Applications of Pulsed Electric Field Treatments
for the Enhancement of Mass Transfer from Vegetable Tissue
Francesco Donsı`•Giovanna Ferrari• Gianpiero Pataro
Received: 29 January 2010 / Accepted: 2 March 2010 / Published online: 18 March 2010
Springer Science+Business Media, LLC 2010
Abstract In the last decades, several non-thermal tech- nologies have been proposed as alternative to the tradi- tional ones to improve the competitiveness of the food industry. The key to success was identified in offering to food industries the opportunity to improve food quality, to introduce new foods in the market, and to optimize the processing procedures while reducing energy costs. Pulsed electric fields (PEF) showed the potential to be one of the most promising novel technologies to reach these objec- tives. The application of PEF as a pretreatment of perme- abilization of vegetable and animal tissue to enhance the efficiency of mass transfer of water or of valuable com- pounds from biological matrices demonstrated its effi- ciency in drying, extraction, and diffusion processes. This article reviews the basic mechanisms of electroporation of plant tissues, discusses the methods of detection of elec- trically induced cell damage, and analyses the influence of process parameters on the efficiency of the treatment.
Furthermore, this article focuses on the applications of PEF, its advantages, and energy costs in different fields of food processing, such as juice expression, drying, and extraction, with special emphasis on the relevance of PEF to the winemaking industry.
Keywords Pulsed electric field (PEF) Cell permeabilization Biological tissue Complex impedance Mass transfer
Introduction
Over the last 50 years, Pulsed electric field (PEF) tech- nology has stimulated intensive research as a non-thermal treatment, in particular with the primary aim of attaining microbial inactivation [19,50,58,105,128,129].
The focus of PEF applications and related studies has recently concentrated on the permeabilization of cell membranes, with the aim of enhancing mass transfer from the inner part of the cells. In fact, in animal and plant cells, which are larger in size than bacterial cells, permeabiliza- tion of the membrane is easier to attain, normally requiring lower electric field intensities, which is reflected in lower energy consumption [68].
Thanks to the reduction of the resistances to mass transfer due to the induced permeabilization of plant cells, PEF technology can be used as a pretreatment to increase the yield in the production of fruit juices, to accelerate the transfer of water during drying operations as well as enhance the extraction of valuable compounds (such as antioxidants, colorants or flavors) from the inner parts of the cells [29,30,69,122].
This paper intends to summarize the applications of PEF as a pretreatment to mass transfer enhancement, high- lighting the potentially achievable results as a function of the intensity of the treatment (energy required and electric field applied) in order to give the reader a perspective of the impact of the technology in product development and process intensification.
Basic Consideration and Mechanism
Electroporation is a physical method of cell (animal or plant) membrane permeabilization caused by externally F. Donsı` G. Ferrari (&) G. Pataro
Department of Chemical and Food Engineering, University of Salerno, Salerno, Italy
e-mail: [email protected] G. Ferrari
ProdAl scarl, Centre of Competence on Agro-Food Productions, University of Salerno, Salerno, Italy
DOI 10.1007/s12393-010-9015-3
applied short and intense electric pulses, widely used in several fields such as biotechnology and medicine for the introduction of different molecules into the cell, electro- fusion, water treatment, and food processing for steriliza- tion, as well as enhancing the efficiency of the pressing, extraction, drying, and diffusion processes.
The exact mechanisms of electroporation are not yet fully understood. Several theories [32, 65, 87, 119, 132, 134] based on the experiments carried out on model sys- tems such as liposomes, planar bilayers, and phospholipid vesicles have been proposed in order to explain the mechanism of the reversible electroporation and/or the electrical membrane breakdown. All of these theories have their relative advantages and disadvantages, but one com- mon feature of great importance is the fact that the mem- brane plays a role in amplifying the applied electric field, as the conductivity of intact membrane is several orders of magnitude lower than the conductivities of extra cellular medium and cell cytoplasm [124].
Hence, when the biological cell is exposed to an external electric field E, the transmembrane potential increases as a result of the charging process at the membrane interfaces.
In Fig.1, the simple case of a sphere shaped biological cell is considered. The potential differences um can be approximated by Eq. (1) which is derived from solving Maxwell’s equation in spherical coordinates assuming several simplifying restrictions [86]:
um¼ 1:5rcellE cosðhÞ ð1Þ
where, rcellis the radius, and h is the angle between the site on the cell membrane where um is measured and the direction of the vector E.
The highest drop of potential occurs at the cell poles (h = 0, p) and decreases to 0 at h = ±p/2. Due to the membrane thickness h (&5 nm) being very thin when compared to a plant cell radius (&100 lm), a selective concentration of the electric field on the membrane occurs, creating a transmembrane electric field, Em= um/h, which is about 105 times higher than the applied field strength [123,124].
If a critical value of the field strength Ecis exceeded, a critical transmembrane potential can be induced (typically 0.2–1.0 V for most cell membranes) that leads to the forma- tion of reversible or irreversible pores in the membrane [133].
The occurrence of reversible or irreversible permeabi- lization of the cell membrane depends on the intensity of the external electric fields as well as the number of pulses applied [68].
When the electric field applied reaches values close to the critical value Ec or when few pulses are applied, reversible permeabilization occurs allowing the cell membrane to recover its structure and functionality. On the contrary, when more intense PEF treatment is applied, irreversible electroporation takes place, resulting in cell
CYTOPLASM CELL
MEMBRANE ELECTRODE
MEDIUM
rθ
+ -
+ -
+ -
E<Ec
E>Ec
E>>Ec REVERSIBLE
IRREVERSIBLE PORES
CYTOPLASM CELL
MEMBRANE ELECTRODE
MEDIUM CYTOPLASM
CELL
MEMBRANE MEDIUM
rθ
+ -
+
+ -
E<Ec
rθ
+
rθ-
+ -
REVERSIBLE
IRREVERSIBLE Fig. 1 Biological cell in an
electric field E. Electroporated area is represented with a dashed line. Ec: critical electric field strength
membrane disintegration as well as loss of cell viability [132].
According to Eq. (1), the critical transmembrane potential is attained with the external electric field decreasing with the cell radius. Due to cell being rather large in plant tissue (&100 lm) when compared to microbial cells (&1–10 lm), the electric field strength required for elecroplasmolysis in plant cells (0.5–5 kV/cm) [66] is lower than that required for inactivation microor- ganisms (10–50 kV/cm) [19].
