Science against microbial pathogens: communicating current research and technological advances
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A. Méndez-Vilas (Ed.)
Factors that Influence the Electric Field Effects on Fungal Cells
Maricica Stoica1, Gabriela Bahrim1 and Geta Cârâc*2
1
Department of Biochemistry and Technologies, Faculty of Food Science and Engineering, “Dunarea de Jos” University
of Galati, 111 Domneasca Street, 800008- Galati, Romania; maricica.stoica@ugal.ro, gabriela.bahrim@ugal.ro
2
Department of Chemistry, Physics and Environment, Faculty of Sciences, “Dunarea de Jos” University of Galati,
111 Domneasca Street, 800008- Galati, Romania; getac@ugal.ro
Fungi are an important group of microorganisms studied due to on their positive biochemical abilities as starter cultures in
biotechnology to positive modify food characteristics and stability and at the same time they play an important role in the
development of products with economical importance such as: enzyme, vitamins, organic acids and antibiotics. On the
other hand, the spoilage of food by fungi is a serious economic issue; therefore the control of fungi is essential and
decisive. In recent years, the use of electric field for microbial inactivation has received much attention in applied
microbiology. Pulsed electric field (PEF), a food preservation method, has proved to inactivate the spoilage
microorganisms and also pathogens. The process consists in the application of a short duration high electric field to food
product, which is placed between two electrodes. Successful application of PEF depends strongly on the cells morphology
and physiological properties, the fluid medium properties, the type and characteristics of the used electric field waveform.
The overall aim of this review is to systematically present the factors that influence the electric field effects on fungal
cells.
Keywords fungi; inactivation; pulsed electric field
1. Introductory remarks
Fungi are eukaryotic microrganisms [1] that include yeasts and molds [2], which are unicellular [3] and multicellular
microorganisms [4]. The fungal cells are larger than bacteria [5] and structurally more complex than other
microorganisms [6]. Some strains are important in food biotechnology as starter cultures with the ability to modify food
characteristics and stability [7], and in industrial biotechnology to produce enzymes [8], and other beneficial byproducts, such as antibiotics [9], vitamins [10], organic acids [11] and others. On the other hand, the food spoilage by
fungi [12, 13] raises an economic issue [14] and it is estimated that between 5 and 10% of the world’s food production
is lost annually due to fungal biodeterioration [15]. The risk of health problems can appear due to mycotoxins produced
as secondary metabolites by fungi during the stationary phase of growth in some specific physic-chemical conditions
[16]. Control of fungal spoilage is therefore, essential and decisive to prevent different biological risks. Different
undertaking methods for food safety and microbial inactivation with thermal processing as the dominant conventional
treatment which extends the shelf-life and at the same time mantaining food safety [17]. The thermal processing leads to
change in the sensory attributes, such as flavours, stability of thermolabil compounds as vitamins, aminoacids as well as
the modifying nutritional quality of the products [17, 18]. The ever-increasing trend towards safety, consumer’s health,
and minimal food processing [19] has challenged food preservation technology to inactivate microorganisms using new
methods instead of the heat treatments [17, 20-22]. The nonthermal methods correspond to the expectations for minimal
processed food and they add higher nutritional value with evident efficiency upon microorganism’s inactivation [23].
Among all emerging nonthermal technologies, high intensity pulsed electric fields (PEF) has been gained interest over
the last decade [24] and it is one of the most appealing technologies due to its short treatment times and reduced heating
effects [25, 26]. High intensity pulsed electric field, as a nonthermal food preservation technology, involves the
discharge of high voltage electric short pulses through the food product [25]. PEF technology is considered more
efficient than traditional heat treatments of food and consequently it presents several advantages over conventional heat
treatments: better retention of flavour, colour and nutritional value, improved protein functionality, increased shelf-life
and reduced pathogen contamination levels [27]. Successful application of PEF depends strongly on biological factors
[28] such as: cell type [17, 29 - 31], size and shape of the cell [29], cells density [32], arrangement and cell position;
dielectric breakdown [33], and physical and chemical properties of food are also considered (conductivity, pH, ionic
strength) [17, 34]. The type and characteristics of the used electric field waveform in PEF [34, 35] are critical for the
outcome of this process [17]. The breakdown of the cell membrane, as effect of the PEF, can be a reversible or
irreversible process [35]. The reversible breakdown has wide applications in biotechnology and medicine [36], while
the irreversible breakdown finds applications in food industry, pharmaceutical research, public health and water
purification [35]. PEF, as an innovative minimal processing, is receiving considerable much attention from research
groups as well as food companies as a new technique with potential to be fully adapted to processing food at larger
industrial users [37].
