**4. Influence of PEF on mass transfer**

The application of pulsed electric fields allows a disintegration of cellular material. The result of disintegrated cells is an improved mass transfer, which affects the following processes:


The drying process can be described as a preservation method removing water from food. Water removal, lowers the water activity and increases the shelf life of the products. It can be used as a pre-treatment to improve nutritional, organoleptic and functional properties of the food products (Torreggiani 1993). An important parameter for the drying process is the mass transfer of water. A high mass transfer rate results in a better quality of dried product due to the shortening of the drying time and a reduction in the drying temperature. The drying process can be regarded as a complex process step including momentum, heat and mass transfer. The moisture in food can be classified in two main groups. First the "immobilized" water, which is retained in fine capillaries and is adsorbed at the surface, and second the "free" water, which retains in voids in foods. The drying process is based on the equilibrium of moisture content. The food looses or gains moisture over a period of time to attain a new equilibrium status (Sharma et al., 2000). For drying of heat sensitive products the osmotic dehydration (OD) can be used as a pre-treatment in order to reduce the drying time. Other pre-treatments, like microwave or conventional thermal heating, lead to a thermal destruction of the nutrients and organoleptic quality of the food (Ade-Omowaye et al., 2001).

For this process, food is placed in an osmotic solution resulting in the formation of a water and a solubility gradient across the cell membrane. The cell membrane separates cell content, mostly water, and the osmotic solution. Consequently two fluxes are formed; the water out of the cell and the osmotic solution into the cell. This flux is a mass transfer process and is a function of the difference in chemical potential. Because of the composition of the cell membrane and the better permeability to water, more water is removed from the cell than less solute goes into the cell. The following Figure (Fig. 4.) represents the flux of water and solute in and out the cell (Torreggiani 1993; Sharma et al., 2000).

The OD process depends on different parameters, like concentration of osmotic solution, contact time and process temperature as well as the exposed surface area. The most widespread problem is the simultaneous solute transfer in the food countercurrent to the water flow. This influences the product quality in a negative way resulting in a candy or salty taste of the product, as well as an altered sugar-to-acid ratio. Different kinds of osmotic solutions can be used. The choice depends on parameters, like organoleptic quality, preservative effect and the product taste. Mostly a sucrose solution is used, because it is very effective and convenient. But because of the sweetness the application is limited to vegetables and fruits. The second most used substance for osmotic solutions is sodium chloride.

In comparison to convection drying the OD process uses less energy, because a lower temperature can be used. Due to the reduced temperature, less heat damage is found on the

The application of pulsed electric fields allows a disintegration of cellular material. The result of disintegrated cells is an improved mass transfer, which affects the following

The drying process can be described as a preservation method removing water from food. Water removal, lowers the water activity and increases the shelf life of the products. It can be used as a pre-treatment to improve nutritional, organoleptic and functional properties of the food products (Torreggiani 1993). An important parameter for the drying process is the mass transfer of water. A high mass transfer rate results in a better quality of dried product due to the shortening of the drying time and a reduction in the drying temperature. The drying process can be regarded as a complex process step including momentum, heat and mass transfer. The moisture in food can be classified in two main groups. First the "immobilized" water, which is retained in fine capillaries and is adsorbed at the surface, and second the "free" water, which retains in voids in foods. The drying process is based on the equilibrium of moisture content. The food looses or gains moisture over a period of time to attain a new equilibrium status (Sharma et al., 2000). For drying of heat sensitive products the osmotic dehydration (OD) can be used as a pre-treatment in order to reduce the drying time. Other pre-treatments, like microwave or conventional thermal heating, lead to a thermal destruction of the nutrients and organoleptic quality of the food (Ade-Omowaye et

For this process, food is placed in an osmotic solution resulting in the formation of a water and a solubility gradient across the cell membrane. The cell membrane separates cell content, mostly water, and the osmotic solution. Consequently two fluxes are formed; the water out of the cell and the osmotic solution into the cell. This flux is a mass transfer process and is a function of the difference in chemical potential. Because of the composition of the cell membrane and the better permeability to water, more water is removed from the cell than less solute goes into the cell. The following Figure (Fig. 4.) represents the flux of

The OD process depends on different parameters, like concentration of osmotic solution, contact time and process temperature as well as the exposed surface area. The most widespread problem is the simultaneous solute transfer in the food countercurrent to the water flow. This influences the product quality in a negative way resulting in a candy or salty taste of the product, as well as an altered sugar-to-acid ratio. Different kinds of osmotic solutions can be used. The choice depends on parameters, like organoleptic quality, preservative effect and the product taste. Mostly a sucrose solution is used, because it is very effective and convenient. But because of the sweetness the application is limited to vegetables and fruits. The second most used substance for osmotic solutions is sodium

In comparison to convection drying the OD process uses less energy, because a lower temperature can be used. Due to the reduced temperature, less heat damage is found on the

water and solute in and out the cell (Torreggiani 1993; Sharma et al., 2000).

