**7. Flux enhancement approaches**

The MD process has some significant advantages over conventional processes, however beside the lack of commercially available MD modules, one of the major technical drawbacks for the use of MD in industrial applications is the low transmembrane flux in comparison with RO. Numerous studies have been performed to reduce polarization effects and to enhance transmembrane flux including cooperation of MD with other membrane processes as well as novel MD module design approaches. Some of mentioned attempts are discussed here.

The combination of MD with other membrane systems such as RO, UF, MF, NF and OD have been well-studied by MD researchers in order to improve transmembrane flux, recovery factors and final product quality (Cabral et al., 2011; Calabro & Drioli, 1997; Cath et al., 2005; Cisse et al., 2011; Conidi et al., 2011; Gomes et al., 2011; Gryta, 2005b; Hogan et al., 1998; Mericq et al., 2009; Wang et al., 2011). Each process is unique and contributes particular advantages to the integrated system design.

UF is a powerful method for removing natural polymers (polysaccharides, proteins) that could increase the viscosity of the feed stream through the MD process. For example, pretreatment of grape juice by UF has been shown to result in an increased flux during subsequent concentration of permeate by OD. The flux increase has been attributed to the reduction in the viscosity of the concentrated juice membrane boundary layer due to protein removal (Bailey et al., 2000). Lukanin et al. (2003) have evaluated the use of an enzymatic pretreatment step before UF of apple juices. The protein level which tend to deposit on the hydrophobic surface during subsequent OD process, have been shown to decrease significantly. Such a deposition improves membrane wetting and can eventually result in a convective flow of liquid through the membrane, which is not allowable in the MD process. Onsekizoglu et al. (2010b) have proposed the use of membrane processes for the production of clarified apple juice concentrate. The efficiency of UF was improved by an additional enzymatic pretreatment and

The increase in permeate temperature results in lower MD flux due to the decrease of the transmembrane vapour pressure difference as soon as the feed temperature kept constant. It is generally agreed upon that the temperature of cold water on the permeate side has smaller effect on the flux than that of the feed solution for the same temperature difference. This is because the vapour pressure increases exponentially with feed temperature (Alklaibi

The increase in permeate flow and/or stirring rate reduces the temperature polarization effect. Consequently, the temperature at the gas/liquid interface approaches to the bulk temperature at the permeate side. This will tend to increase driving force across the membrane; resulting an increase in MD flux (Courel et al., 2000; Hongvaleerat et al., 2008). It is important to note that as the permeate used in the MD is distilled water and in the OD is hypertonic salt solution; the extent of the effect of flow rate is more prominent in the latter configuration. This is because of the contribution of concentration polarization effects on

The MD process has some significant advantages over conventional processes, however beside the lack of commercially available MD modules, one of the major technical drawbacks for the use of MD in industrial applications is the low transmembrane flux in comparison with RO. Numerous studies have been performed to reduce polarization effects and to enhance transmembrane flux including cooperation of MD with other membrane processes as well as novel MD module design approaches. Some of mentioned attempts are

The combination of MD with other membrane systems such as RO, UF, MF, NF and OD have been well-studied by MD researchers in order to improve transmembrane flux, recovery factors and final product quality (Cabral et al., 2011; Calabro & Drioli, 1997; Cath et al., 2005; Cisse et al., 2011; Conidi et al., 2011; Gomes et al., 2011; Gryta, 2005b; Hogan et al., 1998; Mericq et al., 2009; Wang et al., 2011). Each process is unique and contributes

UF is a powerful method for removing natural polymers (polysaccharides, proteins) that could increase the viscosity of the feed stream through the MD process. For example, pretreatment of grape juice by UF has been shown to result in an increased flux during subsequent concentration of permeate by OD. The flux increase has been attributed to the reduction in the viscosity of the concentrated juice membrane boundary layer due to protein removal (Bailey et al., 2000). Lukanin et al. (2003) have evaluated the use of an enzymatic pretreatment step before UF of apple juices. The protein level which tend to deposit on the hydrophobic surface during subsequent OD process, have been shown to decrease significantly. Such a deposition improves membrane wetting and can eventually result in a convective flow of liquid through the membrane, which is not allowable in the MD process. Onsekizoglu et al. (2010b) have proposed the use of membrane processes for the production of clarified apple juice concentrate. The efficiency of UF was improved by an additional enzymatic pretreatment and

*Permeate temperature* 

*Permeate flow rate* 

permeate side in OD.

discussed here.

& Lior, 2005; El-Bourawi et al., 2006).

**7. Flux enhancement approaches** 

particular advantages to the integrated system design.

flocculation step using fining agents such as gelatine and bentonite. Hongvaleerat et al. (2008) obtained flux values of about 7-10 kg/m2h in pineapple juice concentrate production by OD which were higher than those obtained with single-strength juice.

RO or forward osmosis (FO) processes have been proposed as a pre-concentration step before OD or MD promising reduction of processing costs. High quality fruit juice concentrates can be produced economically in this manner. Therefore, an integrated process involving preconcentration of the feed by RO followed by further concentration by OD or MD should yield a high-solids product concentrate of quality comparable to that achieved by OD alone but at significant reduction in processing cost (Martinetti et al., 2009; Nayak & Rastogi, 2010; Wang et al., 2011). The combination of RO and OD processes was evaluated by Cabra et al. (2011) for concentration of Acerola juice, by Kozak et al. (2009) for concentration of Black currant juice, by Galaverna et al. (2008) for concentration of blood orange juice, by Cassano et al. (2003) for concentration of citrus and carrot juices. It is worth mentioning that in all the previously mentioned studies, a clarification pretreatment step (i.e. ultrafiltration of microfiltration) is involved in order to improve both RO and OD flux.