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2. Biological factors
Biological factors are determinant for PEF application [25]. The susceptibility of the microorganisms to high intensity
pulsed electric inactivation is highly related to the morphological and physiological characteristics of treated
microorganisms [25] such as: cell type [17, 29 – 31, 38], cell size and shape [29], cells density [32], the cell
arrangement in suspension; dielectric breakdown [33].
2.1 Type of microorganisms
PEF treatment has been tested on a wide range of microorganisms including different Gram-positive and Gram-negative
bacteria [25, 28, 39], yeasts such as Saccharomyces cerevisiae [25, 39, 40 - 44] and moulds [45, 46]. Barsotti and
Cheftel [28] reported that the efficiency of microbial inactivation depends first on the type of microorganism. Many
authors have found that Gram-positive bacteria are more resistant to PEF than those Gram-negative [25, 31, 35]. The
yeasts show a higher sensitivity to electric fields than vegetative form of bacteria [25, 31, 35] due to their larger size
[25] than most bacteria have, so they may exhibit a lower breakdown transmembrane potential, and therefore, they
would be more sensitive to PEF processing [31]. PEF has only been proven to be effective on vegetative cells [47].
Bacterial spores and mold ascospores are more resistant to PEF exposure than vegetative cells [47, 48], whereas
bacteria spores exhibit a greater resistance to PEF treatment due the dehydrated cytoplasm, which reduces their
electrical conductivity and makes it difficult to develop a sufficient high voltage gradient to breach the surrounding
membrane [49] and also, the mainly the cortex of spores that enclose the cytoplasmatic membrane, probably prevent the
permeabilisation effect of PEF [45]. The degree of resistance of molds or yeast ascospores is not yet established [28].
The yeast ascospore wall did not protect these against the PEF treatment. The resistance of them to PEF exposure is due
to their structure, which includes an extremely thick intermediate space between the cell wall and the cytoplasmic
membrane. The thick space in the cell wall of ascospores has been considered as a possible factor in their high heat
resistance to PEF treatments. Alvarez et al. [45] reported that ascospores and conidiophores were sensitive to PEF
treatment. Taking into account that PEF inactivates microorganisms should vary not only for different species but also
for different growth phase’s characteristics of each species [31]. Cells and membrane properties vary at different time of
cultivation [18]. In the logarithmic phase cells are more sensitive to the applied electric field than lag and stationary
phase [51 – 54]. Treatment parameters for inactivation of the most resistant microorganisms should be carefully
selected without affecting nutritional qualities of culture media [18].
2.2 Cell size and shape
The size and shape of a microorganism play an important role in its inactivation during PEF treatment [40, 55, 56] and
these factors are responsible not only for the different PEF sensitivity of yeast comparing with bacteria but also for the
one among strains of the same species [57]. The cells with larger diameters, such as yeast, were killed at lower electric
direct field than the cells with smaller diameter (typical cells [47, 58, 59]) and conversely, they are more resistant to
alternating current, compared with small cells. Cell size and cell shape, as well as the varying morphological and
biochemical properties of cells, are responsible for the particular behaviour of microorganisms. The influence of the cell
size on the lethal effect of PEF has been related to the transmembrane potential created by external electric field
strengths. Some authors [40, 51, 60] found a decrease in critical breakdown potential when the cell volume increases.