**4. Influence of PEF on mass transfer** 

processes:

al., 2001).

chloride.



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

product and the high concentration of the osmotic solution prevents discoloration (Torreggiani 1993).

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 structural food quality can be obtained.

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

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

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

Mass Transport Improvement by PEF –

volume of solvent (Vauck&Mueller 2000).

easily extracted.

2008).

Applications in the Area of Extraction and Distillation 219

the extract diffuses from the solid in the solvent. This mass transport from the solid in the solvent follows the diffusion law. After this process a mechanical separation of the solid and liquid is required as well as a separation of the extract and the extraction agent. A facilitated extraction can be reached by chopping or mashing the solid with the result of an increased surface area and shorter capillary ways. Another possibility to increase the extraction velocity is an increase of the concentration gradient. This can be achieved by using a high

As described before the principle of the extraction process is based on the mass transfer of the extract from the solid in solvent. The extract is mostly located in cells, which are surrounded by membranes. Consequently, the amount of compounds released to the solvent depends on the degree of the damaged cellular material. The effect of PEF can be defined as electroporation resulting in a disintegration of cellular material and an improved mass transfer. Regarding the extraction process a facilitated extraction is possible using PEF as a pre-treatment, because the extract can easily diffuse into the solvent. There is no force required to open the cellular material. Using PEF as a pre-treatment, the extraction time can be reduced. The extraction yield can be increased and important quality ingredients can be

PEF can be used for an improved extraction process in different application fields. One example is the extraction of calcium from bones (Yin&He 2008). There are different methods used for the extraction of calcium from bones, for example boiling or microwave treatment, often leads to negative effects on the product and the extracted concentration is low. Using PEF, the temperature can be reduced and the calcium concentration increased (Yin&He

Another example for a solid-liquid expression assisted by PEF treatment is sugar beet. Using PEF as a pre-treatment the extraction time and temperature can be lowered (López et al., 2009) as well as a higher sucrose yield can be obtained (Eshtiaghi&Knorr 2002; El-Belghiti&Vorobiev 2004) resulting in more efficient sugar production with lower energy requirements (Lebovka et al., 2007; Loginova et al., 2011). Due to the lower extraction temperature the PEF process offers the possibility to improve the extraction of colorants from red beet root in order to increase the extraction yield and a better stability of the colorants (Fincan et al., 2004; Loginova et al., 2011). An increased juice yield plays an important role in juice industry. In general mechanical, enzymatic or temperature based methods were used to rupture the apple cells for a facilitated extracting, but mostly they lead to a degradation of important juice components as well as a high energy consumption (Toepfl 2006). A PEF treatment of the apples increases the yield from 1,7 to 7,7 % in comparison to an enzymatic treatment (4,2 %) (Schilling et al., 2007) and reduces the energy consumption. Additional the pomace can be used after the treatment for pectin extraction. Another process limited by mass transfer is the distillation process. In this process a high temperature is applied to the product for a defined time in order to separate two components with different boiling points. The high temperature denatures the cellular material and facilitates the extraction of the product. The high heat load degrades the heat sensitive ingredients minimizing the quality of the product. The membrane of the cells represents a semipermeable barrier through which the desired extract cannot diffuse because of the size. After the cells are ruptured the extract can easily diffuse along the

point. During the second phase (phase change period) most of the water in the product crystalizes. The final end temperature is reached during the last phase termed as the tempering phase (Delgado&Sun 2001).

The time, which is required to lower the temperature, is called the freezing time. This time can be predicted with mathematic modeling. Therefore the heat and mass transfer phenomena has to be considered. Two possible models to describe the freezing process are the heat transfer model and coupled heat and mass transfer model. The application of PEF improves the mass transfer. The pre-treatment of potato (Jalté et al., 2009) for example induces a higher freezing rate with a shorter freezing time. Due to PEF treatment the cell membrane gets porous with the result of an enhanced diffusion mass exchange of extracellular and intracellular water as well as a reduced freezing time. Using PEF as a pretreatment can achieve a better quality of the frozen food as smaller ice molecules are also formed. The improved quality can be seen in Fig. 5. The structure and form of the PEF pretreated potato was much better in comparison to the untreated ones.