Overall, the electroporation process consists of different phases [64,123] including:
– charging and polarization of the membranes (charging time of &1 ls);
– temporal destabilization and creation of pores (reported as occurring on time scales of 10 ns);
– expansion of the pores radii and aggregation of different pores (in a time range of hundred of micro- seconds to milliseconds, lasting throughout the duration of pulses);
– resealing of the pores which takes place after electric pulse application (lasting seconds to hours).
The first phase of electroporation (pore formation), which is the cell membrane response to the induced threshold membrane potential, is related to short-lived transient pore formation, which does not contribute to molecular transport. Molecular transport across the per- meabilized cell membrane associated with electroporation is observed from the pore formation phase until membrane resealing is completed [64].
Therefore, in the electropermeabilization effect on bio- logical membranes, the induction and development of the pores after treatment is a dynamic and not an instantaneous process [14].
Detection and Characterization of Cell Disintegration in Vegetable Tissue
The first studies on the degree of cell permeabilization were based on quantifying the release of intracellular metabolites (i.e. pigments) from plant-cultured cells after electroporation induced by the application of PEF [30,36].
The irreversible permeabilization of the cells in vege- table tissue was demonstrated, for the first time on potato tissue (exposed to PEF treatment), determining the release of the intracellular liquid from the treated tissue using a centrifugal method. A liquid leakage from the tissue of PEF-treated samples was detected while no-release occur- red from the control samples. This leakage was therefore interpreted as a consequence of the cellular damage by the electrical pulses inside the cells of the tissue [12].
However, in order to obtain a quantitative measure of the induced cell damage degree P, defined as the ratio of the damaged cells and the total number of cells, several methods have been defined. The direct estimation of the damage degree can be carried out through the microscopic observation of the PEF-treated tissue [44]. However, the procedure is not simple and is ambiguous [123].
Therefore, experimental techniques based on the eval- uation of the indicators that macroscopically registers the complex changes at the membrane level in real biological systems have been introduced.
For example, the value of P could be estimated from diffusion coefficient measurements in the PEF-treated biological materials [62,75]:
P D Dð iÞ= Dð d DiÞ ð2Þ
where D is the measured apparent diffusion coefficient with the subscript i and d referring to the values for intact and totally destroyed material, respectively.
Unfortunately, diffusion techniques are not only indirect and invasive for biological objects, but they may also have an impact on the structure of the tissue. Furthermore, also the validity of the Eq. (2) is still controversial [75,121].
Measurements of the changes in the electrophysical properties such as complex impedance of untreated and treated biological systems have been suggested as a simple and more reliable method to obtaining a measurement of the extent of damaged cells [14].
Biological cells have insulated membranes (the plasma membrane and the tonoplast) that are responsible for the characteristic alternating current frequency dependence on the sample’s impedance. According to Angersbach et al.
[13], the electrical behavior of a single intact plant cell is equivalent to an ohmic-capacitive circuit in which insu- lated cell membranes can be assumed to be a capacitor connected in parallel to a resistor, while the conductive liquid on both sides of the membranes can be introduced to this circuit as two additional resistors. Hence, the electro- physical properties of cell systems, as characterized by the Maxwell–Wagner polarization effect at intact membrane interfaces, can be determined on the basis of impedance measurements in a frequency range between 1 and 100 MHz, which is called b-dispersion [14].
The impedance-frequency spectra of intact and treated samples are typically determined with an impedance mea- surement equipment in which a sample, placed between two parallel plate cylindrical electrodes, is exposed to a sinu- soidal or wave voltage signal of alternative polarity with a fixed amplitude (typically between 1 and 5 V peak to peak) and frequency (f) in the range of 3–50 MHz. However, the range of characteristic low and high frequencies used depends on the cell size in relation to the conductivity of cell liquid and neighboring fluids, as shown in Table1[14].
Electrical impedance is determined as the ratio of the voltage drop across the sample and the current crossing it during the test. The complex impedance Z(jx) is expressed according to Eq. (3):
Z jxð Þ ¼ Z jxj ð Þj ej/ ð3Þ
where j is the imaginary unit, x = 2pf is the angular fre- quency, Z jxj ð Þj is the absolute value of the complex impedance, and u the phase angle between voltage across the sample and the current through it.
As the complex impedance Z(jx) depends on the geometry of the electrode system, the specific conductivity r(x) can be used instead [69, 71, 101]. For the plate electrode system, it has been calculated according to equation:
r xð Þ ¼ ls
AsjZ jxð Þj ð4Þ
where ls is the length of the sample, and As is the area perpendicular to the electric field.
The results of numerous experiments indicate that the impedance or conductivity–frequency spectra of intact and processed plant tissue in a range between 1 and 50 MHz can typically be divided into characteristic zones [13].
Figure2(a) shows a typical frequency-impedance spec- tra for artichoke bracts and the transition from an intact to ruptured state in the frequency range of the measured current of 100 Hz to 10 MHz [22].
The results show that the absolute value of the imped- ance of the intact biological tissue is strongly frequency dependent. This is because in the low frequency field the cell membrane acts as a capacitor preventing the flow of the electric current in the intracellular medium (ohmic- capacitive behavior). Upon increasing the frequency, the cell membrane becomes less and less resistant to the cur- rent flow in the intracellular liquid. At very high frequency values, the membrane is totally shorted out, and the abso- lute value of the complex impedance is representative of
the contribution of both extra and intracellular medium (pure ohmic behavior). Thus, the tissue permeabilization induced by an external stress such as PEF treatment, is detectable in the low frequencies range. In the high fre- quency range, because the cell membrane does not show any resistance to the current flow, there is practically no difference between the impedance of intact cells and cells with ruptured membranes [22,90].
However, the typical electrical behavior of intact and processed plant tissue can be also analyzed in terms of frequency-phase angle spectra [22, 90, 101, 102]. Fig- ure2(b) shows a typical frequency-phase angle spectra for artichoke bracts and the transition from intact to ruptured state in the frequency range of the measured current of 100 Hz to 10 MHz.
According to the ohmic-capacitive behavior of intact biological tissue, a negative value of the phase angle is detected. In particular, at characteristic low and high fre- quencies, the imaginary component of the cell impedance is equal to zero [13,14]. Hence, the phase angle between voltage and current approach to 0 that is typical of a pure ohmic behavior.