Saccharomyces cerevisiae cells have a larger diameter compared to Aspergillus niger spores (Figure 1 and Table 1)
which exhibit a lower critical external electric field for breakdown and were the most sensitive to PEF treatment. The
size and the composition of Aspergillus niger spores (highly melanised) was more favorable to protect the spores of
chemical and electrical stressing [61]. For the cells with a radius of about 2.5÷5μm, an applied electric field, less than
about 1–3 × 105 V/m, would not produce electropermeabilisation [62, 63]. Also, the cell shape influences the
transmembrane potential [64 - 67].
a
b
Figure 1. Scanning electron microscopy of different spoilage fungi colonised on stainless steel
a) Aspergillus niger, b) Saccharomyces cerevisiae
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Table 1. Typical size and shape for the common spoilage fungi
Fungal strain
Cell characteristic
Reference
Aspergillus niger
2.5-4 μm, globose, spinose
[68]
Saccharomyces cerevisiae
5-10 μm, ellipsoidal
[69]
2.3 Density and cells arrangement in suspension
The efficiency of PEF inactivation is dependent on number of microorganisms in the food to be treated [54]. However,
the fact that high cells density decreases the PEF lethal effect is not fully understood. Jayaram et al. [70] found that a
higher transmembrane potential developed across clusters of cells rather than across an individual cell for the same
electric field intensity and the larger number of microbial cells indicated the less effective PEF treatment [71]. Others
authors who studied the inactivation of Escherichia coli in simulated milk ultrafiltrate found that PEF doesn’t affect
when the concentration of this bacterium was variable [72]. Some authors [53] reported that an increasing concentration
of Saccharomyces cerevisiae cells in apple juice has resulted in a slightly lower inactivation with PEF (pulse duration
25 ms at an electric field of 25 kV/cm). Also, Jeantet et al. [73] reported for a greater inoculum size of Saccharomyces
cerevisiae a lower antimicrobial action by PEF treatment. Qin et al. [53] explained that the effect of microbial
concentration on inactivation may be related to the cluster formation of yeast cells and/or possibly concealed
microorganisms in a low electric field region. In recent studies, both the cells arrangement [33] and density in
suspension influence the induced transmembrane voltage [74].
2.4 Dielectric breakdown
The dielectric material is a substrate that is a poor electricity conductor, but it is an efficient supporter of the electric
fields [75]. The dielectric breakdown of a biological compound results from the local combination of an intense electric
field and of neutral, dielectric (non-conductive) constituents or molecules, which suddenly change and become strongly
conductive [76, 77]. Thus the resulting current induced by dielectric breakdown may cause ohmic heating [78]. The
latter, would enhance the modification (unfolding and denaturation) of bioactive macromolecules such as proteins,
polysaccharides and lipids in the treated food. During the PEF treatment, the dielectric breakdown is characterized by a
spark and it is attributed to the presence of chemical compounds that substantially enhance the local electric field due to
the differences in the dielectric properties of the material. Dielectric breakdown is a limiting factor in PEF technology
[79]. The intent of food treatability with PEF processing is to induce the dielectric breakdown of the membrane, rather
than the dielectric breakdown of the fluid food. Therefore, preventing the dielectric breakdown of food is a key to
success of PEF treatment [80].
3. Mechanism of inactivation of cells by PEF in food processing
Microorganisms are inactivated when they are exposed to factors that substantially alter their cell structure or
physiological functionality. Cell functions are altered when the membrane selectivity is disabled [20]. Membrane
structural or functional damage is generally accepted as the cause of cell death during exposure to high-voltage electric
field. The mechanism of inactivation for the exposed biological cells to pulsed electric fields has not been fully clarified
until today [31]. The microorganism’s inactivation by PEF is a multi-step process which may cause cell death through
multiple mechanisms. Saulis [30] suggested that the effect of PEF treatment upon microorganisms during food
processing consists of four main stages: (a) increase in the transmembrane potential due to charging the cell plasma
membrane by the external electric field applied, (b) pore initiation stage, (c) evolution of the pore population during an
electric treatment and (d) post-treatment stage (pore resealing, cell death). Primary effects of PEF on microbial cells
depend on the pulse amplitude and cell size and include structural fatigue due to induced membrane potential and
mechanical stress [46] with duration from nanoseconds to milliseconds [30]. The transmembrane potential generated by
external electric potential field can be done not only for the spherical cell but also for spheroidal [64, 66] ellipsoidal,
cylindrical [65] or the cell with irregular form [67]. The transmembrane potential of non-spherical biological cell will
depend on the cell orientation connected to the external electric field [66]. The cell membrane acts as a capacitor filled
with a material with a low dielectric constant (Figure 2) and is regarded as an insulator shell to the cytoplasm due to its
electrical conductivity, which is six to eight times weaker than that of the cytoplasm [81] as see in Table 2.