Criscuoli & Drioli (1999) presented a detailed energetic and exergetic analysis of both RO– MD and NF–RO–MD integrated systems. They observed an improvement in the performance of the integrated system by introducing NF as water pretreatment for the RO– MD system with almost the same energy.

The coupled operation of MD and OD processes is another promising approach to improve transmembrane flux. In this case, osmotic solution is cooled and the feed solution is slightly heated in order to provide additional driving force. Belafi-Bako & Koroknai (2006) compared MD, OD and coupled operation of OD and MD in terms of flux and final soluble solid concentration in sucrose model solutions and apple juice. Higher water flux and SSC values were achieved with coupled operation confirming an increase in driving force. More recently, Onsekizoglu (2011), have proposed the use of a coupled membrane process capable of concentrating pomegranate juice under very mild conditions. The pomegranate juice was clarified by ultrafiltration in a cross-flow membrane filtration unit (MWCO: 100 kDa). The clarified juice then concentrated by coupled operation of OD and MD, in which the feed solution is gently heated (30.0±2.0°C) and the osmotic solution (CaCl2.H2O) is slightly cooled (10.0±1.0°C). The final step yielded a concentration of the clarified juice (with an initial total soluble solid content of (TSS) 17°Brix) up to 60-62°Brix. The experiments have proven that the driving forces were added in coupled operation, which resulted in enhanced water flux during the operation, thus the coupled process was proposed to be more effective.

Several strategies for reducing temperature polarization through membrane arrangement in MD have been proposed. Some authors have considered the use of spacer-filled channels (Chernyshov et al., 2003; Cipollina et al., 2011; Phattaranawik et al., 2001; Teoh et al., 2008; Wang, 2011). The spacers can improve the flow characteristics at the membrane surface and by promoting regions of turbulence due to the formation of eddies and wakes. Therefore, the temperature polarization can be reduced by improved boundary layer heat transfer. Various surface modification techniques including coating, grafting and plasma polymerization to reduce temperature polarization effect though improvement of membrane surface characteristics have been employed. For example, a novel hollow fiber membrane was proposed by Li & Sirkar (2005) which were commercial porous PP hollow fibres coated with a variety of ultrathin microporous silicone-fluoropolymer layer on surface

Membrane Distillation: Principle, Advances,

maximum flux of permeate.

fouling/cleaning trials were determined.

created two phase flow.

detected.

Limitations and Future Prospects in Food Industry 253

permeate solution into the membrane pores. Therefore the hydrophobic surface of membrane can be partially wetted due to very small nature of the flow channels in MD

It should be emphasized here that although the importance of understanding the fouling phenomena in MD has been pointed out, very few studies have paid attention to long term performance. Most of the performed fouling studies so far examined fouling and scaling in

Gryta (2005a) presented the results of the over 3 years' time research on the direct contact membrane distillation applied for production of the demineralised water using commercial capillary PP membranes. It was found that the membrane was thermally stable, maintaining its morphology and its good separation characteristics throughout the 3 years of DCMD operation. When using permeate of the RO system as DCMD feed solution, membrane pore wetting was not observed; and the DCMD flux was found to be similar to the initial permeate flux. However, precipitation of CaCO3 on the membrane surface was observed when tap water was used directly as a feed. A partial wetting of the membrane was found in this case resulting in a decrease of the permeate flux from 700 to 550 L/m2day. However, the formed deposit was removed every 40–80 h by rinsing the module with a 2–5 wt% HCl solution, permitting the recovery of the initial process efficiency. On the other hand, authors reported that a multiple repetition of this operation resulted in a gradual decline of the

Ding et al. (2008) investigated the fouling resistance in concentrating traditional Chinese medicine (TCM) extract by DCMD. The observed permeate flux decline was attributed to membrane fouling introducing additional thermal resistance in the boundary layer. No considerable membrane wetting due to TCM deposition on the membrane surface was

The membranes used in MD require regular periodic membrane cleaning to remove membrane fouling and keep the permeability loss within a given range. Durham & Nguyen (1994) evaluated the effectiveness of several cleaning agents for OD membranes fouled by tomato paste. The microporous PTFE and cross-linked acrylic-fluoroethane copolymer membranes were used in the study. The cleaning regime was determined by the membrane surface tension. The most effective cleaner for membranes with a surface tension greater than 23 mN/m was determined as 1% NaOH; however, hydrophobic integrity of these membranes was destroyed during repeated fouling/cleaning trials. On the other hand, P3 Ultrasil 56 was the most effective one for membranes with a surface tension less than 23 mN/m. Water vapour flux was maintained and no salt leakage during repeated

Bubbling seems to be an obvious strategy to induce flow and improve shear stress at the membrane surface to control polarization and fouling. Ding et al. (2011) successfully employed the intermittent gas bubbling method to reduce fouling layer formed in concentrating TCM extract through DCMD. To limit membrane fouling or flux decline during concentrating process, intermittent gas bubbling was introduced to the feed side of membrane module. It was confirmed by experimental results that membrane fouling was effectively controlled in the way of removing deposited foulants from membrane surface by

modules (especially in hollow fiber membrane modules) (El-Bourawi et al., 2006).