At medium frequencies, the influence of the capacitive current through the cell membranes on the phase angle is quite high, and a minimum value of the phase angle is Table 1 Characteristic low and high frequency values for different
biological material
Biological material Low
frequency (kHz)a
High frequency (MHz)a Large cells
Animal muscle tissue B3 C15
Fish tissue (mackerel or salmon) B3 C3 Plant cells (apple, potato, or
paprika)
B5 C5
Small cells
Yeast cells (S. cerevisiae) B50 C25
a The indicated frequencies should be used as the preferred frequency bands [14]
Frequency (Hz)
1e+2 1e+3 1e+4 1e+5 1e+6 1e+7
φ
-70 -60 -50 -40 -30 -20 -10
0 intact tissue
1.6 kV/cm - 1kJ/kg 1.6 kV/cm - 5kJ/kg
(b)
Frequency (Hz)
1e+2 1e+3 1e+4 1e+5 1e+6 1e+7
|Z| (Ohm)
10 100 1000 10000
intact tissue 1.6 kV/cm - 1 kJ/kg 1.6 kV/cm - 5kJ/kg
(a)
intact tissue 1.6 kV/cm - 1kJ/kg 1.6 kV/cm - 5kJ/kg intact tissue 1.6 kV/cm - 1kJ/kg 1.6 kV/cm - 5kJ/kg
(b)
intact tissue 1.6 kV/cm - 1 kJ/kg 1.6 kV/cm - 5kJ/kg intact tissue 1.6 kV/cm - 1 kJ/kg 1.6 kV/cm - 5kJ/kg
(a)
Fig. 2 Frequency dependency of the absolute value of complex impedance |Z| (a) and phase angle u (b) before and after PEF treatments of different intensities [22]
detected. As reported in Table2, the minimum phase angle varies with the type of plant material.
While the electroporation progresses, the capacitances of the cell membranes become more and more shortened, and the increase of the phase angle can be taken as a measure for the degree of electroporation. If all cells are opened completely, the phase angle approaches zero in the ideal case [90,102].
In order to quantify the cellular degree of permeabili- zation, a coefficient Zp, the cell disintegration index, has been defined on the basis of the measurement of the elec- trical complex conductivity of intact and permeabilized tissue in the low (&1–5 kHz) and high (3–50 MHz) fre- quency ranges [13]:
Zp¼ rih=rth rtl ril
rih ril ð5Þ
where ril;rtlare the electrical conductivity of untreated and treated material, in a low frequency field, respectively, and rih;rth are the electrical conductivity of untreated and treated samples in a high frequency field, respectively.
The disintegration index characterizes the proportion of damaged (permeabilized) cells within the plant product [69]. It is the average cell disintegration characteristic in the sample and describes the transition of a cell from an intact to ruptured state [3]. For intact cells, Zp= 0; for total cell disintegration, Zp= 1.
Another cell disintegration index Zp was proposed by Lebovka et al. [71] according to the definition of Rogov and Gorbatov [99]:
Zp¼ r ri
rd ri
ð6Þ where r is the measured electrical conductivity value at low frequencies (1–5 kHz), and the subscripts ‘i’ and ‘d’
refer to the conductivities of intact and totally destroyed material, respectively. As in the previous case, Zp= 0 for intact tissue and Zp= 1 for totally disintegrated material.
This method has proved to be a useful tool for the determination of the status of cellular materials as well as the optimization of various processes regarding minimizing cell damage, monitoring the improvement of mass transfer, or for the evaluation of various biochemical synthesis reactions in living systems [13,14].
Unfortunately, there exists no exact relation between the disintegration index Zp and damage degree P, though it may be reasonably approximated by the empirical Archie’s equation [16]:
Zp Pm ð7Þ
where exponent m falls within the range of 1.8–2.5 for biological tissue, such as apple, carrot, and potato [71].
Monitoring of Cell Damage in the Industrial Environment
The methods described above, based on the measurements of impedance at two frequencies (low and high frequency) and on the phase angle criterion, are both reported as reliable indicators for monitoring the extent of cell mem- brane disintegration [13,14,22,71,90,101,102].
However, according to Sack et al. [102], the method based on the impedance measurement at the characteristic low and high frequency works fine for small arrangements, where the inductivities of the leads can be neglected. For a measurement in an industrial flow where large electrode distances have to be used, the influence of the inductance cannot be neglected. It forms a resonant circuit together with stray capacitances. Furthermore, as the measurement device needs a time of 1–2 s to apply the two frequencies one after the other, the sample material changes during the two measurements, resulting in a measurement fault if the material is heterogeneous. A simultaneous application of the two measurement frequencies would be technically possible, but the measurement device would be more complex.
On the other hand, particularly for measurements in an industrial environment, the measurement of the phase angle at only one medium frequency overcomes the mentioned disadvantages of the measurement at two frequencies: at a measurement frequency (Table2), where the phase angle has a minimum, the influence of stray inductance is quite small. Furthermore, as the phase angle between two elec- tric signals can be easily determined by a time measure- ment between the two zero crossings and because the measurement can be carried out at a single frequency, the measurement device can be set up quite simply.
As reported in Table2, the frequency of the minimum phase angle depends on the type of material. Therefore, an adaptation of the frequency to the treated material is of Table 2 Typical frequency value of minimum phase angle for dif-
ferent biological material
Biological material Frequency (kHz)a Reference
Apple 50 [102]
Carrots 100 [102]
Artichoke 110 [22]
Sugar beet 50 [101]
Pinot noir grapes 100 [102]
Alicante grapes 400 [102]
Muskateller mash 300 [102]
Riesling mash 700 [102]
a The indicated frequencies can be considered constant during the permeabilization phase
advantage for the treatment of different type of material with one electroporation device.
The adjustment of the applied energy for an optimum operation can then be based on both the measurement at the output of the reactor to keep the degree of cell disinte- gration constant and additionally on the measurement of the untreated material to determine the initial degree of denaturation [101,102].
Influence of PEF-Process Parameters
According to electroporation theory, the extent of cell membrane damage of biological material is mainly influ- enced by the electric treatment conditions. Typically, as well as electric field strength E, pulse width spand number of pulses np(or treatment time t¼ sp np) are reported as the most important electric pulses parameters affecting the electroporation process.
In general, increasing the intensity of these parameters enhances the degree of membrane permeabilization even if beyond a certain value a saturation level of the disinte- gration index is generally reached [71].
For example, the disintegration index of potato tissue was found markedly increased by increasing either field strength or the number of pulses [15,66,69].
The effect of the applied field strength (between 0.1 and 0.4 kV/cm) and pulse width (between 10 and 1,000 ls) on the efficiency of disintegration of apple tissue by pulsed electric fields (PEF) has been also studied [34]. The char- acteristic damage time s, estimated as a time when the saturation disintegration index Zp attains one-half of a maximal value, i.e. Zp= 0.5 [71], decreased with the increasing of the field strength and pulse width. In partic- ular, longer pulses were more effective, and their effect was particularly pronounced at room temperature and moderate electric fields (E = 0.1 kV/cm).