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Figure 2. The intact cell membrane (modeled by a capacitor and a resistor placed in parallel) has a potential Vm [82]
Table 2. Electrical conductivity values for cell membrane and cytoplasm
Cell
component
Membrane
Cytoplasm
Electrical
conductivity, μS/cm
30
3÷5·107
References
[83]
[84 - 85]
When the cells are exposed to an electric field, the free charges generated on the membrane surfaces are attracted to
each other due to the difference in the signs (- and +) which causes a compression [86]. The accumulation of negative
and positive charges in cell membranes forms a transmembrane potential (Vm). Vi potential induced by field application
is superimposed onto the initial transmembrane potential Vm (Figure 3). Induced potential on the cell membrane is
important for studying the effects of the electric field on cells and can be calculated analytically or numerically [33]. In
the spherical coordinate system, a biological cell can be represented by a sphere surrounded by a shell. Figure 3 shows a
cross section of such a sphere. A cell membrane is presented as a shell, where d denotes the membrane thickness and r
the cell radius (Figure 3a).
Figure 3. Distribution of the transmembrane potential in a spherical cell [82]: a) transmembrane potential (Vi) induced by the electric
field, adds on to the initial transmembrane potential (Vm ). Breakdown potential VR is first reached at the pole, where potential vectors
point to the same direction, b) breakdown (rupture) potential VR is reached in other membrane zones when field E >> ER is applied
For physiological conditions where d << r, Vm, can be calculated with Eq. 1, favorable for potential on a surface of a
nonconducting sphere [33]:
Vm ≈ frEcosθ (1)
The further membrane thinning increases the electrostatic attraction between the two sides of the membrane. At a
given time and location, electrocompression supersedes the elastic resistance of the membrane. Local membrane
breakdown with pore formation takes place for a given value of the applied field (E). The transmembrane potential
reaches a critical value, referred to as the breakdown potential (VR). Experimental data clearly indicate that the electric
field necessary for membrane disruption increases when the cell radius decreases. The breakdown potential VR induced
on the cell membrane due to an external electric field is given as [82]:
VR ≈ frERcosθ
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(2)
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A. Méndez-Vilas (Ed.)
where ER is rupture electric field, θ is the angle between the vector of the electric field and the vector of the cell radius r
at the relevant point on the membrane and f is a parameter depending on the cell shape (f =1.5 for the spherical cell).
This equation is compatible with a nearly constant VR value, whatever the type of cell, while ER is inversely related to
the value of the cell radius. More precisely, membrane rupture occurs when and where the sum of the initial and
induced potentials equals the breakdown potential VR. In the membrane zone where the two potential vectors are
oriented in the same direction, rupture occurs at a lower electric field than in the region where these vectors are
oriented. High transmembrane potential exerts pressure on the cell membrane; this pressure decreases membrane
thickness and ultimately causes pore formation. Cell lysis with loss of membrane integrity occurred when
transmembrane potential was approximately 1 V [87]. The electrical breakdown of the cell membrane, as effect of the
PEF, can be reversible or irreversible [35]. Reversible breakdown has wide applications such as applied biotechnology,
biochemistry, molecular biology, medicine and other biological research domains [36, 88, 89] while the irreversible
breakdown finds applications in the food industry to preserve the quality of foods [47, 54, 90 – 93] pharmaceutical
research and public health [35] and in water purification [35, 94].
