Inactivation kinetics of pulsed electric field-resistant strains of Listeria monocytogenes and Staphylococcus aureus in media of different pH

A study of the effect of pulsed electric fields (PEF) on the inactivation of Listeria monocytogenes STCC 5672 and Staphylococcus aureus STCC 4459 in McIlvaine buffer covering a range from pH 3.5 to 7.0 was conducted. Mathematical models based on the Weibull distribution were developed to describe the influence of the electric field strength, treatment time and pH of the treatment medium on the lethality of both Gram positive pathogenic bacteria after PEF treatments. Both microorganisms were more sensitive to PEF in media of low pH, although the influence of the pH on the PEF resistance was more significant in S. aureus. In the best cases scenario, the highest inactivation levels achieved were 3.3 and 6.1 log₁₀ cycles for L. monocytogenes and S. aureus respectively in pH 3.5 after 500 μs of 35 kV/cm. Based on these results and those observed in literature, L. monocytogenes STCC 5672 at any pH investigated has been shown as one of the most PEF resistant microorganism. Therefore, this microorganism should be considered as a possible target microorganism to define process criterion for PEF pasteurization.     

Modeling inactivation kinetics and occurrence of sublethal injury of a pulsed electric field-resistant strain of Escherichia coli and Salmonella Typhimurium in media of different pH

A study of the effect of pulsed electric fields (PEF) on the kinetics of inactivation and the occurrence of cell damage in Escherichia coli O157:H7 and Salmonella Typhimurium 878 treated in McIlvaine buffer covering a range from pH 3.5 to 7.0 was conducted. Mathematical equations based on the Weibull distribution were developed to describe the influence of the electric field strength, treatment time and pH of the treatment medium on the lethality and generation of cell damage of both Gram negative pathogenic bacteria after the application of PEF treatments. E. coli O157:H7 was more PEF resistant than Salmonella Typhimurium at all pH investigated. PEF resistance of E. coli was influenced by the pH but the pH hardly affected the PEF resistance of Salmonella Typhimurium 878. After 150 μs at 35 kV/cm, 1 and 5 log₁₀ cycles of inactivation of E. coli O157:H7 were observed in the range of pH 3.5–4.5 and 5.5–6.5, respectively. Cell damage increased with the field strength and treatment time. A maximum cell damage level of 4.2 and 2.7 log₁₀ cycles for E. coli O157:H7 and Salmonella Typhimurium was observed respectively after a treatment of 30 kV/cm at pH 3.5. PEF induced cell damage was not detected at pH higher than 5.0 for both microorganisms. The developed equations can be applied to design combining processes which can increase the lethality of PEF or to reduce the intensity of PEF treatments to achieve a determine level of microbial inactivation.Industrial relevanceThis study demonstrates that when the influence of several factors on the microbial behavior is investigated, the development of mathematical models is a very useful tool to evaluate the influence of each parameter and their interactions. In this study, it has been mathematically described for first time the influence of the pH of the treatment medium and the occurrence of sublethal injury in a wide range of electric field strengths and treatment times in two Gram negative pathogenic bacteria, Escherichia coli O157:H7 and Salmonella Typhimurium 878. These models would also be of interest for engineering design, evaluation and optimization of PEF process as a new technique for food preservation.     

The influence of process parameters for the inactivation of Listeria monocytogenes by pulsed electric fields

The influence of the electric field strength, the treatment time, the total specific energy and the conductivity of the treatment medium on the Listeria monocytogenes inactivation by pulsed electric fields (PEF) has been investigated. L. monocytogenes inactivation increased with the field strength, treatment time and specific energy. A maximum inactivation of 4.77 log(10) cycles was observed after a treatment of 28 kV/cm, 2000 micros and 3490 kJ/kg. The lethal effect of PEF treatments on L. monocytogenes was not influenced by the conductivity of the treatment medium in a range of 2, 3 and 4 mS/cm when the total specific energy was used as a PEF control parameter. A mathematical model based on the Weibull distribution was fitted to the experimental data when the field strength (15-28 kV/cm), treatment time (0-2000 micros) and specific energy (0-3490 kJ/kg) were used as PEF control parameters. A linear relationship was obtained between the log(10) of the scale factor (b) and the electric field strength when the treatment time and the total specific energy were used to control the process. The total specific energy, in addition to the electric field strength and the treatment time, should be reported in order to evaluate the microbial inactivation by PEF.     