Fig. 5. Pre-treatment of potato discs in comparison to untreated discs after freezing (Jalté et al., 2009)

Besides drying and freezing, PEF treatment influences the extraction process. In general, extraction is a separation process separating a substance from a matrix. Two main extraction processes are liquid-liquid extraction and solid-liquid extraction. In the following the soliliquid extraction is described in detail as well as the impact of PEF on the solid-liquid expression.

The aim of a solid-liquid extraction is the separation of the desired extract located in the solid by using a solvent in which the extract is soluble. Mostly, the extract is located in the pore or cell structure of the solid. This technique has been used a long time for the extraction of plant oils, for example sucrose from sugar beet or tanning- or color extracts. Today the solid-liquid extraction is mostly used for (Bouzrara&Vorobiev 2003)


The principle of an extraction process is based on the diffusion characteristics of the solid and the solubility of the extract. The solvent is added to the solid and during the extraction

point. During the second phase (phase change period) most of the water in the product crystalizes. The final end temperature is reached during the last phase termed as the

The time, which is required to lower the temperature, is called the freezing time. This time can be predicted with mathematic modeling. Therefore the heat and mass transfer phenomena has to be considered. Two possible models to describe the freezing process are the heat transfer model and coupled heat and mass transfer model. The application of PEF improves the mass transfer. The pre-treatment of potato (Jalté et al., 2009) for example induces a higher freezing rate with a shorter freezing time. Due to PEF treatment the cell membrane gets porous with the result of an enhanced diffusion mass exchange of extracellular and intracellular water as well as a reduced freezing time. Using PEF as a pretreatment can achieve a better quality of the frozen food as smaller ice molecules are also formed. The improved quality can be seen in Fig. 5. The structure and form of the PEF pre-

Fig. 5. Pre-treatment of potato discs in comparison to untreated discs after freezing (Jalté et

Besides drying and freezing, PEF treatment influences the extraction process. In general, extraction is a separation process separating a substance from a matrix. Two main extraction processes are liquid-liquid extraction and solid-liquid extraction. In the following the soliliquid extraction is described in detail as well as the impact of PEF on the solid-liquid

The aim of a solid-liquid extraction is the separation of the desired extract located in the solid by using a solvent in which the extract is soluble. Mostly, the extract is located in the pore or cell structure of the solid. This technique has been used a long time for the extraction of plant oils, for example sucrose from sugar beet or tanning- or color extracts. Today the

The principle of an extraction process is based on the diffusion characteristics of the solid and the solubility of the extract. The solvent is added to the solid and during the extraction

treated potato was much better in comparison to the untreated ones.

solid-liquid extraction is mostly used for (Bouzrara&Vorobiev 2003)



tempering phase (Delgado&Sun 2001).

al., 2009)

expression.



the extract diffuses from the solid in the solvent. This mass transport from the solid in the solvent follows the diffusion law. After this process a mechanical separation of the solid and liquid is required as well as a separation of the extract and the extraction agent. A facilitated extraction can be reached by chopping or mashing the solid with the result of an increased surface area and shorter capillary ways. Another possibility to increase the extraction velocity is an increase of the concentration gradient. This can be achieved by using a high volume of solvent (Vauck&Mueller 2000).

As described before the principle of the extraction process is based on the mass transfer of the extract from the solid in solvent. The extract is mostly located in cells, which are surrounded by membranes. Consequently, the amount of compounds released to the solvent depends on the degree of the damaged cellular material. The effect of PEF can be defined as electroporation resulting in a disintegration of cellular material and an improved mass transfer. Regarding the extraction process a facilitated extraction is possible using PEF as a pre-treatment, because the extract can easily diffuse into the solvent. There is no force required to open the cellular material. Using PEF as a pre-treatment, the extraction time can be reduced. The extraction yield can be increased and important quality ingredients can be easily extracted.

PEF can be used for an improved extraction process in different application fields. One example is the extraction of calcium from bones (Yin&He 2008). There are different methods used for the extraction of calcium from bones, for example boiling or microwave treatment, often leads to negative effects on the product and the extracted concentration is low. Using PEF, the temperature can be reduced and the calcium concentration increased (Yin&He 2008).