Following exposure to PEF treatment, the microorganism either survives or dies [95]. When an intact cell is exposed
to electrical field (Figure 4a) the lipid bi-layer and proteins of the cell are temporarily destabilised and perforation
ensues, but if the electric field is removed, the pores reseal [96, 97] electroporation can be completely reversible [96]
and cells retain their viability [98] (Figure 4b). The total area of induced pores is small in relation to the total surface
area of the membrane and the pores are able to close again mainly due to the diffusion of lipid molecules and
rearrangement of the proteins. The mechanistic details of the membrane restructuring that follows electric field
exposure in living cells have not been definitively established. If the intensity of the electric fields is higher, the
membrane will not be able to recover the pores, an irreversible breakdown occurs and small compounds will start
leaking undergo lysis [99] (Figure 4c).
Figure 4. Reversible and irreversible breakdown: (a) intact cell, (b) cell membrane reversible permeabilised, (c) cell membrane
irreversibly permeabilised (inactivated cell)
The attempts to describe the dependences the of inactivation efficiency on the different process variables have been
done [30]. The magnitude of the breakdown potential depends on the microorganism type, cell size and shape, growth
phase of microorganism and the physical and chemical characteristics of the medium in which the cells are present. The
fraction of survived microorganisms, S, is correlated with the applied field intensity E and treatment time t can be
represented by [47, 50]:
S= (t/tc)(E-Ec)/k
(3)
where tc is the critical treatment time, Ec is the critical electric field strength and k is the kinetic constant. This equation
implies that increasing the amplitude of electric pulses is much more effective than lengthening the treatment time
[100]. If logarithms to base 10 on both sides of Eq. 3 are taken, the left-hand side of such equation would represent the
log reduction (90% reduction in the initial population of microorganisms) [31]. The transformed equation depends
linearly on the applied field strength and on the treatment time, the electric field strength having a more pronounced
effect than treatment time, but both are important factors [31].
For the efficiency of food processing as well as other applications, the size and the evolution of the created pores is
especially important [30]. The pore behaviour is affected by the more factors such as: pore resealing capacity [101]
dependence of pore formation energy on pore density the variations of the transmembrane potential [102, 103] the ionic
strength of the medium [30, 104] membrane fluidity [105]. The number and pores distribution depend on both time and
position on the cell surface [104].
The behaviour of pores after PEF treatment has not been examined in sufficient detail yet, neither theoretically nor
experimentally. Microorganisms will retain their viability or die [30, 95, 106]. Recent theoretical analysis and
experimental data show that the resealing of the pore consists of a few stages of the quick (microseconds - milliseconds,
minutes) reduction in pore size and the stage of the slow complete pore closure [107]. Analysis shows that the time
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necessary for the pores resealing varies from cell to cell [30, 108]. Dutreux et al. [109] consider that the most important
reasons of cell death after PEF treatment are the increased permeability or even structural damage of the cell membrane
causing the release of the intracellular components outside of the cell. Others authors suggest that the cell dye after PEF
processing through apoptosis or necrosis [110 – 112] and/or osmotic swelling [113]. To understand the mechanism for
the cells inactivation by PEF technology, it is not enough to know the changes occurring in the cell membrane during
and after the PEF [30]. The factors causing the different cell death such as mammalian cells, yeasts, and bacteria treated
by PEF have to be considered as well [30]. Knowing the mechanisms of different cell death types could help in
increasing the efficiency of PEF treatment technology.
4. Fluid medium properties
In general, microbial inactivation through PEF technology depends on process parameters, but few data reports that the
prediction PEF inactivation of food spoilage microorganisms is a function of process parameters and also environmental
factors [21]. Fluid (food product matrix) properties, such as electrical conductivity, ionic strength and pH strongly
influence the PEF sensitivity of present microorganisms as specific and unspecific microbiota [28, 31].