However, Knorr and Angersbach [69], utilizing the dis- integration index for the quantification of cell permeabiliza- tion of potato tissue, found that, at a fixed number of pulses, the application of variable electric field strength and pulse width, but constant electrical energy per pulse W, resulted in the same degree of cell disintegration. Thus, the authors suggested that the specific energy per pulse can be considered a suitable process parameter for the optimization of mem- brane permeabilization as well as for PEF-process develop- ment.For exponential decay pulses, W can be calculated by:
W¼kE2maxsp
q ½kJ= kgð pulseÞ ð8Þ
where Emax is the peak electric field strength (kV/m), r is the electrical conductivity (S/m), spis the pulse width (s), and q is the density of the product (kg/m3).
The relationship between W and cell permeabilization was evaluated systematically by examining the variation of specific energy input per pulse (from 2.5 to 22,000 J/kg) and the number of pulses (np= 1–200; pulse repetition, 1 Hz). The treatment-induced Zp increased continuously with the pulse energy as well as with the pulse numbers.
Theoretically, the total cell permeabilization of plant tissue was obtained by applying either one very high energy pulse or a large number of pulses of low energy per pulse [69].
Based on these results and due to its integrated char- acter, total specific energy input WT, defined as WT ¼ W np(in kJ/kg), should be used, next to field strength, as a suitable treatment intensity parameter in order to compare the results obtained using different electric pulse protocols and PEF devices. In addition, the use of the energy input required to achieve for a given matrix complete cell dis- integration also provides an indication of the operational costs.
A criterion for energy optimization, based on the rela- tionship between the characteristic damage time s and the electric field intensity E, has been proposed by Lebovka et al. [71].
According to Eq. (8), the energy input during the treat- ment time t = s(E) can be characterized by the product s(E)E2. The s(E) decreases by increasing the electric field intensity E, and the product of s(E)E2goes through a mini- mum. Criteria of energy optimization require a minimum of this product. This minimum corresponds to the minimum power consumption for material treatment during charac- teristic time t = s(E). A further increase of E results in a progressive increase of the optimization product s(E)E2and energy input but gives no additional increase in conductivity disintegration index Zp. An optimal value of the electric field intensity Eopt& 400 V/cm, which results in maximal material disintegration at the minimal energy input, was estimated for apple, carrot, and potato tissue. Based on this value, the characteristic time s was estimated as 2 9 10-3 s for apple, 7 9 10-4s for carrot, and 2 9 10-4 s for potato, and the energy consumption decreased in the same order:
apple ? carrot ? potato [71].
Impact of Pulsed Electric Field on Juice Expression Pretreatments that cause plasmolysis, cellular damage, or permeabilization of plant cells can be used to enhance the efficiency of hydraulic pressing, which is widely used in the food industry [110,111], as well as to increase the yield in the production of fruit juices and vegetable oils [29,30, 42,69].
Traditional methods used to obtain raw material plas- molysis include heating, osmotic dehydration or freezing dehydration, alkaline breakage, and enzymatic treatment
[7,18,97,120]. For many years, PEF treatment was pro- posed for cellular material plasmolysis, known as electro- plasmolysis, and applied in order to obtain the intensification of juice yield production as well as improve product quality [83,89,107]. However, pioneering appli- cations, due to the high intensity of the electric field applied, suffered from an uncontrolled increase in food temperature and product quality deterioration [25].
Recently, effective permeabilization of cellular mem- branes has been demonstrated to be possible at a moderate pulsed electric field intensity [13,20,21,69,70], and since then numerous applications of PEF treatment to juice expression enhancement were proposed, which are sum- marized in Table3. The efficiency of PEF treatment was demonstrated for juice expression from alfalfa [48], from apple mash [25,72,93,106], from carrot gratings [52,70, 93], from peppers [4], from potatoes [31, 73], and from sugar beet [43,61,91,94,108,109].
The PEF-assisted expression of fruit juices has been shown to increase yields to similar values that are reached using commercial enzymes, but in a faster continuous process, where products are more similar to freshly pressed products [43]. Further developments have shown the advantages of applying the PEF simultaneously to pressing, due to the dependency of the PEF treatment on the uniform and tight packing of raw materials between electrodes and removal of excess liquid for the extracellular volume at the initial steps of compression [25].
Sugar Beets
Sugar beet expression by PEF with the application of electric fields of 1.2–2.5 kV/cm upon pressing at 20–50 bar at room temperature led to a twofold increase of the solid concentration in the obtained juice, in comparison with traditional methods [43]. Improved yield constituted a Table 3 Summary of the applications of PEF treatment to vegetable tissue expression
Tissue E (kV/cm) Energy (kJ/kg) Expression method Main effects of PEF treatment Reference Alfalfa 1.25–2.5 4–6 40 bar Increased of extractable protein by 57% and mineral
extraction by 73%
[48]
Apple 0.1–0.5 – 3 bar intermediate PEF Enhancement of juice yield and improvement of quality by simultaneous application of PEF and pressure
[25]
0.5 – 5 bar intermediate PEF Higher juice yield than non-treated and thermally pretreated samples
[72]
1–5 1.1–27 250 bar Increase of juice yield from 2 to 8%, whereas enzymatic mash treatment resulted in a 4% growth
[106]
0.25–0.4 – 5 bar intermediate PEF Permeation of cell membranes and increase of portion of juice from internal pores of particles
[93]
Carrot 2.6 – 100 bar Increase of juice yield from 30 to 50% (depending on
grating size) for untreated samples to 70% after PEF [70]
0.25–1 5–10 5 bar intermediate PEF ? washing
Increase of juice yield from 51 to 67% after PEF, higher content of b-carotene
[52]
0.25–0.4 – 5 bar intermediate PEF Enhancement of juice yield and improvement of qualitative characteristics of the juice
[93]
Pepper 1.7 15 100 bar Juice yield 10% higher than control (comparable to
enzyme treatments); 60% increase of b-carotene in the juice (compared to 44% for enzyme)
[3,4]
Potato 0.3 – 5 bar intermediate PEF Highest juice yield for PEF application to the pre- compressed samples
[73]
0.1–0.7 – 15–30 bar Increase of yield with increased PEF intensity [31]
Sugar beet 1.2–2.5 – 20–50 bar Increase of the solid concentration twofold in the obtained juice
[43]
Up to 300 2–10 32 bar Same juice yield of thermal denaturation at 72C (174 kJ/kg), but with significantly lower energy consumption
[108,109]
0.5 – 5 bar intermediate PEF Highest yield for intermediate PEF application [94]
0.6 5 5 bar intermediate
PEF ? ohmic heating
Synergetic effect of ohmic heating and PEF: promotion of 85% of juice extraction from coarse cuts
[91]
– 2–3.6 5–15 bar intermediate
PEF ? washing
Yield increase from 30 to 80% after PEF and to 97%
after PEF ? washing, higher purity
[61]
sufficient driving force for the development of pilot systems, such as the test device Karlsruher Elektroporations Anlage (KEA), which consisted of a 300 kV Marx generator oper- ating at 10 Hz and delivering its pulses to a cylindrical reaction chamber, large enough to treat entire sugar beets in a continuous stream, with axially and azimuthally distributed electrodes. KEA was used to demonstrate the advantages of PEF treatment for the production of sugar from beets com- pared with conventional techniques. The same juice yield was obtained for both thermal denaturation treatment at 72C and PEF treatment, but with a significantly lower energy consumption (174 kJ/kg for thermal denaturation vs.