seawater desalination or wastewater treatment applications.

by plasma polymerization. The coated fibres were arranged in a rectangular cross-flow module design, permitting the hot feed solution to flow over the outside surface of the fibres with a higher Reynolds value. Therefore, reduced temperature polarization inducing higher permeate fluxes have been reported. The reason for applying the coating layer was to provide an additional porous layer having higher hydrophobicity than PP, which itself is one of the polymeric materials with very low surface energy (Li et al., 2008). In recent years, a novel multiple-layered composite membrane have been proposed by Qtaishat et al. (2009) comprising a thin hydrophobic microporous layer and a thin hydrophilic layer. The hydrophobic side of the membrane was maintained adjacent to the hot feed, whereas the hydrophilic layer of the membrane was kept adjacent to cold water, which penetrates into the pores of the hydrophilic layer. Such membranes were found to be promising as they combine the low resistance to mass flux, achieved by the diminution of the water vapour transport path length through the hydrophobic thin top layer and a low conductive heat loss through the membrane, obtained by using a thicker hydrophilic sublayer.

#### **8. Long-term performance**

#### *Membrane fouling & Cleaning procedures*

Membrane fouling refers to the loss of membrane performance due to deposition of suspended or dissolved substances on the membrane surface and/or within its pores. There are several types of fouling in the membrane systems including inorganic fouling or scaling, particulate/colloidal fouling, organic fouling and biological fouling (biofouling) (Gryta, 2008). Inorganic fouling or scaling is caused by the accumulation of inorganic precipitates, such as calcium salts (CaCO3, CaSO4), and magnesium carbonates on membrane surface or within pore structure. Precipitates are formed when the concentration of these sparingly soluble salts exceeds their saturation concentrations. Particulate/colloidal fouling is mainly associated with accumulation of biologically inert particles and colloids on the membrane surface. Organic fouling is related with the deposition or adsorption of organic matters on the pores of the membrane surface. Microbial fouling however is formed due to the formation of biofilms on membrane surfaces. Such films (bacterial, algal, or fungal) grow and release biopolymers (polysaccharides, proteins, and amino sugars) as a result of microbial activity (Pabby et al., 2009).

Even though the general agreement is that the fouling phenomena is significantly lower than those encountered in other pressure-driven membrane separation processes, it is one of the major drawbacks in membrane distillation (Gryta, 2005b). The extensive research on membrane fouling has revealed that the efficiency of MD installation can be reduced by more than 50 percent after 50–100 h of process operation due to the presence of fouling effects. In fact, all of the known types of fouling have been determined to occur practically in MD operations (Gryta, 2008).

Kullab & Martin (2011) pointed out that fouling and scaling may result pore clogging in MD membranes, leading to a decrease in effective membrane area, and therefore the permeate flux. Moreover, the flow channel area may be reduced resulting higher temperature polarization due to the pressure drop across the membrane. The increased deposition of the foulant species at the membrane surface would eventually lead to an increase in the pressure drop to levels that the hydrostatic pressure may exceed the LEP of the feed or

by plasma polymerization. The coated fibres were arranged in a rectangular cross-flow module design, permitting the hot feed solution to flow over the outside surface of the fibres with a higher Reynolds value. Therefore, reduced temperature polarization inducing higher permeate fluxes have been reported. The reason for applying the coating layer was to provide an additional porous layer having higher hydrophobicity than PP, which itself is one of the polymeric materials with very low surface energy (Li et al., 2008). In recent years, a novel multiple-layered composite membrane have been proposed by Qtaishat et al. (2009) comprising a thin hydrophobic microporous layer and a thin hydrophilic layer. The hydrophobic side of the membrane was maintained adjacent to the hot feed, whereas the hydrophilic layer of the membrane was kept adjacent to cold water, which penetrates into the pores of the hydrophilic layer. Such membranes were found to be promising as they combine the low resistance to mass flux, achieved by the diminution of the water vapour transport path length through the hydrophobic thin top layer and a low conductive heat loss

Membrane fouling refers to the loss of membrane performance due to deposition of suspended or dissolved substances on the membrane surface and/or within its pores. There are several types of fouling in the membrane systems including inorganic fouling or scaling, particulate/colloidal fouling, organic fouling and biological fouling (biofouling) (Gryta, 2008). Inorganic fouling or scaling is caused by the accumulation of inorganic precipitates, such as calcium salts (CaCO3, CaSO4), and magnesium carbonates on membrane surface or within pore structure. Precipitates are formed when the concentration of these sparingly soluble salts exceeds their saturation concentrations. Particulate/colloidal fouling is mainly associated with accumulation of biologically inert particles and colloids on the membrane surface. Organic fouling is related with the deposition or adsorption of organic matters on the pores of the membrane surface. Microbial fouling however is formed due to the formation of biofilms on membrane surfaces. Such films (bacterial, algal, or fungal) grow and release biopolymers (polysaccharides, proteins, and amino sugars) as a result of

Even though the general agreement is that the fouling phenomena is significantly lower than those encountered in other pressure-driven membrane separation processes, it is one of the major drawbacks in membrane distillation (Gryta, 2005b). The extensive research on membrane fouling has revealed that the efficiency of MD installation can be reduced by more than 50 percent after 50–100 h of process operation due to the presence of fouling effects. In fact, all of the known types of fouling have been determined to occur practically in

Kullab & Martin (2011) pointed out that fouling and scaling may result pore clogging in MD membranes, leading to a decrease in effective membrane area, and therefore the permeate flux. Moreover, the flow channel area may be reduced resulting higher temperature polarization due to the pressure drop across the membrane. The increased deposition of the foulant species at the membrane surface would eventually lead to an increase in the pressure drop to levels that the hydrostatic pressure may exceed the LEP of the feed or

through the membrane, obtained by using a thicker hydrophilic sublayer.