4.1 Medium conductivity
The medium conductivity influences significantly the action of the electric field which passes through that medium
[114-116] and in this case there are living cells, the medium conductivity is an important factor that influences the
biological properties (electropermeabilisation, motility and microbial inactivation) [54]. The electric medium
conductivity is an important parameter in PEF treatment [86]. The correlation between microorganism’s inactivation
and medium electrical conductivity has been investigated by different authors [50, 91, 117 – 125]. A significant
increase in the microorganism’s inactivation at lower ionic strength and conductivity was shown, whereas other authors
[124] did not confirm this theory. Some researchers [86, 118] argued that the PEF treatment is more effective in
medium with lower conductivity due to a larger difference on the ionic concentration between the suspension and the
cell cytoplasm. The large ionic gradient facilitates an increase by ionic substances across the cell membrane, which
weakens the membrane structure and makes it more susceptible to the PEF [54, 76, 86]. Thus, more articles argue that
the microorganism’s inactivation increases with decreasing the medium conductivity [70]. Other authors have reported
that by decreasing the medium conductivity it is possible to increase the inactivation level of yeast strains, such as
Saccharomyces cerevisiae or other microorganisms [47, 120, 126]. In fact, food which have high electrical conductivity
are difficult to process by PEF since they generate low peak electric fields across their treatment chambers due to the
high current that is typically generated during PEF technology [54]. A lower medium conductivity is suitable to obtain
greater microbial inactivation for the same applied electric field. The input voltage can be increased to obtain the same
applied electric field for a different conductivity level, but however, it did not influence cells inactivation.
4.2 Ionic strength and medium pH
The microbial inactivation by PEF technology is highly influenced by ionic strength and medium pH [31]. The
inactivation ratio is generally enhanced when the medium has a low ionic strength [31, 50, 54, 118, 127]. VegaMercado et al. [119] reported that the ionic strength was responsible for electroporation and compression of the cells
while the pH influenced the cytoplasmatic system when the poration was completed. These authors consider that the
ionic strength and pH disturb the homeostasis of the microorganisms leading to an increase of the inactivation ratio. The
increasing of ionic strength has as results an increasing in the electron mobility through solution and a decreasing in the
microorganism inactivation by PEF treatment. Tsong [128] considered that the reduced inactivation rate in high ionic
strength solutions can be explained by stability of the cells membranes when they are exposed to a medium which
contains several ions. The ions dissolved in the treated medium have been found to produce effects on microbial
inactivation with PEF. Ions such as Ca2+, Na+, K+ and Mg2+ can be influence the membrane and the cellular physiology,
while ions as Na+ and K+ do not influence inactivation of microorganisms; Ca2+ and Mg2+, however, have been shown
to induce a protective effect against PEF [50]. Bruhn et al. [129] showed that the presence of ions in a medium appears
to be necessary to increase the transmembrane potential. The pH of the PEF treatment medium is one of the factors that
generated most controversy in literature. Some researchers have observed that microorganisms were more PEF sensitive
in acidic medium [43, 91, 119]. Other authors have indicated that microbial resistance was lower at neutral pH [43, 73,
124, 130] and with no influence on microbial PEF inactivation [125, 131, 132]. These differences have not been
elucidated yet but they could be correlated with the increase number of pulses and electric field strength applied at the
lower medium pH, the microorganism’s type [45] and a change in the cell capability to maintain a transmembrane pH
gradient due to membrane electroporation [45]. The medium pH plays an important role in microbial inactivation when
PEF is combined with organic acids treatment having antimicrobial effect [18]. The strong synergic inactivation by
combination of organic acids and PEF treatment at lower pH (e.g. 3.4) indicated that entry of undissociated acids into
microbiological cells was enhanced [133]. Wouters et al. [91] also showed that the PEF inactivation of some
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microorganisms (Listeria innocua, Saccharomyces cerevisiae) in phosphate buffer were more effective when the pH of
the buffer was decreased from 6.8 to 5.0. It is possible that the inactivation levels achieved were due to not only the
combined effect of pH and PEF, but also the antimicrobial potential of the organic acids [86]. Since the pH have
influences the microbial inactivation by PEF, the acidic food such as fruit juices are good candidates for processing by
PEF technology.