Evaluation of a static treatment chamber to investigate kinetics of microbial inactivation by pulsed electric fields at different temperatures at quasi-isothermal conditions

A static parallel electrode treatment chamber with tempered electrodes has been designed to obtain kinetics data on microbial inactivation by pulsed electric fields (PEF) at different temperatures at quasi-isothermal conditions. Distribution of the electric field strength and temperature within the treatment zone was estimated by a finite element method. A good agreement was observed between the temperatures estimated by numerical simulation and temperatures measured by a thermocouple in the treatment zone before and after the PEF treatments (values of RMSE below 3%). Influence of the treatment temperature on PEF inactivation (30 kV/cm) of Salmonella typhimurium was investigated at temperatures between 4 and 50 °C in media of pH 3.5 and 7.0. Treatment temperature had an important effect on microbial inactivation for both values of pH. At pH 3.5 the inactivation of S. typhimurium was irrelevant at 4 °C but about 1.5, 2.9, 4.0 and 5.0 Log₁₀ reductions were obtained after 30 pulses (90 μs) at 15, 27, 38 and 50 °C, respectively. At pH 7.0, around two Log₁₀ cycles of inactivation were observed after 50 pulses (150 μs) at 4 °C. At temperatures in the range between 15 and 50 °C the treatment temperature practically did not influence PEF resistance of S. typhimurium. A model based on the Weibull distribution adequately described kinetics of inactivation of S. typhimurium at different temperatures. The treatment chamber designed in the investigation could be useful to obtain kinetics data on PEF destruction of microorganisms or other components of interest at a uniform distribution of electric field strength and homogeneous and quasi-isothermal conditions in a wide range of temperatures.     

Effect of environmental factors and cell physiological state on Pulsed Electric Fields resistance and repair capacity of various strains of Escherichia coli

The aim was to determine the resistance variation of four strains of Escherichia coli to Pulsed Electric Fields (PEF), the role of the sigma factor RpoS in PEF resistance, as well as the influence of several environmental factors and the cell physiological state on the PEF resistance and repair capacity. The rpoS null mutant, E. coli BJ4L1, exhibited decreased PEF resistance as compared with its wild-type parent, BJ4. W3110 and O157:H7 were the most PEF-resistant strains: whereas 2 and more than 3 Log10 cycles of BJ4 and BJ4L1 cells, respectively, were inactivated after 50 pulses at 35 kV/cm, only 0.5 Log10 cycle of inactivation of W3110 and O157:H7 was attained. A different pattern was observed and the resistance variation among strains was largely reduced, when selective recovery media were used. At exponential growth phase, the resistance of the four strains was lower, and more than 4 Log10 cycles of inactivation of all strains tested were attained at 30 kV/cm. Previous heat and cold shock treatments scarcely influenced cell PEF resistance. PEF survival increased with the reduction in water activity of the treatment medium to 0.94: the occurrence of sublethally injured cells was negligible, and less than 1 Log10 cycle of inactivation was attained at 35 kV/cm. PEF-treated cells were sensitive to a subsequent storage at pH 4.0 or in the presence of sorbic acid, attaining a final inactivation of 4-5 Log10 cycles after 24 hour-incubation. In conclusion, the work confirms the role of rpoS in PEF resistance. E. coli strains exhibit large differences in PEF resistance. These differences were less important when cells were recovered under selective conditions. Both resistance variation among strains and occurrence of sublethal damage were noticeably influenced by the environmental factors tested.     