2–10 kJ/kg for PEF treatment) [109].
Further developments have led to a semi-industrial device for sugar beet pretreatment, with a maximum electric field strength of 300 kV/cm with microsecond pulse lengths, 1 kJ stored energy, and a repetition rate of 20 Hz [108].
Process development instead has showed the advantages of PEF treatment simultaneously to pressing. The first experiments were conducted in lab-scale, using a laboratory filter-press cell connected to a PEF treatment system. The application of PEF to non-pressurized cake resulted in an increase of energy consumption and a higher applied voltage.
The PEF treatment of excessively pressurized cakes delayed the juice expression, while the best results were obtained when the sugar beet tissue was treated with PEF at 1.5–5 bars [28,94]. In particular, it was observed that an electric field intensity of 0.5 kV/cm and duration of PEF application 0.03–0.05 s were the optimal treatment conditions [94].
On lab-scale, it has also been demonstrated that the combination of PEF with a moderate heat treatment (50C), delivered to the sugar beets by ohmic heating is also possible. The combination of ohmic heating (60 V/cm, 50 Hz) and PEF treatment (0.6 kV/cm, 0.04 s) leads to a synergetic effect, with an 85% enhancement of juice extraction, due to the combination of the electropermea- bilization of cell membranes and the thermal softening of tissues [91].
The results obtained by simultaneous PEF and pressing were validated in a pilot-scale multi-plate and frame pressing equipment (pressure of 5–15 bar and particles filling up to 15 kg) for the PEF-assisted cold pressing of sugar beet cossettes. The juice yield enhancement depen- ded on the energy delivered by PEF, with the efficiency steeply increasing to an energy input of about 2 kJ/kg, and leveled off beyond 3.6 kJ/kg [61].
Apples
Pioneering studies have showed that PEF treatment was able to enhance juice expression from apple mash simi- larly to heating [83] as well as increase yield from 67 to
73% in comparison with cellulases enzyme [20], with the advantages of lower energy and better quality juice. The PEF field applied between 0.05 and 0.4 kV/cm for instance required an energy consumption comprised between 0.06 and 0.7 kJ/kg [89], while the reduction in the required amount of enzyme produced a clearer color juice [20].
A more intense PEF treatment of apple mash, at an electric field from 1 to 5 kV/cm and specific energy ranging from 1 to 27 kJ/kg before pressing, resulted in an increased juice yields from 2 to 8%, whereas enzymatic treatment induced a 4% growth. In addition, sugar, malic acid, and pectin contents as well as the antioxidant capacity of the juices were not affected [106]. On the other hand, milder PEF treatment (0.5 kV/cm for 10-2 s, pulse dura- tion of 10 ls) required the combination with moderate heating of apple mash (50 C for 10 min) in order to cause softening of the tissue. The PEF treatment also increased the juice yield for non-thermally pretreated samples, but the enhancement was significantly higher in combination with thermal pretreatment [72].
In the case of apples, the simultaneous application of PEF (0.1 to 0.5 kV/cm, pulse duration of 100 ls) and pressing (3 bar) showed that the highest juice yield when compared to untreated samples can be achieved at the lowest applied electric field if PEF is applied when the pressure in the system reaches a constant value [25].
Since the simultaneous pressure and PEF treatment application promotes the damage of defective cells, enhances diffusion migration of moisture, and depresses the cell resealing processes [25], the expressed juice can significantly change during pressing. A first portion of the juice can be associated with the juice exhausted from cells during the cutting of the plant tissue. The second portion, that can instead be associated to juice expressed from the internal pores of sliced particles [93,110,111], is significantly enhanced by PEF due to the permeation of the cell membranes, with this portion being significantly clearer and less cloudy than the PEF-untreated samples [93].
Carrots
Juice expression from carrots appears to be highly depen- dent on carrot mash particle size, giving upon pressing at 100 bar, a higher juice yield for 3 mm (30%) size than for 1.5 mm (50%). Nevertheless, PEF-assisted carrot juice expression resulted independent on size, giving a yield of about 70% for the application of 50 pulses at 2.6 kV/cm [70]. In addition, a higher availability of b-carotene was measured in comparison with juice produced by traditional methods [70].
Combined pressing and moderate PEF treatment (0.25–
0.4 kV/cm) was able to enhance the juice yields from carrots as well as regulate the qualitative characteristics of the juice expressed from the soft plant tissue [93]. PEF treatments of a moderate electric field strength (0.25–1 kV/
cm) coupled with the application of constant pressure (5 bar) and intermediate washing steps, showed that PEF increased the juice yield and Brix, but the effects were noticeable mainly for larger slices with a small degree of initial damage. Specific energy was comprised between 5 and 10 kJ/kg [52].
Other Vegetable Tissue
Juice expression from mechanically comminuted Paprika at a pressure of 10 MPa for 4 min was enhanced by either PEF pretreatment application (electric field strength = 1.7 kV/cm and specific energy = 15 kJ/kg) or by using a pectolytic enzyme. PEF and enzyme treatments resulted in an approximate 10% juice yield increase, with comparable quality. In relation to color, and in particular redness, which is one of the main characteristics of Paprika, PEF induced higher a* values than enzymes (?18 vs ?15.5), with the amount of b-carotene extracted being higher for PEF (60 vs. 44%) [3,4].