**8. Long-term performance** 

*Membrane fouling & Cleaning procedures* 

microbial activity (Pabby et al., 2009).

MD operations (Gryta, 2008).

permeate solution into the membrane pores. Therefore the hydrophobic surface of membrane can be partially wetted due to very small nature of the flow channels in MD modules (especially in hollow fiber membrane modules) (El-Bourawi et al., 2006).

It should be emphasized here that although the importance of understanding the fouling phenomena in MD has been pointed out, very few studies have paid attention to long term performance. Most of the performed fouling studies so far examined fouling and scaling in seawater desalination or wastewater treatment applications.

Gryta (2005a) presented the results of the over 3 years' time research on the direct contact membrane distillation applied for production of the demineralised water using commercial capillary PP membranes. It was found that the membrane was thermally stable, maintaining its morphology and its good separation characteristics throughout the 3 years of DCMD operation. When using permeate of the RO system as DCMD feed solution, membrane pore wetting was not observed; and the DCMD flux was found to be similar to the initial permeate flux. However, precipitation of CaCO3 on the membrane surface was observed when tap water was used directly as a feed. A partial wetting of the membrane was found in this case resulting in a decrease of the permeate flux from 700 to 550 L/m2day. However, the formed deposit was removed every 40–80 h by rinsing the module with a 2–5 wt% HCl solution, permitting the recovery of the initial process efficiency. On the other hand, authors reported that a multiple repetition of this operation resulted in a gradual decline of the maximum flux of permeate.

Ding et al. (2008) investigated the fouling resistance in concentrating traditional Chinese medicine (TCM) extract by DCMD. The observed permeate flux decline was attributed to membrane fouling introducing additional thermal resistance in the boundary layer. No considerable membrane wetting due to TCM deposition on the membrane surface was detected.

The membranes used in MD require regular periodic membrane cleaning to remove membrane fouling and keep the permeability loss within a given range. Durham & Nguyen (1994) evaluated the effectiveness of several cleaning agents for OD membranes fouled by tomato paste. The microporous PTFE and cross-linked acrylic-fluoroethane copolymer membranes were used in the study. The cleaning regime was determined by the membrane surface tension. The most effective cleaner for membranes with a surface tension greater than 23 mN/m was determined as 1% NaOH; however, hydrophobic integrity of these membranes was destroyed during repeated fouling/cleaning trials. On the other hand, P3 Ultrasil 56 was the most effective one for membranes with a surface tension less than 23 mN/m. Water vapour flux was maintained and no salt leakage during repeated fouling/cleaning trials were determined.

Bubbling seems to be an obvious strategy to induce flow and improve shear stress at the membrane surface to control polarization and fouling. Ding et al. (2011) successfully employed the intermittent gas bubbling method to reduce fouling layer formed in concentrating TCM extract through DCMD. To limit membrane fouling or flux decline during concentrating process, intermittent gas bubbling was introduced to the feed side of membrane module. It was confirmed by experimental results that membrane fouling was effectively controlled in the way of removing deposited foulants from membrane surface by created two phase flow.

Membrane Distillation: Principle, Advances,

volatile compounds in black currant juice.

the base membrane.

Limitations and Future Prospects in Food Industry 255

recovery of black currant and cherry juice aroma compounds. The influence of the sweeping gas flow rate (SGMD only), feed temperature and feed flow rate on the permeate flux and the concentrations factors of 12 selected aroma compounds were examined on an aroma model solution and on black currant juice in a laboratory scale set-up. At 45 °C the most volatile and hydrophobic aroma compounds was obtained with the highest concentration factors: 12.1–9.3 (black currant juice) and 17.2–12.8 (model solution). A volume reduction of 13.7% (vol.%) at 45 °C, 400 L/h, resulted in an aroma recovery of 73–84 vol.% for the most

In concentration of fruit juices containing oily constituents (such as limonene in orange juice), membrane wetting may occur due to high affinity of hydrophobic membrane material with such compounds. Coating of membrane with hydrophilic polymers such as polyvinyl alcohol (PVA) (Mansouri & Fane, 1999) and alginate (Xu et al., 2004) has been proposed to overcome this problem. Recently, Chanachai et al. (2010) studied the coating of hydrophobic membrane PVDF with chitosan, a highly hydrophilic polymer, for protection against wetting by oils from fruit juice. The results indicated that the coated membrane well protected the membrane against wetting-out and could maintain stable flux. Coated membranes used to concentrate the oil solution (limonene 2%, v/v) for 5 h were not wetted out during flux measurement and no visual damage was observed indicating the stability on