Effect of the medium characteristics and the heating and cooling rates on the nonisothermal heat resistance of Bacillus sporothermodurans IC4 spores

In recent years, highly thermo-resistant mesophilic spore-forming bacteria belonging to the species Bacillus sporothermodurans have caused non-sterility problems in industrial sterilization processes. The aim of this research was to evaluate the effect of the heating medium characteristics (pH and buffer/food) on the thermal inactivation of B. sporothermodurans spores when exposed to isothermal and non-isothermal heating and cooling treatments and the suitability of non-linear Weibull and Geeraaerd models to predict the survivors of these thermal treatments. Thermal treatments were carried out in pH 3, 5 and 7 McIlvaine buffer and in a courgette soup. Isothermal survival curves showed shoulders that were accurately characterized by means of both models. A clear effect of the pH of the heating medium was observed, decreasing the D₁₂₀ value from pH 7 to pH 3 buffer down to one third. Differences in heat resistance were similar, regardless of the model used and were kept at all temperatures tested. The heat resistance in courgette soup was similar to that shown in pH 7 buffer. When the heat resistance values obtained under isothermal conditions were used to predict the survival in the non-isothermical experiments, the predictions estimated the experimental data quite accurately, both with Weibull and Geeraerd models.     

Inactivation of Yersinia enterocolitica by pulsed electric fields

The influence of growth conditions, treatment medium characteristics and PEF process parameters on the lethal effect on Yersinia enterocolitica of pulsed electric fields (PEF) treatments in batch has been investigated. Growth phase, temperature of growth, pH, conductivity of the treatment medium, pulse width and frequency of pulses did not influence the sensitivity of Y. enterocolitica to PEF. However, an A_w decrease from >0.99 to 0.93 of the treatment medium increased the PEF resistance of Y. enterocolitica with 3.5 log₁₀ cycles after a treatment of 22 kV/cm, 800 μs and 880 kJ/kg. Inactivation of Y. enterocolitica increased with the field strength, treatment time and total specific energy up to a maximum of 6 log₁₀ cycles after 28 kV/cm, 2000 μs and 3559 kJ/kg. A nonlinear relationship was found among the survival fraction and the treatment time or the specific energy that was accurately described by a mathematical model based on the Weibull distribution. The inactivation of Y. enterocolitica by PEF was characterized by maximum field strength thresholds. Above these thresholds, specific energy necessary to obtain a given level of inactivation scarcely decreased by increasing the electric field strength, and inactivation of Y. enterocolitica only depended on the specific energy applied.     

Predicting inactivation of Salmonella senftenberg by pulsed electric fields

Pulsed electric field inactivation of Salmonella senftenberg suspended in McIlvaine buffer of pH 7 and conductivity 2 mS/cm was investigated. In this study, square wave waveform pulses were used. After the same treatment time, inactivation of S. senftenberg depended neither on pulse width (1–15 μs) nor frequency of treatment (1–5 Hz). Survivor curves of S. senftenberg at different electric field strengths did not follow first-order kinetics. These survival curves were described by the log-logistic model proposed by Cole et al. [Cole, M. B., Davies, K. W., Munro, G., Holyoak, C. D., and Kilsby, D. C. (1993). A vitalistic model to describe the thermal inactivation of Listeria monocytogenes. Journal of Industrial Microbiology, 12, 232–239]. Comparison of measured and estimated values showed that this model accurately described the inactivation of S. senftenberg by high electric field pulses in the range of 12–28 kV/cm.     

