Inactivation of Escherichia coli by combining pH, ionic strength and pulsed electric fields hurdles (original) (raw)
Related papers
2010
The aim of this work was to study the efficiency of inactivation of Escherichia coli cells in aqueous suspensions using combined moderate pulsed electric field (PEF) and thermal treatments. The inactivation kinetics of E. coli cells in aqueous suspensions (1 wt%) was monitored using conductometric technique. The electric field strength E was within 5-7.5 kV/cm, the effective PEF treatment time PEF t was within 0-0.75 s, the pulse duration t i was within 0.3-1 ms, the medium temperature was 30-50°C, and the time of thermal treatment t T was within 0-7000 s. The organic acid concentration was within 0-0.5 g/L.The damage of E. coli was accompanied by release of intracellular components. The synergy between the PEF and thermal treatments in E. coli inactivation was clearly demonstrated. The damage efficiency was noticeably improved by addition of organic acids, especially lactic acid.
Food Microbiology, 2002
The inactivation of Escherichia coli O157:H7 by pulsed electric ¢eld (PEF) processing as a function of electric ¢eld strength (15^30 kVcm À1), pulse number (1^20), temperature (5^651C) and pH (3?5 and 6?8) was studied using a commercially available pulser, a static chamber and gellan gum gel as a suspension medium.The custom-designed static chamber achieved near-isothermal treatment conditions while eliminating £ow ¢eld e¡ects. Gellan gum gel was used to suspend the bacteria for treatment. It allowed uniform distribution of bacteria and neither inhibited nor promoted bacterial growth.The combination of equipment design and experimental protocol allowed the contribution of electrical (PEF) and thermal energy to be measured separately. In water-based gel, a maximum of 3 log reductions were achieved by PEF energy. Greater inactivation was observed at a treatment temperature of 55 o C, but the additional inactivation was attributable entirely to thermal energy. Microbial injury was also observed at this temperature. At 60 o C and above, complete inactivation was achieved, but this was attributable entirely to thermal energy. In water-based gellan gum gel adjusted to pH 3?5, again a 3 log reduction was achieved by PEF. In gel made from freshly squeezed apple juice naturally having the same pH, however, a maximum of 1?5 log reduction was observed.
Applied Sciences
The paper investigated studies on the application of pulsed electric fields for the treatment of liquid media in a continuous manner in a co-field treatment chamber with elliptic insulator profiles. The electric field distribution and the temperature rise in the treatment chamber were evaluated via the finite element method. A non-uniform electric field was found at the elliptical insulator edges, while the electric field distribution on the insulator surface was rather uniform. The maximum temperature rise in the liquid media was located slightly behind the elliptic insulator due to the accumulated heat in the flowing liquid media. In the optimized treatment chamber, the average electric field intensity could be as high as 12.21 kV/cm at the moderate voltage at 7.5 kV. As a strategy to improve the inactivation while limiting the temperature rise, a series of treatment chambers was verified by experiments under the conditions of 7.5 kV, a 2.5% duty cycle, and 250 Hz. It was found th...
Journal of Food Protection, 2003
This investigation was undertaken to study the inactivation of Escherichia coli O157:H7 by pulsed electric field (PEF) treatment and heat treatment after exposure to different stresses. E. coli O157:H7 cells exposed to different pHs (3.6, 5.2, and 7.0 for 6 h), different temperatures (4, 35, and 40°C for 6 h), and different pre-PEF treatments (10, 15, and 20 kV/cm) were treated with PEFs (20, 25, and 30 kV/cm) or heat (60°C for 3 min). The results of these experiments demonstrated that a pH of 3.6 and temperatures of 4 and 40°C caused significant decreases in the inactivation of E. coli O157:H7 by PEF treatment and heat treatment (P < 0.05). Pre-PEF treatments, pHs of 5.2 and 7.0, and a temperature of 35°C, on the other hand, did not result in any resistance of E. coli O157:H7 cells to inactivation by PEF treatment and heat treatment (P > 0.05).
International Journal of Food Microbiology, 2007
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.
Using pulse electric fields (PEF) for selective inactivation of coliform bacteria
Biologija
Coliform bacteria usually originate from the intestine, and the contamination typically occurs when there is a lack of sterility in food industry. PEF-induced selective nonthermal pasteurisation method might pasteurise coliform bacteria while leaving the needed bacteria intact. To evaluate this hypothesis, we chose Escherichia coli dh5α (E. coli) strain as a representation of coliform bacteria for this study. We also used Streptococcus thermophilus bacteria strain as a representation of lactobacteria used in milk by-products. The obtained results of PEF application showed that selective death of bacteria after PEF treatment can be induced. PEF was applied on bacteria. Then the clonogenic assay, the metabolic activity, and bacterial growth in the bioreactor were evaluated. By applying PEF treatment on E. coli and S. thermophilus their survival was monitored. We found the PEF parameters under which coliform bacteria E. coli were killed more than 100 times effectively than S. thermophi...