It has been well-established that the combination of MD with other membrane technologies offers important benefits over stand alone use of MD in the concentration of various types of juices including grape juice (Rektor et al., 2007), pineapple juice (Hongvaleerat et al., 2008), kiwi fruit juice (Cassano & Drioli, 2007), camu-camu juice (Rodrigues et al., 2004), sugarcane juice (Nene et al., 2004) and cactus pear juice (Cassano et al., 2007). The integration of MD with other membrane operations such as MF, UF, NF, RO and OD permits advantage of achieving high quality fruit juice concentrates with higher economic feasibility. The use of integrated membrane processes for clarification and concentration of citrus (orange and lemon) and carrot juices have been proposed by Cassano et al. (2003). A limpid phase has been produced by ultrafiltration pilot unit. The clarified permeate coming from UF has been concentrated up to 15-20 °Brix by RO with a laboratory scale unit. Finally, OD step was applied to yield 60-63°Brix concentrate with a transmembrane flux of 1kg/m2h. A slight decrease in the total antioxidant activity has been reported during RO treatment, whereas no significant change was observed during OD treatment. Kozak et. al (2009) investigated an integrated approach for black currant juice concentration. The juice samples were prefiltered by MF and preconcentrated to 22°Brix by RO. A further concentration of the retentate coming from RO was obtained by MD and black currant concentrate with 58.2 °Brix was produced. Onsekizoglu et al. (2010b) have proposed the use of membrane processes for the production of clarified apple juice concentrate. The efficiency of UF was improved by an additional enzymatic pretreatment and flocculation step using fining agents such as gelatine and bentonite. The permeate coming from the UF with initial TSS contents of ca. 12 °Brix were subsequently concentrated up to TSS contents of 65 °Brix by MD, OD and coupled operation of MD & OD processes. The effect of clarification and concentration processes on formation of 5-hydroxymethylfurfural (HMF), retention of bioactive compounds (phenolic compounds, organic acids, glucose, fructose and sucrose) and their efficiency in preserving natural color and aroma (trans-2-hexenal, the most relevant compound in apple juice aroma) were evaluated in order to maintain a high quality product. The new membrane based

As can be concluded from the expressed results, there is a lack of data and understanding in fouling phenomena in MD especially in the food processing field. However, the risk of fouling and wetting of membrane pores compromises the durability of the membranes limiting their applications in food industry. The long term MD performance needs to be extensively studied so as to make the MD process more challenging in food industry.

### **9. Applications in food industry**

The main food-related applications of membrane distillation are the desalination and production of high purity water from brackish water and seawater. The major advantage of MD in desalination is the ability to achieve high rejection factors which cannot be accomplished by RO at high permeate fluxes. Production of high purity water is wellestablished with rejection factors of almost 100% of non-volatile compounds (Khayet & Matsuura, 2011). The MD process has been successfully studied for purification of waste waters of pharmaceutical (Ding et al., 2011) and textile (Criscuoli et al., 2008) industries as well as underground waters contaminated with heavy metals (Zolotarev et al., 1994) and sulfuric acid solutions (Tomaszewska, 2000). Very recently, the feasibility of applying membrane distillation process for recovering potable water from arsenic, uranium and fluoride contaminated brackish waters has been proposed (Yarlagadda et al., 2011). A high quality permeate with dissolved solids concentrations less than 20 ppm (>99% rejection of salts) along with arsenic, fluoride and uranium contaminant reductions in the range of 96.5– 99.9% were reported. Ke He et al. (2011) reported a flux value of 14.36 L/m2h over one month DCMD operation of sea water at the following conditions: hot side inlet temperature of 60 °C, cold side inlet temperature of 20 °C, and hot and cold side flow rate of 0.6 L/min for PTFE pore size 0.22 μm membranes. The electrical conductivity values of permeate were determined below 8 μS/cm. Gryta (2010) evaluated the desalination of water containing up to 12 g/L of soluble salts MD for 250 h by using PP membranes. Electrical conductivity values of produced water were in the range of 3.4–4.1 μS/cm despite a ten-fold increase of salt concentration. The permeate flux during MD process lasting 250 h decreased slightly from 543 to 498 L/m2h. Thermal water pretreatment was used to prevent scaling which was formed due to decomposition of bicarbonates dissolved in water. On the other hand, the operation was found to be beneficial only for underground waters with high hardness.

One of the main advantages of MD in water purification is the lower energy consumption. Like any other distillation process MD also requires energy for evaporation of water, as stated earlier. However, MD process can effectively operate at low temperatures, which makes it possible to utilize low-grade waste and/or alternative energy sources, such as solar and geothermal energy.

MD and OD are proposed as very challenging technologies for concentration of fruit juice allowing to overcome the drawbacks of conventional thermal evaporation encountered by application of high temperatures (Ali et al., 2003; Bui & Nguyen, 2005; Cisse et al., 2005; Pabby et al., 2009; Shaw et al., 2002; Vaillant et al., 2001). The preliminary study of effective concentration of orange juice by MD was presented by Calabro et al. (1994) using a microporous PVDF membrane. Alves & Coelhoso (2006) compared MD and OD in terms of water flux and aroma retention in model orange juice. A higher retention per amount of water removal was observed with OD together with higher flux values. Very recently, Jorgensen et al. (2011) evaluated the potential of SGMD and VMD configurations for

As can be concluded from the expressed results, there is a lack of data and understanding in fouling phenomena in MD especially in the food processing field. However, the risk of fouling and wetting of membrane pores compromises the durability of the membranes limiting their applications in food industry. The long term MD performance needs to be