Pulsed electric fields cause bacterial envelopes permeabilization depending on the treatment intensity, the treatment medium pH and the microorganism investigated

The relationship between membrane permeabilization and loss of viability by pulsed electric fields (PEF) depending on the treatment intensity and the treatment media pH in two gram-positive (Lactobacillus plantarum, Listeria monocytogenes) and two gram-negative (Escherichia coli, Salmonella senftenberg 775W) bacterial species has been investigated. Loss of membrane integrity was measured as increased uptake of the fluorescent dye propidium iodide (PI). Non-permanent/reversible permeabilization was detected when cells stained with PI during PEF resulted in higher fluorescence than that measured in cells stained after PEF. Whereas loss of viability of the two gram-negative bacteria was correlated with the sum of non-permanent and permanent membrane permeabilization when treated at pH 7.0, in the case of the two gram-positives, loss of viability was correlated with a permanent loss of membrane integrity. At pH 7.0, the four bacteria exhibited reversible permeabilization. However, whereas the gram-positives capable of reversing permeabilization survived, the gram-negative cells died, despite their capacity to reverse permeabilization immediately after PEF. Thus, resealing is not necessarily related to the survival of PEF-treated cells. In contrast, when cells were PEF-treated at pH 4.0 a more complicated picture emerged. Whereas loss of viability was correlated with a permanent loss of membrane integrity in L. monocytogenes cells, in L. plantarum the degree of permeabilization was higher, and in the gram-negative strains, much lower than the percentage of inactivated cells. These results support the view that membrane permeabilization is involved in the mechanism of bacterial inactivation by PEF, but the nature of membrane damage and its relationship with cell death depends on the bacterial species and the treatment medium pH.     

Screening for Listeria monocytogenes surrogate strains applicable to food processing by ultrahigh pressure and pulsed electric field

Ultrahigh pressure (UHP) and pulsed electric field (PEF) are emerging processing technologies developed to enhance the safety while maintaining the fresh-like quality of food. For each food and process combination, a pathogen of concern (i.e., target pathogen) must be determined, and a low-risk microorganism that serves as the pathogen surrogate for process validation must be identified. The objective of this study was to identify a surrogate for Listeria monocytogenes for UHP and PEF process validation. Potential surrogates tested include four Lactobacillus spp., a Pediococcus sp., and a Listeria innocua strain. These were compared with nine L. monocytogenes strains, with regard to sensitivity to UHP and PEF processing. For UHP treatment, the strains were suspended in citrate-phosphate buffer (pH 7.0 or 4.5), sweet whey, or acidified whey and pressure processed at 500 MPa for 1 min. For PEF treatment, the strains were suspended in NaCl solution, acid whey, or sweet whey and processed at 25 kV/cm. The lethality of UHP or PEF treatment varied considerably, depending on medium types and pH and the treated strain. Treating the tested microorganisms with UHP inactivated 0.3 to 6.9 log CFU/ml for L. monocytogenes strains and 0.0 to 4.7 log CFU/ml for the potential surrogates. When PEF was employed, populations of tested microorganisms decreased < 1.0 to 5.3 log CFU/ml. L. monocytogenes V7 and OSY-8578 were among the most resistant strains to UHP and PEF treatments, and thus are candidate target strains. Lactobacillus plantarum ATCC 8014 demonstrated similar or greater resistance compared with the target organisms; therefore, the bacterium is proposed as a surrogate of L. monocytogenes for both processes under the conditions specified in the food matrices tested in this study.     

高压脉冲电场作用下亚致死酵母的存在与检测

为探究高压脉冲电场(PEF)对酿酒酵母的灭菌机理,研究了从亚细胞水平揭示PEF处理下的损伤亚致死酵母细胞的存在及产生规律。采用pH7.2的磷酸盐缓冲液(PBS)模拟体系,30 kV/cm电场强度下循环处理100～500μs,通过选择性培养基与非选择性培养基菌落计数对PEF处理前后亚致死酵母细胞进行了研究。结果显示,亚致死酵母的临界渗透压为4%NaCl,PEF处理后,酿酒酵母的亚致死率可达到4.5个对数,亚致死细胞数量达到1.5个对数,且亚致死程度随处理时间的延长而加大。选择性培养基可用于PEF处理过程中亚致死细胞的定量检测。     