Juice yield obtained from the compression of potato tissue was increased when increasing PEF intensity (elec- tric field strength from 0.13 to 0.68 kV/cm) and decreased deformation rates. For example, the compressive force was reduced by a factor of 5 when pressing potatoes pretreated with an electric field strength of 0.68 kV/cm and duration of 1 ms, at a relative deformation of 0.5 and a deformation rate of 0.1 mm/min [31]. The efficiency of PEF is even higher when treatment is applied to pre-compressed sam- ples. Since in the uncompressed samples, there is a lot of external air, the efficiency of the PEF application is greatly reduced, and due to the reduction of sample thickness (approximately to half-size), lower voltages are required in the PEF pretreatment case in order to reach the required electric field [73].
Alfalfa juice concentrate is widely used as a supplement in some health food products and can therefore be con- sidered a nutraceutical or functional food [48]. A PEF treatment from 1.25 to 2.5 kV/cm, followed by pressing at 40 bar for 2 min resulted in an increase of the extracted protein (57%) as well as minerals (73%) in comparison with untreated samples [49]. Furthermore, since no sig- nificant variation in the quantities of extracted juice was observed once the maximum damage degree was reached, higher electric fields required lower energy. For instance, the same level of damage degree, and therefore the same juice yield, was achieved at 1.25 kV/cm, using 150 kJ/kg and at 2.5 kV/cm, using 110 kJ/kg [48,49].
Impact of Pulsed Electric Fields on Extraction and Recovery of Valuable Compounds
The electroporation of plant cells can be used to enhance the extraction of intracellular metabolites of commercial interest, such as pigments, antioxidants, or flavors, due to the permeabilization not only of the cell membrane [29,30, 42,69], but also of vacuoles [44], where some metabolites are contained.
Different applications have been proposed for PEF- assisted extraction and recovery of valuable compounds, which are summarized in Table4. In addition to the extraction of sugar from sugar beet [38, 39, 60, 74, 79, 109], PEF has been used to enhance the recovery of soluble substances from apple tissue [59], from carrots [40], and chicory [76], polyphenols and antioxidants from artichoke bracts [22] and from fennel [37], betulin from mushrooms [126], betanine from red beetroot [46, 78], and oil from rapeseed [53] and seed maize [54]. Some applications of PEF were also proposed for the recovery of polysaccha- rides from frog tissue [125] as well as of dissoluble calcium from bones [127].
Betanine
Red beetroot is one of the main sources of betanines, natural colorants used to enhance the redness of different food products [114]. Betanines are usually recovered through the aqueous extraction of shredded beetroots, with a yield of 45–70% [78]. Since the amount of compounds released during solid–liquid extraction depends on the degree of cell disintegration, mechanical fragmentation, heating, pressing or a combination of several solvents in the extraction medium are often used. However, these processes may affect the color stability of betanines, which is sensitive to pH, temperature, light, and oxygen [55].
In contrast, the use of PEF allows high degree of cell permeabilization with minimal thermal stresses. Two main approaches have been proposed, both consisting in deliv- ering a moderate amount of energy to the beetroot tissue (between 2.5 and 7 kJ/kg), but using a different electric field intensity. For instance, 270 monopolar rectangular pulses of 10 ls duration at 1 kV/cm field strength (specific energy = 7 kJ/kg) enhanced the release of 90% of total red coloring and ionic content after 1 h aqueous extraction in comparison with less than 5% of the untreated sample [46].
Treatments of higher electric field intensity (7 kV/cm) and shorter pulse duration (2 ls) resulted in release of 90%
of the total betanine in McIlvaine buffer (pH 3.5) after 300 min with a fivefold increase in comparison with the control [78]. In this case, the specific energy delivered to the beetroot was lower (2.5 kJ/kg) due to the fact that betanine extraction was no longer dependent on pulse
Table 4 Summary of the applications of PEF treatment to extraction of valuable compounds from vegetable and non-vegetable cells
Tissue Extracted
compound
E (kV/cm) Energy (kJ/k)
Extraction method Main effects of PEF treatment
Reference
Apple Soluble
substances
\1 10–70 Water
T: 20–75C Large excess
Significant increase of the diffusion coefficient
[59]
Artichoke Polyphenols 0.75–1.5 1–5 McIlvaine buffer (pH 2.8) T: 30C
L:S = 20
Threefold increase of polyphenols extraction over 5 h,
Optimal conditions at 1.5 kV/cm, 1 kJ/kg
[22]
Carrot Soluble
substances
0.67 – Water
T: 18–35C L:S = 3 Centrifugal
Maximum yield obtained upon tissue preheating at 50C and mild PEF treatment (300 V/cm–
30 pulses)
[40]
Chicory Soluble
substances
0.1–0.6 \10 Water
T: 20–60C L:S = 3
At 40C, 5 times higher yield than control. Smaller differences at higher T
[76]
Fennel Antioxidants 0.6 5–40 Water
T: 20, 60–90C.
L:S = 2
Yield increase from 60 to 98% for coarse gratings
[37]
Inonotus obl iquus Betulin 10–70 – Ethanol solution (35–100%) Room T L:S = 5–30
Betulin yield increased by 20%
and the extracting time was much shorter for 2 pulses at 40 kV/cm
[126]
Rapeseed Oil 5–7 42–84 Hexane
room T L:S = 20
Slight increase in oil recovery, but increase of functional food ingredients
[53]
Red Beetroot Betanine 1 7 Water ? 0.25 M mannitol
Room T L:S = 1
Release of about 90% of total red coloring after 60 min compared to \ 5% of the control
[46]
7 2.5 McIlvaine buffer (pH 3.5) T: 30C
L:S = 0.25
Fivefold quicker release of 90% of total betanine than non-PEF- treated samples
[78]
Maize Oil 0.6–7.3 0.6–90 Hexane
room T L:S = 20
Increased yield of phytosterols (32%) and of the isoflavonoids genistein and daidzein (20–21%)
[54]
Sugar Beet Sugar 0.2–0.8 0.5–24 Water circulating at
3.3 g/min on 20 g Room T
Significant enhancement of soluble transfer from 150 V/cm to 500 V/cm before stable value
[60]
0.2–1.2 6–7 Water
Room T L:S = 1
Increase in yield from 5 to 30% [38]
*15 0.75 Hot water Sugar extraction at much lower
temperatures (\70C), with higher purity (no thermal denaturation of cell walls)
[109]
0.7–0.9 5–6 Water
room T L/S = 3
Centrifugal (150–9,6609g)
Under centrifugation, 90% release after 100 min, compared to 300 min of untreated
[39]
0.1 – Water
T = 20–60C L:S = 3
Complete release after 1 h at 40C, in comparison to 50%
of the untreated
[74]
number at higher field strengths ([5 kV/cm), and appar- ently a higher degree of permeabilization can be obtained, even when fewer pulses were applied [78]. Moreover, the combination of PEF at 7 kV/cm and pressing at 14 bar shortened the extraction time by 18-fold [78].