The main food-related applications of membrane distillation are the desalination and production of high purity water from brackish water and seawater. The major advantage of MD in desalination is the ability to achieve high rejection factors which cannot be accomplished by RO at high permeate fluxes. Production of high purity water is wellestablished with rejection factors of almost 100% of non-volatile compounds (Khayet & Matsuura, 2011). The MD process has been successfully studied for purification of waste waters of pharmaceutical (Ding et al., 2011) and textile (Criscuoli et al., 2008) industries as well as underground waters contaminated with heavy metals (Zolotarev et al., 1994) and sulfuric acid solutions (Tomaszewska, 2000). Very recently, the feasibility of applying membrane distillation process for recovering potable water from arsenic, uranium and fluoride contaminated brackish waters has been proposed (Yarlagadda et al., 2011). A high quality permeate with dissolved solids concentrations less than 20 ppm (>99% rejection of salts) along with arsenic, fluoride and uranium contaminant reductions in the range of 96.5– 99.9% were reported. Ke He et al. (2011) reported a flux value of 14.36 L/m2h over one month DCMD operation of sea water at the following conditions: hot side inlet temperature of 60 °C, cold side inlet temperature of 20 °C, and hot and cold side flow rate of 0.6 L/min for PTFE pore size 0.22 μm membranes. The electrical conductivity values of permeate were determined below 8 μS/cm. Gryta (2010) evaluated the desalination of water containing up to 12 g/L of soluble salts MD for 250 h by using PP membranes. Electrical conductivity values of produced water were in the range of 3.4–4.1 μS/cm despite a ten-fold increase of salt concentration. The permeate flux during MD process lasting 250 h decreased slightly from 543 to 498 L/m2h. Thermal water pretreatment was used to prevent scaling which was formed due to decomposition of bicarbonates dissolved in water. On the other hand, the operation was found to be beneficial only for underground waters with high hardness.

One of the main advantages of MD in water purification is the lower energy consumption. Like any other distillation process MD also requires energy for evaporation of water, as stated earlier. However, MD process can effectively operate at low temperatures, which makes it possible to utilize low-grade waste and/or alternative energy sources, such as solar

MD and OD are proposed as very challenging technologies for concentration of fruit juice allowing to overcome the drawbacks of conventional thermal evaporation encountered by application of high temperatures (Ali et al., 2003; Bui & Nguyen, 2005; Cisse et al., 2005; Pabby et al., 2009; Shaw et al., 2002; Vaillant et al., 2001). The preliminary study of effective concentration of orange juice by MD was presented by Calabro et al. (1994) using a microporous PVDF membrane. Alves & Coelhoso (2006) compared MD and OD in terms of water flux and aroma retention in model orange juice. A higher retention per amount of water removal was observed with OD together with higher flux values. Very recently, Jorgensen et al. (2011) evaluated the potential of SGMD and VMD configurations for

extensively studied so as to make the MD process more challenging in food industry.

**9. Applications in food industry** 

and geothermal energy.

recovery of black currant and cherry juice aroma compounds. The influence of the sweeping gas flow rate (SGMD only), feed temperature and feed flow rate on the permeate flux and the concentrations factors of 12 selected aroma compounds were examined on an aroma model solution and on black currant juice in a laboratory scale set-up. At 45 °C the most volatile and hydrophobic aroma compounds was obtained with the highest concentration factors: 12.1–9.3 (black currant juice) and 17.2–12.8 (model solution). A volume reduction of 13.7% (vol.%) at 45 °C, 400 L/h, resulted in an aroma recovery of 73–84 vol.% for the most volatile compounds in black currant juice.

In concentration of fruit juices containing oily constituents (such as limonene in orange juice), membrane wetting may occur due to high affinity of hydrophobic membrane material with such compounds. Coating of membrane with hydrophilic polymers such as polyvinyl alcohol (PVA) (Mansouri & Fane, 1999) and alginate (Xu et al., 2004) has been proposed to overcome this problem. Recently, Chanachai et al. (2010) studied the coating of hydrophobic membrane PVDF with chitosan, a highly hydrophilic polymer, for protection against wetting by oils from fruit juice. The results indicated that the coated membrane well protected the membrane against wetting-out and could maintain stable flux. Coated membranes used to concentrate the oil solution (limonene 2%, v/v) for 5 h were not wetted out during flux measurement and no visual damage was observed indicating the stability on the base membrane.

It has been well-established that the combination of MD with other membrane technologies offers important benefits over stand alone use of MD in the concentration of various types of juices including grape juice (Rektor et al., 2007), pineapple juice (Hongvaleerat et al., 2008), kiwi fruit juice (Cassano & Drioli, 2007), camu-camu juice (Rodrigues et al., 2004), sugarcane juice (Nene et al., 2004) and cactus pear juice (Cassano et al., 2007). The integration of MD with other membrane operations such as MF, UF, NF, RO and OD permits advantage of achieving high quality fruit juice concentrates with higher economic feasibility. The use of integrated membrane processes for clarification and concentration of citrus (orange and lemon) and carrot juices have been proposed by Cassano et al. (2003). A limpid phase has been produced by ultrafiltration pilot unit. The clarified permeate coming from UF has been concentrated up to 15-20 °Brix by RO with a laboratory scale unit. Finally, OD step was applied to yield 60-63°Brix concentrate with a transmembrane flux of 1kg/m2h. A slight decrease in the total antioxidant activity has been reported during RO treatment, whereas no significant change was observed during OD treatment. Kozak et. al (2009) investigated an integrated approach for black currant juice concentration. The juice samples were prefiltered by MF and preconcentrated to 22°Brix by RO. A further concentration of the retentate coming from RO was obtained by MD and black currant concentrate with 58.2 °Brix was produced. Onsekizoglu et al. (2010b) have proposed the use of membrane processes for the production of clarified apple juice concentrate. The efficiency of UF was improved by an additional enzymatic pretreatment and flocculation step using fining agents such as gelatine and bentonite. The permeate coming from the UF with initial TSS contents of ca. 12 °Brix were subsequently concentrated up to TSS contents of 65 °Brix by MD, OD and coupled operation of MD & OD processes. The effect of clarification and concentration processes on formation of 5-hydroxymethylfurfural (HMF), retention of bioactive compounds (phenolic compounds, organic acids, glucose, fructose and sucrose) and their efficiency in preserving natural color and aroma (trans-2-hexenal, the most relevant compound in apple juice aroma) were evaluated in order to maintain a high quality product. The new membrane based

Membrane Distillation: Principle, Advances,

implementation.