Resistance of Campylobacter jejuni to heat and to pulsed electric fields

The resistance of Campylobacter jejuni NCTC 11351 to heat and to pulsed electric fields (PEF) was studied at different treatment intensities (temperatures between 52 and 60 °C, and electric field strengths between 15 and 35 kV/cm, respectively). The influence of the growth phase, the pH of the treatment medium and the presence of sodium pyruvate in the recovery medium was also examined. A model based on the Weibull distribution was used to describe the inactivation curves, and times for the first decimal reduction were calculated (δ values). C. jejuni cells did not increase their resistance to heat nor to PEF upon entrance into stationary phase. The acidification of the treatment medium from 7.0 to 4.5 caused a sensitization of C. jejuni to heat (δ value at 55 °C × 1/4); on the contrary, resistance to PEF was increased (δ value at 25 kV/cm × 2.5). The absence of pyruvate in the recovery medium prevented recovery of a high percentage of heat-treated cells, but did not affect PEF survival. Whereas C. jejuni can be considered a heat sensitive organism (δ value at 55 °C and buffer of pH 7.0 of 2 min, z value 4.40 °C), it showed a relatively high resistance to PEF as compared to other vegetative cells (δ value at 25 kV/cm and buffer of pH 7.0 of 7 pulses, z_PEF value 8.20 kV/cm). Results obtained in this investigation indicate that Campylobacter spp. should be taken into account for the design of PEF treatments for food hygienization.Industrial relevanceBefore PEF can be commercially implemented it is necessary to determine its efficacy on pathogenic microorganisms of interest in order to ensure safety of food. There is no data available about the resistance of C. jejuni to pulsed electric fields, although it is now recognised as the leading cause of bacterial food-borne gastroenteritis throughout the world. In this research we characterize the resistance to heat and to PEF of C. jejuni NCTC 11351. Physiological factors affecting its survival to both agents are also explored.     

Effects of Pulsed Electric Field Processing on Quality Characteristics and Microbial Inactivation of Soymilk

Effects of pulsed electric fields (PEF) on quality characteristics and microbial inactivation of soymilk were studied with different PEF parameters. PEF did not affect significantly the values of pH, “a” (an indicator of redness ranging from “?a” to “+a”, ?a?=?green, +a?=?red) and electric conductivity. The values of “L” (white if “L”?=?100, black if “L”?=?0) increased slightly, whereas values of viscosity and “b” (an indicator of yellowness ranging from “?b” to “+b”, ?b?=?blue, +b?=?yellow) decreased slightly as PEF time increased from 0 to 547 μs. Cysteine, tyrosine, phenylalanine, and serine reduced with the increase of PEF time. The relative activities of soybean lipoxygenase (SLOX) decreased with PEF time increasing from 0 to 1,036 μs. When PEF time and strength increased, the inactivation of Escherichia coli and Staphylococus aureus increased significantly (p?<?0.05), achieving a maximum of 5.20 and 3.51 log₁₀ cycles reduction at PEF time 547 μs and pulsed electric strength 40 kV/cm, respectively. The inactivation of E. coli, S. aureus, and SLOX as a function PEF time followed Weibull distribution. This study demonstrated that PEF could inactivate efficiently E. coli, S. aureus, and SLOX without affecting the quality characteristics of soymilk. Thus, this technique could be an advantageous alternative to heat treatment for pasteurization of soymilk.     