Another example for a solid-liquid expression assisted by PEF treatment is sugar beet. Using PEF as a pre-treatment the extraction time and temperature can be lowered (López et al., 2009) as well as a higher sucrose yield can be obtained (Eshtiaghi&Knorr 2002; El-Belghiti&Vorobiev 2004) resulting in more efficient sugar production with lower energy requirements (Lebovka et al., 2007; Loginova et al., 2011). Due to the lower extraction temperature the PEF process offers the possibility to improve the extraction of colorants from red beet root in order to increase the extraction yield and a better stability of the colorants (Fincan et al., 2004; Loginova et al., 2011). An increased juice yield plays an important role in juice industry. In general mechanical, enzymatic or temperature based methods were used to rupture the apple cells for a facilitated extracting, but mostly they lead to a degradation of important juice components as well as a high energy consumption (Toepfl 2006). A PEF treatment of the apples increases the yield from 1,7 to 7,7 % in comparison to an enzymatic treatment (4,2 %) (Schilling et al., 2007) and reduces the energy consumption. Additional the pomace can be used after the treatment for pectin extraction.

Another process limited by mass transfer is the distillation process. In this process a high temperature is applied to the product for a defined time in order to separate two components with different boiling points. The high temperature denatures the cellular material and facilitates the extraction of the product. The high heat load degrades the heat sensitive ingredients minimizing the quality of the product. The membrane of the cells represents a semipermeable barrier through which the desired extract cannot diffuse because of the size. After the cells are ruptured the extract can easily diffuse along the

Mass Transport Improvement by PEF –

sucrose solution (Loginova et al., 2011)

with less color degradation (Loginova et al., 2011).

Fig. 7. below shows a current process for apple juice extraction.

70 °C.

Applications in the Area of Extraction and Distillation 221

Fig. 6. Influence of temperature and PEF on the sucrose concentration and the purity of the

improved filter capability of the product in comparison to the thermally treated product at

The thermal treatment is dependent on the holding time (equal with treatment time for the extraction process) and aiming on an almost complete thermo-break. The application of PEF reduces the treatment time resulting in a high cell disintegration index of approximately 0,95 in several milliseconds in contrast to a cell disintegration index of 0,85 in more than 100 s for a thermal treatment. A higher cell disintegration index can be reached by using a higher electric field strength (Lebovka et al., 2007). The researchers Eshtiaghi et al. (Eshtiaghi&Knorr 2002) indicated a cell disintegration index of approximately 0,7 by treating the slices with an electric field strength of 2,4 kV/cm and 20 pulses at 20 °C. The same cell disintegration index can be achieved by a heat treatment at 72 °C for 15 min.

In addition to the extraction of sucrose from sugar beet the extraction of industry relevant colorants from red beet roots can be improved. Red beet concentrate can be used as a food additive to color food. Betalains are the major colorants, and are composed of the red –violet substance betacyanin and the yellow-orange betaxanthin. The main function of betalains beside the color, is antiviral and antimicrobial activity (Azeredo 2009). Most extraction processes contains a thermal treatment leading to a lower extraction yield and high color degradation, because of the sensitivity of the betalains towards high temperature. For example a thermal treatment at 30 °C for 5 h leads to a degradation of 20 % of the betalains. In comparison an extraction at 80 °C for 1 h results in a complete degradation of the betalains (Loginova et al., 2011). The extraction assisted by PEF lowers the temperature without decreasing the extraction yield and destruction of the color (Fincan et al., 2004; Loginova et al., 2011). The same extraction rate can be achieved using a pre-treatment with PEF at 30 °C of the red beet using a field strength of 1,5 kV/cm and 20 pulses with a duration of 100 µs in comparison to a thermal treatment at 80 °C for 40 min and in addition

Another main product function is the solid-liquid juice extraction process assisted by PEF. The effects of PEF on the extraction of apple juice will be described. The left side of the

concentration gradient. To rupture the cells, PEF can be used. Using PEF a disintegration of the cells could be observed at moderate temperatures. Consequently, the heat load could be minimized and no change of quality of the extract could be detected. In addition to that, the extraction yield could also be increased. In summary, the result of using pulsed electric fields instead of high temperature treatment is a reduced temperature and distillation time. A possible application being the treatment of roses to gain a higher yield of rose oil while reducing the distillation time (Dobreva et al., 2010).
