**3. Determination of cell disintegration index**

The application of PEF induces a polarization of the cellular membrane resulting in an increased trans membrane potential. The increase leads to a rapid electrical breakdown and to local structural changes in the membrane. The result of the applied external electric field is the formation of pores related to an increase of permeability of the membrane. A permeabilisation influences the diffusion processes, for example increasing the extraction yield and shortening the distillation time. To make the best use of the permeabilisation, a maximum of cell disintegration is required. But it has to be noted that cell permeabilisation by PEF leads to a softer structure. An optimum for the PEF settings (process parameters) has to be defined as for some liquid solid separation techniques a too soft texture may be limiting. Up to now different methods are available to measure the permeability of membranes. Microscopic methods can be

Mass Transport Improvement by PEF –

(Angersbach et al.,2000)

Applications in the Area of Extraction and Distillation 215

Besides the relation between the conductivity and the frequency, the Fig. 2. also displays the impact of pulse number on the effect of the permeabilisation of cellular material. A higher number of applied pulses leads to a higher conductivity, which shows a higher permeabilisation of the membrane (Knorr&Angersbach 1998). Not only does the number of pulses affect the efficiency of the PEF treatment, the applied electric field strength amplitude also plays a part. Higher electric field strength causes a more rapid dynamics of pore formation and a faster initiation of conductive channels (Angersbach et al., 2000). To compare the relation between number of pulses and electric field strength; typically a lower number of pulses is necessary to reach a high disintegration of cellular material when high electric field strength is applied (Knorr&Angersbach 1998; Angersbach et al., 2000). The effect of the electric

field strength in relation to the permeability of the membrane is illustrated in Fig. 3.

Fig. 3. Relation between the electric field strength amplitude and the relative permeability

Using low field strength amplitudes no significant differences between the samples were observed, because the electric field strength did not reach the critical value and no electrical breakdown is induced. This phenomenon occurs if the applied field strength is higher than the critical field strength. With increasing electric field strength the relative permeability of

Fig. 3. also displays the dependence of the applied electric field amplitude to the type of product. Four different tissue types (potato-, apple- and fish tissue and potato cell culture) were tested and different values of electric field strength were necessary for a disintegration of these tissues. The reason for this is based on dependence of the membrane to rupture on

By measuring the conductivity the optimum electric field strength can be determined. The aim is to reach a maximum cell disintegration. Lebovka et al. 2002 (Lebovka et al., 2002) detected an electric field strength of 400 V/cm for maximum of cell disintegration of apple-, carrot and potato tissue. Comparing these tissues treated with 400 V/cm different treatment times are necessary to reach a high cell disintegration. Apple tissue required the highest treatment time and consequently the highest energy consumption in contrast to potato,

In conclusion, by measuring the conductivity changes induced by PEF treatment it is

the membrane increases up to the maximum, where all cells are disintegrated.

the trans membrane voltage and the cell size distribution (Angersbach et al., 2000).

which has the lowest energy consumption (Lebovka et al., 2002).

possible to determine the amount of disintegrated cells.

used when the analysed tissue was stained. It is also possible to measure the content of the extracted material (sugar, ions) (Angersbach&Heinz 1997). Mostly these two methods are not exact enough to determine the disintegration of biological membranes. A better method for determining the cell disintegration is the electrical method based on measuring the frequency dependent conductivity.

An external electric field induces a polarization of the membranes. A result of the polarization is an increase of the membrane current and a simultaneously decrease of the resistance of the membrane, which corresponds to an increase of the conductivity (Angersbach et al., 2000). The increase of the conductivity up to 5 mS in less than 5 µs (Angersbach et al., 2000) can be measured and be set in relation to the conductivity of intact membrane. Consequently analyzing the frequency dependent conductivity offers the possibility to estimate the amount of intact cellular material. The relation of the conductivity of the intact and the disintegrated cells is termed as the cell disintegration index Zp. The cell disintegration index allows an exact definition of the amount of ruptured cellular material. If there is no difference between the conductivity of the untreated and PEF treated cell membrane, the Zp value is 0 and the cell can be defined as an intact cell. In contrast to that a Zp value of 1 indicates the maximum of cell disintegration due to the PEF treatment (Angersbach&Heinz 1997).

The frequency dependent conductivity allows a detection of product properties. Fig. 2. illustrates an example of conductivity of various treated and untreated tissues at different frequencies.

Fig. 2. Measured conductivity at different frequencies (Knorr&Angersbach 1998)

As it can be seen in Fig. 2. the conductivity changes with increasing frequency. At low frequency ranges the conductivity of PEF treated cell material and intact tissue is different, because of the capacitive properties of intact tissue. The highest value for conductivity relates to mechanically ruptured cells. From the graph it can also be seen that as the frequency increases, the conductivity of all cellular material reaches the same conductivity value. The reason being the relationship between conductivity and the frequency. At high frequency ranges (5-10 MHz) the cell membranes, whether damaged or not, are not resistant to the electric current (Angersbach&Heinz 1997; Knorr&Angersbach 1998).

used when the analysed tissue was stained. It is also possible to measure the content of the extracted material (sugar, ions) (Angersbach&Heinz 1997). Mostly these two methods are not exact enough to determine the disintegration of biological membranes. A better method for determining the cell disintegration is the electrical method based on measuring the frequency

An external electric field induces a polarization of the membranes. A result of the polarization is an increase of the membrane current and a simultaneously decrease of the resistance of the membrane, which corresponds to an increase of the conductivity (Angersbach et al., 2000). The increase of the conductivity up to 5 mS in less than 5 µs (Angersbach et al., 2000) can be measured and be set in relation to the conductivity of intact membrane. Consequently analyzing the frequency dependent conductivity offers the possibility to estimate the amount of intact cellular material. The relation of the conductivity of the intact and the disintegrated cells is termed as the cell disintegration index Zp. The cell disintegration index allows an exact definition of the amount of ruptured cellular material. If there is no difference between the conductivity of the untreated and PEF treated cell membrane, the Zp value is 0 and the cell can be defined as an intact cell. In contrast to that a Zp value of 1 indicates the maximum of cell

The frequency dependent conductivity allows a detection of product properties. Fig. 2. illustrates an example of conductivity of various treated and untreated tissues at different

Fig. 2. Measured conductivity at different frequencies (Knorr&Angersbach 1998)

to the electric current (Angersbach&Heinz 1997; Knorr&Angersbach 1998).