Comparing the PEF resistance and occurrence of sublethal injury on different strains of Escherichia coli,Salmonella Typhimurium,Listeria monocytogenes and Staphylococcus aureus in media of pH 4 and 7

The effect of pulsed electric fields (PEF) on the reduction of the population and on the occurrence of sublethal injury in five strains of two Gram-positive (Listeria monocytogenes and Staphylococcus aureus) and two Gram-negative (Escherichia coli and Salmonella Typhimurium) microorganisms was investigated in media of pH 4.0 and 7.0. Samples were subjected to 50 exponential waveform pulses of 15, 20, 25 and 30 kV/cm at a repetition rate of 1 Hz.PEF resistance and occurrence of sublethal injury depended on the electric field strength and pH of the treatment medium and varied widely among the 20 strains investigated. In general, the increment of the electric field strength from 20 to 30 kV/cm had a significant effect on the viability loss of all the strains. However this increment hardly affected the viability of E. coli W3110 and O157:H7 in the medium of pH 4.0 and of L. monocytogenes 4031, 5672 and 4032 in the medium of pH 7.0. At 30 kV/cm a population reduction higher than 4-Log₁₀ cycles was observed in two strains of S. Typhimurium (878, 4594) and in three strains of S. aureus (976, 4465, 4466) in the medium of pH 4.0. In the medium of pH 7.0, this level of reduction was observed in two strains of E. coli (471, BJ4L1), in three strains of S. Typhimurium (443, 880, 722) and in three strains of S. aureus (976, 4465, 4466).In general, important sublethal injury was not observed for the strains of the two-Gram positive microorganisms in the media of pH 4.0 and 7.0 and for the strains of S. Typhimurium treated in the media of pH 7.0. For E. coli sublethal injury was detected for all the strains investigated, except the strain BJ4L1 treated at 30 kV/cm in the medium of pH 7.0.Industrial relevance. The identification of the most PEF resistant microorganisms of public health concern is necessary to establish the treatment conditions for PEF pasteurization. Data obtained in this investigation demonstrates that the PEF resistance and the occurrence of sublethal injury may vary greatly among different strains of a bacteria and both depend on the pH of the treatment medium. Therefore the most resistant microorganisms of public health significance can be expected to vary for different foods depending of their pH.     

Heat and acid tolerance of Listeria monocytogenes after exposure to single and multiple sublethal stresses

The majority of published studies on the adaptive heat or acid tolerance response of Listeria monocytogenes have been performed with a single strain exposed to a single adaptation treatment; however, in food ecosystems, microorganisms commonly exist as multi-species communities and encounter multiple stresses, which may result in "stress hardening". Therefore, the present study evaluated the adaptive responses to heat (52, 57 and 63 degrees C) or lactic acid (pH 3.5) of a 10-strain composite of L. monocytogenes meat and human isolates at stationary phase, following exposure to combinations of osmotic (10% NaCl), acidic (pH 5.0 with HCl) and thermal (T; 46 degrees C) stresses, sequentially or simultaneously within 1.5h, in tryptic soy broth with 0.6% yeast extract (TSBYE). All treatments induced adaptive responses on L. monocytogenes at 57 degrees C, while no such cross-protection was observed at 52 and 63 degrees C. Survivor curves at 57 degrees C appeared convex with profound shoulders determined by a Weibull model. The highest thermotolerance was observed after combined exposure to acid and heat shock (pH-T), followed by exposure to osmotic shock, and by the combination of osmotic with heat shock (NaCl-T). Regarding acid tolerance, prior exposure to low pH, pH-T, or a combination of NaCl, pH and T resulted in a marked increase of resistance to pH 3.5, showing concave inactivation curves with tails at higher levels of survivors (log(10)CFU ml(-1)) than the control cultures. The sequence of exposure to sublethal stresses did not affect the thermotolerance of L. monocytogenes, whereas simultaneous exposure to most multiple stresses (e.g., NaCl-pH-T, NaCl-T and NaCl-pH) resulted in higher survivors of L. monocytogenes at pH 3.5 than exposure to the same stresses sequentially. The results indicate that combinations and sequences of sublethal hurdles may affect L. monocytogenes acid and heat tolerance, especially in acidic environments with mild heating or in low moisture environments.     

