C). Tailing was observed in all survival curves, and sigmoidal survival curves were observed at relatively high pressure (500-600 MPa). Four modeling methods (linear and nonlinear including Weibull, modified Gompertz, and log-logistic models) were fitted to these data at 500, 550, and 600 MPa. Performances of the modeling methods were compared using mean square error (MSE). The linear regression model at these three pressure levels had a mean square error (MSE) of 1.260-2.263. Nonlinear regressions using Weibull, modified Gompertz, and log-logistic models had MSE values in the range of 0.334-0.764, 0.601-1.479, and 0.359-0.523, respectively. Modeling results indicated that first-order kinetics could not accurately describe pressure inactivation of S. typhimurium DT 104 in UHT milk; the log-logistic model produced the best fit to data.     

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.     

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.     

Inactivation of Listeria innocua in skim milk by pulsed electric fields and nisin

Pulsed electric fields (PEF) is an emerging nonthermal processing technology used to inactivate microorganisms in liquid foods such as milk. PEF results in loss of cell membrane functionality that leads to inactivation of the microorganism. There are many processes that aid in the stability and safety of foods. The combination of different preservation factors, such as nisin and PEF, to control microorganisms should be explored. The objective of this research was to study the inactivation of Listeria innocua suspended in skim milk by PEF as well as the sensitization of PEF treated L. innocua to nisin. The selected electric field intensity was 30, 40 and 50 kV/cm and the number of pulses applied was 10.6, 21.3 and 32. The sensitization exhibited by PEF treated L. innocua to nisin was assessed for 10 or 100 IU nisin/ml. A progressive decrease in the population of L. innocua was observed for the selected field intensities, with the greatest reduction being 2 1/2 log cycles (U). The exposure of L. innocua to nisin after PEF had an additive effect on the inactivation of the microorganism as that exhibited by the PEF alone. As the electric field, number of pulses and nisin concentration increased, synergism was observed in the inactivation of L. innocua as a result of exposure to nisin after PEF. The reduction of L. innocua accomplished by exposure to 10 IU nisin/ml after 32 pulsed electric fields was 2, 2.7, and 3.4 U for an electric field intensity of 30, 40, and 50 kV/cm, respectively. Population of L. innocua subjected to 100 IU nisin/ml after PEF was 2.8-3.8 U for the selected electric field intensities and 32 pulses. The designed model for the inactivation of L. innocua as a result of the PEF followed by exposure to nisin proved to be accurate in the prediction of the inactivation of L. innocua in skim milk containing 1.2 or 37 IU nisin/ml. Inactivation of L. innocua in skim milk containing 37 IU nisin/ml resulted in a decrease in population of 3.7 U.