5. Electric field waveforms
The electric-field is the most important treatment parameters affecting the performance of microbial inactivation by
PEF [90]. The PEF technology consists of delivering an electric field in pulses into a food [134] to reduce the most
resistant microorganisms which are significant to the public health to level that is not likely to present a health risk
under normal distribution and storage conditions [135]. The PEF process involves the application of pulses of high
voltage, typically of 20-80 kV/cm for short periods of time (less than 1 second) to fluid food placed between two
electrodes [54]. Once the applied electric field exceeds a critical value for a certain period of time, the transmembrane
potential is induced which leads to cells death. The factors that affect the microbial inactivation with PEF depend not
only the biological factors and fluid medium properties, but also they on the processing factors. From the electric field
strength and the treatment time, some other variables such as pulse characteristics can also influence the inactivation
ratio and reaction kinetics in PEF treatment [31]. The pulse waveshapes is briefly taken into account below. The square
wave and exponential decay pulses (in bipolar or monopolar form) (Figure 5) are generally used for PEF cells treatment
[25, 46, 86].
a
b
c
d
Figure 5. Pulse waveforms commonly used in PEF technology adapted from [134]: (a) exponentially decaying pulse, (b) bipolar
exponential, (c) square pulse and, (d) bipolar square
Qin et al. [35] and Zhang et al. [136] reported the effects of different voltage waveforms on the inactivation rate of
bacteria and yeasts. Authors reported that the square wave pulses are more energy and lethally efficient in the microbial
inactivation that exponentially decay pulses. However, exponentially decaying pulses are easier to generate and change
in direction than square pulses [137]. In terms of pulse polarization, the bipolar pulses (square or exponential decay) are
more lethal than monopolar pulses [35, 138] science the alternating stress produced by bipolar pulses causes intense
structural changes of the membrane increasing the risk of the cell membrane to electrical breakdown [31]. The bipolar
pulses produce alternating changes in the movement of charged molecules, which cause a stress in the cells membrane
and enhance cells lysis. Moreover, bipolar pulse reduces deposition of solids on the electrode surface, decreases food
electrolysis, and economical efficiency is increased by reduce of the energy consumed [27, 54].
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5. Final Remarks
The biological risks linked to the consumption of unpasteurized food as vegetal product, fruit juices, milk and dairy
products due to microorganisms are affecting various countries worldwide. Consumers today are demanding high
quality, minimal food processing and food safety. PEF technology opens up new possibilities with respect to utilisation
of raw products and reduction of microbial contaminants from food microbiota. The efficiency of the microorganism
inactivation by PEF treatment depends on a variety of factors which can be divided into some main: biological
properties of treated microorganisms (type, size and shape; cells density; association and cells position; dielectric
breakdown) and physical and chemical properties of foods (medium conductivity, ionic strength, ions, pH and electric
treatment, shape of the pulses). These factors affect the different stages of microorganism’s inactivation. Consequently,
it is clear that in order to produce safe and stable food products more researches are required to establish the correct
minimal processing techniques parameters. The processing parameters used for the inactivation of microorganisms
should be clearly highlighted to allow comparisons and scale up the technology at the industrial level. Thus, the
intrinsic factors referring to microorganisms, fluid characteristic of processed food and processing conditions (pulse
waveforms) should be reported to facilitate the optimization and standardization of the PEF treatment. The authors hope
that the data presented in this chapter can be helpful for a better understanding of the technology to applied PEF
treatment as food preservation technique.
Acknowledgements. This work has benefited from financial support through the 2010 POSDRU/89/1.5/S/52432 project,
ORGANIZING THE NATIONAL INTEREST POSTDOCTORAL SCHOOL OF APPLIED BIOTECHNOLOGIES WITH
IMPACT ON ROMANIAN BIOECONOMY, project co-financed by the European Social Fund through the Sectorial Operational
Programme Human Resources Development 2007-2013.
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