Effect of electric and flow parameters on PEF treatment efficiency

The effects of both the electric and flow parameters on the lethality and energy efficiency of a pulsed electric fields (PEF) treatment were studied. An experimental plan was designed in order to study the microbial inactivation of Saccharomyces cerevisiae and Escherichia coli cells inoculated in a buffer solution. The following process parameters were taken into consideration: electric field strength (13-30 kV/cm), total specific energy input (20-110 J/mL), flow rate of the processed stream (1-4 L/h) and number of passes through the chamber (up to 5).The results showed that, at a fixed flow rate (2 L/h), microbial inactivation of both microbial strains increased with increasing field strength and applied energy input. The maximum inactivation level (5.9 Log-cycles for S. cerevisiae and 7.0 Log-cycles for E. coli) corresponded to the more intensive PEF treatment (30 kV/cm and 110 J/mL). However, for any given field strength applied, the inactivation rate decreased by increasing the energy input. This behavior was attributed to the presence of heterogeneous treatment conditions due, for example, to a different morphology (size and shape) or cell membrane (composition, structure), a local variation of the electric field strength in the treatment chamber, the tendency of microbial cells to form clusters, or a non-uniform distribution of the residence time of the product in the PEF chamber.A more effective stirring of the microbial suspensions which was achieved, at a fixed field strength (18 kV/cm), either by increasing the flow rate with a single pass operation through the PEF chamber, or by operating in re-circulating mode at a constant flow rate, provided a significant increase in the effectiveness and energy efficiency of the pulse treatment.A mathematical model based on the Weibull distribution adequately described the inactivation kinetics of both microbial strains under different flow dynamic conditions.     

Bacterial resistance after pulsed electric fields depending on the treatment medium pH

The objective was to evaluate and compare the pulsed electric field (PEF) resistance of four Gram-positive (Bacillus subtilis, Listeria monocytogenes, Lactobacillus plantarum, Staphylococcus aureus) and four Gram-negative (Escherichia coli, E. coli O157:H7, Salmonella serotype Senftenberg 775W, Yersinia enterocolitica) bacterial strains under the same treatment conditions. Microbial characteristics such as cell size, shape or type of the cell envelopes did not exert the expected influence on microbial PEF resistance. The most PEF resistant bacteria depended on the treatment medium pH. For instance, L. monocytogenes, which showed the highest PEF resistance at pH 7.0, was one of the most sensitive at pH 4.0. The most PEF resistant strains at pH 4.0 were the Gram-negatives E. coli O157:H7 and S. Senftenberg. A subsequent holding of PEF-treated cells in pH 4.0 for 2 h increased the degree of inactivation up to 4 extra Log₁₀ cycles depending on the bacterial strain investigated. Under these treatment conditions, the most PEF resistant bacterial strains were still the pathogens S. Senftenberg and E. coli O157:H7.

Industrial relevance

The design of appropriate food preservation processes by PEF requires the selection of an adequate target bacterial strain, which should correspond to the most PEF resistant microorganism contaminating food. This study indicates that the pH of the treatment medium plays an important role in determining this target bacterial strain. On the other hand, the combination of PEF and subsequent holding under acidic conditions has been proven to be an effective method in order to achieve a higher level of microbial inactivation.     

Modelling thermal inactivation of Listeria monocytogenes in sucrose solutions of various water activities

The heat resistance of Listeria monocytogenes was determined in sucrose solutions with water activity (a(w)) ranging from 0.99 to 0.90. At all temperatures investigated shape of the survival curves depended on the a(w) of the treatment medium. The survival curves for a(w)=0.99 appeared to be linear, for a(w)=0.96 were slightly upwardly concaved and for a(w)=0.93 and 0.90 were markedly concave upward. A mathematical model based on the Weibull distribution provided a good fit for all the survival curves obtained in this investigation. The effect of the temperature and a(w) on the Weibull model parameters was also studied. The shape parameter (p) depended on the a(w) of the treatment medium but in each medium of different a(w) the temperature did not have a significant effect on this parameter. The p parameter followed a linear relationship with a(w). The scale parameter (delta) decreased with the temperature following an exponential relationship and increased by decreasing the a(w) in the range from 0.99 to 0.93. However the delta parameter of survival curves obtained at a(w)=0.90 were lower than those obtained at a(w)=0.93. A mathematical model based on the Weibull parameters was built to describe the joint effect of temperature and a(w) on thermal inactivation of L. monocytogenes. This model provides a more complete information on the influence of the a(w) on the L. monocytogenes than the data initially generated. The model developed indicated that the effect of the a(w) on the thermal resistance of L. monocytogenes varied depending upon the temperature of treatment.     