**11. References** 

216.

133.

*Desalination,* 171, 111-131.

*Membrane Science,* 272, 58-69.

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*Technologies,* 12, 388-397.

Limitations and Future Prospects in Food Industry 257

possibility of integrating MD to other separation techniques in order to improve the efficiency of the overall system and to make the process economically viable for industrial applications. For fruit juice concentration, coupled operation of MD and OD seems promising to overcome high temperature related problems (i.e. aroma and colour loss) encountered in MD. However, integration of MD with other MD variants as well as conventional distillation techniques has not yet been investigated. Hence more focus on such combinations is required. In recent years, coupling MD with solar energy systems has

The ability to effectively operate at low temperatures makes MD process possible to utilize low-grade waste and/or alternative energy sources. In recent years, coupling MD with solar, geothermal and waste energy systems has been proposed to decrease energy consumption in desalination systems. Such an approach may be crucial for food processing systems. For example, in the case of fruit juice concentration, much lower temperatures should be applied in order to obtain stable products able to retain as much possible the uniqueness of the fresh fruit, its original color, aroma, nutritional value and structural characteristics. Thus, the possibility of operating under very mild conditions enables MD to utilize various alternative energy sources, making it more promising for industrial application. Further efforts need to be concentrated in this field, especially in utilization of waste energy and/or other renewable energy sources in the view of industrial

Agashichev, S. P. (2006) Modeling of the concentration polarization in a cylindrical channel

Ali, F., Dornier, M., Duquenoy, A. & Reynes, M. (2003) Evaluating transfers of aroma

Alklaibi, A. M. & Lior, N. (2005) Membrane-distillation desalination: status and potential.

Alves, V. D. & Coelhoso, I. M. (2006) Orange juice concentration by osmotic evaporation and

Babu, B. R., Rastogi, N. K. & Raghavarao, K. S. M. S. (2006) Mass transfer in osmotic

Babu, B. R., Rastogi, N. X. & Raghavarao, K. S. M. S. (2008) Concentration and temperature

Bagger-Jorgensen, R., Meyer, A. S., Pinelo, M., Varming, C. & Jonsson, G. (2011) Recovery of

Bailey, A. F. G., Barbe, A. M., Hogan, P. A., Johnson, R. A. & Sheng, J. (2000) The effect of

distillation. *Journal of Membrane Science,* 164, 195-204.

a batch-type pilot plant. *Journal of Food Engineering,* 60, 1-8.

of an ultrafiltration module. *Theoretical Foundations of Chemical Engineering,* 40, 215-

compounds during the concentration of sucrose solutions by osmotic distillation in

membrane distillation: A comparative study. *Journal of Food Engineering,* 74, 125-

membrane distillation of phycocyanin colorant and sweet-lime juice. *Journal of* 

polarization effects during osmotic membrane distillation. *Journal of Membrane* 

volatile fruit juice aroma compounds by membrane technology: Sweeping gas versus vacuum membrane distillation. *Innovative Food Science & Emerging* 

ultrafiltration on the subsequent concentration of grape juice by osmotic

been well studied by various researchers for desalination of sea water.

concentration techniques have been reported to be very efficient since the concentrated juice presented nutritional and sensorial quality very similar to that of the original juice especially regarding the retention of bright natural color and pleasant aroma, which were considerably lost during thermal evaporation. Further analysis have shown that the subsequent concentration treatments by MD, OD and coupled operation of MD & OD processes did not induce any significant changes in phenolic compounds, organic acids and sugars independently on the final concentration achieved.

The MD process can be successfully applied to remove ethanol and the other volatile metabolites from the fermentation broth (Banat & Al-Shannag, 2000; Gryta, 2001; Gryta & Barancewicz, 2011; Gryta et al., 2000a; Tomaszewska & Bialonczyk, 2011). The fermentation of sugar with *Saccharomyces cerevisiae* proceeds with the formation of by-products, which tend to inhibit the yeast productivity. The removal of ethanol is usually carried out by distillation. The primary disadvantages of the conventional process of ethanol generation include high energy consumption and excessive amount of wastewater discharged from the distillation columns. The MD process provides an economical alternative to the existing distillation technique for continuous removal of fermented products. The removal of volatile metabolites from the fermentation broth by MD process enables reduction of the inhibitory effect of these compounds on microbial culture together with an increased rate of sugar conversion to ethanol and hence the cost of further concentration of alcohol can be reduced. The main advantage of MD over conventional distillation processes is that membrane distillation takes place at a temperature below the normal boiling point of broth solutions.

Other food-related applications of MD include concentration of natural food colorants (Nayak & Rastogi, 2010), dealcoholization of wine (Varavuth et al., 2009) and concentration of herbal and plant extracts (Cisse et al., 2011; Dornier et al., 2011; Johnson et al., 2002; Zhao et al., 2011).