Oil
PEF treatment has also been proposed to enhance the extraction of oil from harder tissues, such as maize, olives, and soybeans [54] as well as rapeseed [53]. In order for PEF treatment to be effective for such vegetable tissues, a long pulse duration (of the order of 100 ls) and high specific energy (up to 90 kJ/kg) are required, and in any case, the enhancement of oil yield is only marginal, while a more substantial variation of the functional components was observed [53,54]. For instance, a mild application of PEF (0.6 kV/cm) on maize germs, increased oil yield by 7%, in comparison with an 8% increase induced by freeze- thawing with a significantly higher energy expenditure.
Additionally, the composition profile of extracted oil resulted in the enrichement of phytosterols (up to 30%) as well as isoflavonoids. Genistein and daidzein in soybeans increased by 20% after PEF treatment in comparison with the untreated samples [54].
Rapeseed is the most important oilseed in Europe, due to it being used for the industrial applications of biodiesel [53]. Nevertheless, there are also numerous food applica- tions, due to its high content of essential linolenic acid (x-3 fatty acid) and of monounsaturated fatty acids, polyphe- nols, phytosterols as well as tocopherols, which gives rapeseed oil an excellent nutritive value. PEF treatment of intensity 5–7 kV/cm and 42–84 kJ/kg induced irreversible permeabilization. Nevertheless, after hexane extraction for 2 h at room temperature, only a slight increase in oil recovery was observed [53]. On the other hand, antioxidant capacity increased by about 10% due to the increased polyphenol contents [53].
Sugar
Aqueous extraction is the standard method to recover sugar from beets. It is usually performed in industry by an immersion of beet cuts (cossettes) in hot water (70–75 C).
This leads to an alteration of the cell tissue and loss of pectin into the juice, which consequently needs compli- cated purification. The application of PEF ensures a non- thermal permeabilization of the cellular membrane that allows extraction at lower temperatures [38].
A patent was filed in 1999 to protect a method for treating sugar beets with an electric field, followed by extraction or pressing [42]. Later, a pilot-scale electropo- rator was developed, using a Marx generator, capable of achieving appreciable energy savings since the treated beets could be extracted at much lower temperatures, for instance below 70C, with the same sugar extraction, but higher purity since thermal denaturation of the cell walls is prevented [109].
The application of moderate PEF (\1 kV/cm) as pre- treatment to the laboratory scale solid–liquid extraction of sugar from sugar beets showed that a minimal value of 0.15 kV/cm is required in order to obtain a measurable enhancement of soluble transfer, with a stable value increase being reached above 0.5 kV/cm [60]. Moreover, increasing the number of pulses at 0.94 kV/cm from 100 to 250 increased the yield of solute from 15 to nearly 30%
after 8 h, while a further increase up to 1,000 did not affect the yield of solute [38]. In all cases, the specific energy requirements were below 10 kJ/kg, and the energy input was found to be almost the same (5–6 kJ/kg) for different shapes and sizes, such as disks (30 mm diameter and 8.5 thickness) as well as 1.5 mm thick gratings [39]. Electric field intensity can be further reduced when PEF treatment is applied in combination with thermal plasmolysis at moderate temperature. PEF treatment accelerated the dif- fusion process, which, to be completed in 1 h, required mild heating at 40C, while for untreated sample heating Table 4continued
Tissue Extracted
compound
E (kV/cm) Energy (kJ/k)
Extraction method Main effects of PEF treatment
Reference
1–7 3.7 Water
T: 20, 40, 70C L:S = 5
Maximum yield compared to untreated increased by 7 at 20C and 1.6 times at 40C
[79]
Rana temporaria Poly-saccharide 20 – KOH solution Room T
55% higher extraction yield, less impurities
[125]
Bone Dissoluble calcium 0–70 – Citric acid solution (0–2%) room T
L:S = 40
Yield 20 times higher than control, 9 than boiling, 3 than microwave
[127]
at 70C was required. In addition, in comparison with untreated samples, no loss of solution purity is induced by PEF at comparable extraction temperatures, but purity is measurably increased at comparable yields, since lower extraction temperatures are required for PEF-treated sam- ples [74].
A kinetics investigation of PEF-assisted sucrose extraction showed that the efficiency of the solid–liquid extraction is independent of frequency, pulse width, and shape at 7 kV/cm, with it being influenced by the electric field strength applied as well as the temperature of the extracting medium. The application of 20 pulses at 7 kV/
cm (3.9 kJ/kg) increased the maximum yield by 7 and 1.6 times, compared to non-PEF-treated samples, at 20 and 40C, respectively, while on the other hand, reduced the extraction temperature for 80% of sucrose in 60 min, from 70 to 40C [79].
Soluble Matter
The extraction of soluble substances of different kinds has been enhanced through PEF, in combination with a low thermal treatment, from apple slices [59], from carrots [40]
as well as from chicory [76]. In the case of carrots, tissue preheating (30–50C) contributed additionally to the effectiveness of PEF treatment, due to tissue softening and changes in the cell membrane fluidity. For instance, pre- heating at 50C allowed cell electroporation at low PEF intensity (0.3 kV/cm) and short treatment time (30 pulses) [40,41]. In the case of chicory, treatments at a low electric field intensity (0.4–0.6 kV/cm) and with moderate electric energy consumption (\10 kJ/kg) induced a high level of tissue disintegration even at room temperature. Complete solute extraction (measured inBrix) was obtained even at room temperatures (20 or 30C) within 3 h, while for untreated samples at least extraction at 60C was required [76].
The effect of grating size (thickness varying from 0.1 to 1.8 mm) was investigated for fennel. The PEF treatment (up to 0.6 kV/cm, pulse duration of 100 ls and up to 50 pulses) was more effective for coarse gratings than for thin gratings, in terms of juice yield and purity. In fact, while the extraction rate was faster for smaller gratings of untreated fennel, extraction from PEF-treated sample occurred with the same rate, independent of grating size.
Furthermore, extracts obtained from coarse gratings treated by PEF contained more vitamins C and E than extracts obtained by thermal extraction. On the other hand, in order to reach maximum yield, energy required depended on grating size and reached values as high as 40 kJ/kg for largest size [37].