Food Research International, 1995
Inactivation of microorganisms exposed to high-voltage pulsed electric fields is a promising non-thermal food preservation technology. This paper demonstrates and validates the inactivation of Escherichia coli, a Gram-negative bacterium and Staphylococcus aureus, a Gram-positive bacterium, subjected to high-voltage electric field pulses. A four-log cycle reduction in microbial population is achieved in model foods such as simulated milk ultrafiltrate (SMUF) with a peak electric field strength of 16 kV/cm and 60 pulses with a pulse width ranging between 200 and 300 μs. The temperature of the cell suspension was kept below the lethal temperature, demonstrating that inactivation is not due to thermal effects induced by the pulses of high-voltage electricity. Thermal food preservation causes undesirable changes in the physical character, quality and nutrient content of foods. Non-thermal preservation techniques minimize the undesirable changes in foods. A comparison between the inactivation of microorganisms by high-voltage pulsed electric fields and thermal methods of food preservation is also discussed.
Bacterial resistance after pulsed electric fields depending on the treatment medium pH
Innovative Food Science & Emerging Technologies, 2005
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 Log 10 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. D 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.
Bacterial inactivation of liquid food and water using high-intensity alternate electric field
Journal of Food Process Engineering, 2016
Bacteria contaminated liquid food and water, along with the harmful byproducts from chemical disinfection methods, raise concerns to the public health. This article investigated the inactivation of bacteria in liquid food and water using the pilot-scale concentrated high intensity electric field (CHIEF) system. By generating a strong alternate electric field in a small region of the reactor, the pilot CHIEF system was able to inactivate microbes at a continuous operation. Compared with other disinfection methods, the CHIEF system used relatively less energy density to decontaminate and required a shorter treatment time. The mildest energy density required was 500 kJ/L to obtain a 5log reduction for Escherichia coli, at the minimum treatment time of 4.7 ms. An electric field analysis was performed using both an equivalent-circuit model and numerical simulation. A logarithm function model and a Weibull model were used to describe the effects of energy density and treatment time on the disinfection effectiveness of the CHIEF system. The CHIEF system was compared with other water disinfection methods in multiple aspects, such as treatment time and energy consumption. Practical applications The concentrated high intensity electric field (CHIEF) process presented in this study is a continuous process that can be used to decontaminate liquid food or water. The process operates under non-thermal conditions, which protects the nutrient and flavor of the food. The process uses alternate electric supply at a relatively low voltage (10 kV) and frequency (60 Hz). The reactor design and material selection enable a strong electric field to form within the treatment region, allowing the inactivation of microbes without using the pulsed energy. As a result, the process can achieve effective decontamination under a relatively short treatment time (4.7 ms) and moderate energy cost (500 kJ/L). Since the process eliminates the use of pulse generators, it requires less capital investment and has more scale-up potentials. K E Y W O R D S bacteria decontamination, continuous liquid food treatment, high-intensity alternate electric field, low energy requirement, short treatment time Nomenclature: E L , Electric field strength (kV/m); U L , Voltage at the liquid channel (kV); I, Current (A); d L , Diameter of the liquid channel (mm); U 0 , Applied voltage (kV); R L , Resistance (X); C D , Capacitance (F); T, Temperature (8C); A L , Area of the liquid channel (mm 2); A D , Area of the dielectric material (mm 2); d D , Distance between the dielectric plates mm ð Þ; Q, Power density (kJ/L); V, Volume in the liquid channel (mm 3); U, Electric potential (kV); P dis , Dissipation power density (kW/ L); P spec , Specific power density (kW/L); P UV , Powder density of UV disinfection (kW/m 2); S, Log reduction (-log CFU/ml); N 0 and, N t 5 Initial and final bacteria populations; k, Inactivation constant; n, Degree of polynomial.
Moderate electric fields can inactivate Escherichia coli at room temperature
Journal of Food Engineering, 2010
The inactivation of Escherichia coli using moderate electric fields (MEF) below 25°C, was investigated. Keeping the temperature always below 25°C demonstrated that electric fields are involved in the inactivation of E. coli, without possible synergistic temperature effects. Electric fields above 220 V cm À1 promoted death rates of 3 log 10 cycles of E. coli in less than 6 min, and even higher rates at greater electric fields, while presumably overcoming the thermal degradation caused by conventional high temperature treatments. A non-thermal model was proposed that successfully describes the E. coli death kinetics under this treatment. SEM observations of E. coli cells after the exposure to the MEF treatment, revealed changes at the cell membrane level, indicating a possible cause for the cell death rates. These results show that this treatment holds potential for sterilization of thermolabile products (e.g. serum and other physiological fluids, food products), by itself or as a complement of the traditional heat-dependent techniques.