Effect of temperature,pH and presence of nisin on inactivation of Salmonella Typhimurium and Escherichia coli O157:H7 by pulsed electric fields

The aim of this investigation is to evaluate the concurrent influence of temperature (4–50 °C), pH (3.5–7.0), and the presence of nisin (up to 200 μg/mL) on the inactivation of two PEF-resistant Gram-negative, pathogenic bacteria, Salmonella Typhimurium STCC 878 and Escherichia coli O157:H7, using a PEF treatment of 30 kV/cm and 99 μs. A response surface model using a central composite design was developed for the purpose of understanding the individual effects and interactions of these factors.The models showed that temperature was the factor with the greatest influence on the PEF inactivation in the two strains investigated. Increasing the treatment temperature from 4 to 50 °C increased the lethality of PEF up to at least 4 Log₁₀ reductions for both microorganisms at all pH levels investigated. PEF lethality varied with the square of the pH observing the highest microbial PEF sensitivity at pH 5.25 at all temperatures. The addition of nisin to the treatment medium did not influence the PEF lethality independently of the temperature.PEF induced 1.0–1.5 Log₁₀ cycles of damaged cells at pH 3.5 for Salmonella Typhimurium STCC 878 and at pH 5.25 for E. coli O157:H7, independently of the temperature or the presence of nisin in the treatment medium.The application of PEF at 50 °C permitted the achievement of 5 Log₁₀ reductions of Salmonella Typhimurium STCC 878 and E. coli O157:H7 in a range of pH from 4.2 to 6.7 and from 4.5 to 6.0, respectively. Therefore, the application of PEF at moderate temperatures has great potential for achieving effective control of Gram-negative pathogenic microorganisms in the range of pH found in most foods.     

Inactivation of Escherichia coli by high hydrostatic pressure at different temperatures in buffer and carrot juice

The inactivation of Escherichia coli MG1655 was studied at 256 different pressure (150-600 MPa)-temperature (5-45 degrees C) combinations under isobaric and isothermal conditions in Hepes-KOH buffer (10 mM, pH 7.0) and in fresh carrot juice. A linear relationship was found between the log10 of inactivation and holding time for all pressure-temperature combinations in carrot juice, with R2-values>or=0.91. Decimal reduction times (D-values), calculated for each pressure-temperature combination, decreased with pressure at constant temperature and with temperature at constant pressure. Further, a linear relationship was found between log10D and pressure and temperature. A first order kinetic model, describing log10D in carrot juice as a function of pressure and temperature was formulated that allows to identify process conditions (pressure, temperature, holding time) resulting in a desired level of inactivation of E. coli. For Hepes-KOH buffer, the Weibull model more accurately described the entire set of inactivation curves of E. coli MG1655 compared to the log-linear or the biphasic model. Several secondary models (first and second order polynomial and Weibull) were evaluated, but all had poor fitting capacities. When the Hepes-KOH dataset was limited to 22 of the 34 pressure-temperature combinations, a first order model was appropriate and enabled us to use the same model structure as for carrot juice, for comparative purposes. The major difference in kinetic behaviour of E. coli in buffer and in carrot juice was that inactivation rate as a function of temperature showed a minimum around 20-30 degrees C in buffer, whereas it increased with temperature over the entire studied temperature range in carrot juice.