As it can be seen in Fig. 2. the conductivity changes with increasing frequency. At low frequency ranges the conductivity of PEF treated cell material and intact tissue is different, because of the capacitive properties of intact tissue. The highest value for conductivity relates to mechanically ruptured cells. From the graph it can also be seen that as the frequency increases, the conductivity of all cellular material reaches the same conductivity value. The reason being the relationship between conductivity and the frequency. At high frequency ranges (5-10 MHz) the cell membranes, whether damaged or not, are not resistant

disintegration due to the PEF treatment (Angersbach&Heinz 1997).

dependent conductivity.

frequencies.

Besides the relation between the conductivity and the frequency, the Fig. 2. also displays the impact of pulse number on the effect of the permeabilisation of cellular material. A higher number of applied pulses leads to a higher conductivity, which shows a higher permeabilisation of the membrane (Knorr&Angersbach 1998). Not only does the number of pulses affect the efficiency of the PEF treatment, the applied electric field strength amplitude also plays a part. Higher electric field strength causes a more rapid dynamics of pore formation and a faster initiation of conductive channels (Angersbach et al., 2000). To compare the relation between number of pulses and electric field strength; typically a lower number of pulses is necessary to reach a high disintegration of cellular material when high electric field strength is applied (Knorr&Angersbach 1998; Angersbach et al., 2000). The effect of the electric field strength in relation to the permeability of the membrane is illustrated in Fig. 3.

Fig. 3. Relation between the electric field strength amplitude and the relative permeability (Angersbach et al.,2000)

Using low field strength amplitudes no significant differences between the samples were observed, because the electric field strength did not reach the critical value and no electrical breakdown is induced. This phenomenon occurs if the applied field strength is higher than the critical field strength. With increasing electric field strength the relative permeability of the membrane increases up to the maximum, where all cells are disintegrated.

Fig. 3. also displays the dependence of the applied electric field amplitude to the type of product. Four different tissue types (potato-, apple- and fish tissue and potato cell culture) were tested and different values of electric field strength were necessary for a disintegration of these tissues. The reason for this is based on dependence of the membrane to rupture on the trans membrane voltage and the cell size distribution (Angersbach et al., 2000).

By measuring the conductivity the optimum electric field strength can be determined. The aim is to reach a maximum cell disintegration. Lebovka et al. 2002 (Lebovka et al., 2002) detected an electric field strength of 400 V/cm for maximum of cell disintegration of apple-, carrot and potato tissue. Comparing these tissues treated with 400 V/cm different treatment times are necessary to reach a high cell disintegration. Apple tissue required the highest treatment time and consequently the highest energy consumption in contrast to potato, which has the lowest energy consumption (Lebovka et al., 2002).

In conclusion, by measuring the conductivity changes induced by PEF treatment it is possible to determine the amount of disintegrated cells.

Mass Transport Improvement by PEF –

(Torreggiani 1993).

structural food quality can be obtained.

for the treatment of apple tissue (Amami et al., 2006).

Fig. 4. Mass transfer during OD process (Torreggiani 1993)

Applications in the Area of Extraction and Distillation 217

product and the high concentration of the osmotic solution prevents discoloration

As the OD process is directly dependent on the mass transfer, a PEF treatment can enhance OD. The first time facilitated mass transfer during OD using PEF was reported by Rastogi et al. (Rastogi et al., 1999). Because of the structural changes of the cell membrane due to PEF application, the mass transfer is facilitated (Ade-Omowaye et al., 2001). Consequently a facilitated, fast exit of the water in the osmotic solution is possible, but no solid uptake, because of the selective permeabilisation of the cell membrane (Taiwo et al., 2003). Using PEF before the OD process the drying time can be significantly reduced and a better

Different product types were treated with PEF before OD. For example, the pre-treatment of red pepper with PEF shows a facilitated moisture removal and an improvement of the quality of the dried products. Using OD in combination with PEF a preserved color quality of the red pepper could be obtained (Ade-Omowaye 2003). Carrots can also be pre treated with PEF resulting in an increased diffusion coefficient and a reduction of the OD time from 4 h to 2 h (Rastogi et al., 1999). The PEF induced enhanced mass transfer depends on the electric field strength and the applied number of pulses (Rastogi et al., 1999). The same reduction in OD was reported (Rastogi et al., 1999; Amami et al., 2007a; Amami et al., 2007b)

In addition to drying process the freezing process can be regarded as a mass transport dependent preservation method. Suitable products for this type of treatment are fruits (strawberries and raspberries), vegetables (peas, green beans) as well as fish and meat products (Sharma et al., 2000). The process is based on a decrease of temperature under the freezing point and combines the effect of low temperature with the conversion of water into ice (Delgado&Sun 2001). The advantage of the freezing process is the reduced chemical

In general the freezing process can be separated into three main phases. The first phase is called the pre-cooling or chilling phase, where the product is cooled down to the freezing

reactions and the delay of cellular metabolic reactions (Delgado&Sun 2001).