Polysaccharides from Rana temporaria chensinensis David, a Chinese frog [125], and dissoluble calcium from
bones [127] were recovered with the aid of PEF in a continuous system. For example, compared to the con- ventional extraction in KOH solution, PEF at 20 kV/cm and an overall pulse duration of 6 ls increased the extraction yield of polysaccharides from Rana temporaria by 55% and improved extract purity [125]. Similarly, dis- soluble calcium was recovered from bones by PEF-assisted extraction in citric acid solution (1.25%), with a maximum yield with an electric field of 70 kV/cm and 12 pulses. The calcium extraction was increased with a greater pulse number and higher electric field intensity. In comparison with traditional extraction methods, such as boiling and microwave, the extraction time by PEF was shortened, and the content of dissoluble calcium by PEF was 20 times faster than untreated sample, 9 times than boiled, and 3 times than microwaved [127].
Antioxidants
Over the last few years, the market-driven development of foods with not only nutritive functions but also health benefits has focused interest on bioactive compounds, such as polyphenols (anthocyanins, tannins, catechins, and flavones), which can modulate biochemical reactions as well as the organism’s defenses, with beneficial effects on human health, due to the antioxidant, anti-inflammatory, anti-tumor properties as well as contributing to the pre- vention of heart diseases [56,57].
Since bioactive compounds are in general intracellular metabolites produced by vegetables or animals, a key issue is represented by their extraction under mild conditions, in order to preserve their activity.
One of the first attempts to recover bioactive compounds consisted of the application of PEF (0 to 1.6 kV/cm, 0 to 30 pulses) to Chenopodium rubrum and Morinda citrifolia cells. Depending on the treatment intensity, up to 85% of the total amaranthin content and 6% of total content of anthraquinones of the cells were released. However, cells were killed at release values higher than 16 and 2%, respectively [36].
More recently, PEF-assisted extraction of betulin from Inonotus Obliquus (white rot fungus) was conducted by applying exponentially decaying bipolar pulses with a pulse duration of 2 ls at 10–70 kV/cm, in a continuous device. In comparison with conventional method, betulin extraction by PEF was effective at 2 pulses at 40 kV/cm, with betulin yield increased by 20% and shorter extraction times [126].
PEF treatment was also applied to induce an irreversible disintegration on the cellular membrane of raspberry cells for the extraction of anthocyanins. The extraction rate was linearly correlated to the number of pulses of PEF [82], but increasing intensity degraded Cyanidin-3-glucoside, the
major anthocyanins in raspberry, into chalcone [130,131].
Nevertheless, in general, only a marginal reduction of bioactives, such as flavonoids, anthocyanins, carotenoids, and vitamin, were reported to be induced by PEF [112].
The involucral bracts of artichokes are a valuable by- products of artichoke processing, rich in polyphenolic compounds. A PEF pretreatment (applied electric field of 1.5 kV/cm for 500 pulses of the duration of 10 ls each), which required an energy consumption of 5 kJ/kg enhanced the extraction yield in water of 150% as well as the rate of release. Milder pre-treatment conditions (0.75 kV/cm for 500 pulses of the duration of 10 ls each) required less energy (1 kJ/kg) but gave a yield increase of only 30% with respect to untreated samples. Typical extraction curves are reported in Fig.3. Extraction enhancement was correlated to tissue permeabilization (reported in Fig.2) by the measurable increase of con- ductivity in the low frequency range of the treated tissue [22].
Another field of intensive research of PEF application to enhance the extraction of antioxidants is represented by polyphenols release from grape skins in winemaking. This topic will be discussed in details in the following paragraph.
Impact of Pulsed Electric Fields on Winemaking Grapes contain large amounts of different phenolic com- pounds, especially located in the skin, that are only par- tially extracted during traditional winemaking processes, due to the resistances to mass transfer of the cell walls and cytoplasmatic membranes. In red wine, the main phenolic compounds are anthocyanins, responsible for the color of red wine, tannins and their polymers that instead give the bitterness and astringency to the wines [84]. In addition, the presence of phenolic compounds is also responsible of
the beneficial health properties of the wine, as undoubtedly highlighted by recent studies in vitro and in vivo on their antioxidant and free radical-scavenging properties [47,88].
The phenolic content and composition of wines depends on the initial content in grapes, which is a function of variety and cultivation factors [63], but also on the wine- making techniques [84]. For instance, increasing fermen- tation temperature, thermovinification, must or grape freezing, saigne´e as well as use of maceration enzymes can enhance the extraction of phenolic compounds through the degradation or permeabilization of the grape skin cells [85, 100]. Nevertheless, permeabilization techniques suffer from some drawbacks, such as higher energy costs and a lower stability of valuable compound at higher temperature (thermovinification), or the introduction of extraneous compounds and general worsening of the wine quality [113].
On the other hand, traditional techniques for increasing phenolic content of wines include the extension of the maceration times beyond the time required for fermenta- tion, up to 3 or 4 weeks [23] but are limited by three- to fourfold longer maceration times.
Therefore, PEF treatment may represent a viable option for enhancing the extraction of phenolic compounds from skin cells during the maceration steps, without altering wine quality and with moderate energy consumption. An initial confirmation of this concept was reported for PEF- assisted juice expression from blue grapes, which resulted in the increase of anthocyanin concentration [67]. Also when applied to white grapes (Muscadelle, Sauvignon and Semillon) prior or simultaneously to pressing (5 bar), PEF resulted beneficial not only in the increase of juice yield from 50 to 80% after 45 min of pressing, but also in the significant improvement of juice quality. Optimum treat- ment conditions required an energy input of 20 kJ/kg at electric field strength 0.75 kV/cm [92]. Similar mild con- ditions (15 kJ/kg at electric field strength 0.4 kV/cm) were used for the permeabilization of another white grape, such as Chardonnay variety. The PEF pre-treatment resulted in the increase of juice yield from 67 to 75%, in the elevation of the content of polyphenols (more than 15%) as well as a reduction of turbidity [51]. When PEF-treated grape skins of Chardonnay grapes are subjected to aqueous extraction at ambient temperature, the quantity of extracted poly- phenols was increased by about 10%, in comparison with untreated grapes [27].
In comparison with other non-thermal technologies, the effect of PEF treatment at 3 kV/cm was comparable with High Hydrostatic Pressure at 600 MPa for 1 h and Ultra- sounds at 35 kHz and 70C also for 1 h, in terms of the extraction of bioactive substances such as anthocyanins from grape by-products. In fact, the measured enhancement of total phenolic content was of 50% with respect to time (h)
0.0 0.5 1.0 12.0 24.0
concentration (mg/g bracts)
0.0 0.5 1.0 1.5
control
PEF 0.75kV/cm 0.5kJ/kg PEF 1.5kV/cm 5kJ/kg
Fig. 3 Effect of PEF treatments of different intensity on total polyphenols release [22]