## **10. Concluding remarks and future prospects**

As a promising alternative to replace other separation processes, MD has gained much interest for its lower energy requirement in comparison with conventional distillation, lower operating pressures and higher rejection factors than in pressure driven processes such as NF, and RO. Although MD has been known for more than 40 years, a number of problems exist when MD is considered for industrial implementation. Most of the conducted MD studies are still in the laboratory scale. In recent years, some pilot plant studies have been proposed for desalination (Blanco et al., 2011; Farmani et al., 2008; Song et al., 2008; Xu et al., 2006), however long term evaluations of pilot plant applications for the concentration and recovery of aqueous solutions containing volatile solutes especially in the food industry are still scarce. Therefore, achievement of high concentration levels in certain fruit juice samples taking into account the effects on mass and heat transfer mechanisms, membrane characteristics and the quality parameters together with a detailed economical analysis should be examined on a large scale.

On the other hand, there is a lack of commercially available MD units; practically all membrane modules are designed for other membrane operations (i.e. microfiltration) rather than MD. Novel membranes specifically designed for MD applications should be fabricated in an economically feasible way. Research on transmembrane flux enhancement (i.e. acoustic field) for large scale applications is required. More attention should be paid to the

concentration techniques have been reported to be very efficient since the concentrated juice presented nutritional and sensorial quality very similar to that of the original juice especially regarding the retention of bright natural color and pleasant aroma, which were considerably lost during thermal evaporation. Further analysis have shown that the subsequent concentration treatments by MD, OD and coupled operation of MD & OD processes did not induce any significant changes in phenolic compounds, organic acids and sugars

The MD process can be successfully applied to remove ethanol and the other volatile metabolites from the fermentation broth (Banat & Al-Shannag, 2000; Gryta, 2001; Gryta & Barancewicz, 2011; Gryta et al., 2000a; Tomaszewska & Bialonczyk, 2011). The fermentation of sugar with *Saccharomyces cerevisiae* proceeds with the formation of by-products, which tend to inhibit the yeast productivity. The removal of ethanol is usually carried out by distillation. The primary disadvantages of the conventional process of ethanol generation include high energy consumption and excessive amount of wastewater discharged from the distillation columns. The MD process provides an economical alternative to the existing distillation technique for continuous removal of fermented products. The removal of volatile metabolites from the fermentation broth by MD process enables reduction of the inhibitory effect of these compounds on microbial culture together with an increased rate of sugar conversion to ethanol and hence the cost of further concentration of alcohol can be reduced. The main advantage of MD over conventional distillation processes is that membrane distillation takes place at a temperature below the normal boiling point of broth solutions. Other food-related applications of MD include concentration of natural food colorants (Nayak & Rastogi, 2010), dealcoholization of wine (Varavuth et al., 2009) and concentration of herbal and plant extracts (Cisse et al., 2011; Dornier et al., 2011; Johnson et al., 2002; Zhao

As a promising alternative to replace other separation processes, MD has gained much interest for its lower energy requirement in comparison with conventional distillation, lower operating pressures and higher rejection factors than in pressure driven processes such as NF, and RO. Although MD has been known for more than 40 years, a number of problems exist when MD is considered for industrial implementation. Most of the conducted MD studies are still in the laboratory scale. In recent years, some pilot plant studies have been proposed for desalination (Blanco et al., 2011; Farmani et al., 2008; Song et al., 2008; Xu et al., 2006), however long term evaluations of pilot plant applications for the concentration and recovery of aqueous solutions containing volatile solutes especially in the food industry are still scarce. Therefore, achievement of high concentration levels in certain fruit juice samples taking into account the effects on mass and heat transfer mechanisms, membrane characteristics and the quality parameters together with a detailed economical analysis

On the other hand, there is a lack of commercially available MD units; practically all membrane modules are designed for other membrane operations (i.e. microfiltration) rather than MD. Novel membranes specifically designed for MD applications should be fabricated in an economically feasible way. Research on transmembrane flux enhancement (i.e. acoustic field) for large scale applications is required. More attention should be paid to the

independently on the final concentration achieved.

**10. Concluding remarks and future prospects** 

should be examined on a large scale.

et al., 2011).

possibility of integrating MD to other separation techniques in order to improve the efficiency of the overall system and to make the process economically viable for industrial applications. For fruit juice concentration, coupled operation of MD and OD seems promising to overcome high temperature related problems (i.e. aroma and colour loss) encountered in MD. However, integration of MD with other MD variants as well as conventional distillation techniques has not yet been investigated. Hence more focus on such combinations is required. In recent years, coupling MD with solar energy systems has been well studied by various researchers for desalination of sea water.

The ability to effectively operate at low temperatures makes MD process possible to utilize low-grade waste and/or alternative energy sources. In recent years, coupling MD with solar, geothermal and waste energy systems has been proposed to decrease energy consumption in desalination systems. Such an approach may be crucial for food processing systems. For example, in the case of fruit juice concentration, much lower temperatures should be applied in order to obtain stable products able to retain as much possible the uniqueness of the fresh fruit, its original color, aroma, nutritional value and structural characteristics. Thus, the possibility of operating under very mild conditions enables MD to utilize various alternative energy sources, making it more promising for industrial application. Further efforts need to be concentrated in this field, especially in utilization of waste energy and/or other renewable energy sources in the view of industrial implementation.
