Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours K S M S Raghavarao1, Naveen Nagaraj1, Ganapathi Patil1, B Ravindra Babu1 and K Niranjan2 1

Department of Food Engineering, Central Food Technological Research Institute, Mysore, India 2 School of Food Biosciences, The University of Reading, Reading, Berkshire, UK Liquid foods and natural colours are concentrated in order to reduce the costs of storage, packaging, handling and transportation. However, both liquid foods and natural colours are sensitive to temperature and concentration by conventional methods, such as evaporation, results in product deterioration. Alternative processes, such as freeze concentration, have the drawback with respect to the maximum achievable concentration (only up to 40–45°Brix). In recent years membrane processes such as microfiltration, ultrafiltration and reverse osmosis are gaining importance for the concentration of liquid foods and natural colours. These existing membrane processes have limitations of concentration polarization, membrane fouling, shear damage (in the case of protein) and maximum achievable concentration (only up to 25°Brix). Recently, technological advances related to the development of athermal membrane processes such as osmotic membrane distillation and direct osmosis have shown the potential to overcome the above limitations. Furthermore, these processes can be employed as a pre-concentration step to reduce water load on subsequent processing steps and can be easily scaled up. Recent advances and developments in these athermal membrane processes used for concentration of liquid foods and natural colours along with theoretical aspects are discussed in this chapter.

1 Introduction Liquid foods such as fruit juices are of high nutritive value as they are naturally enriched with minerals, vitamins and other beneficial components required for human health. Emerging technologies for food processing ISBN: 0-12-676757-2

Copyright © 2005 Elsevier Ltd All rights of reproduction in any form reserved

10

252 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

In recent years, food colours derived from natural sources (plant/marine) are gaining importance over their synthetic counterparts as food additives since they are non-toxic and non-carcinogenic. When extracted from their sources, both fruit juices and natural colours have low solid content, colour strength and high water load. Water, being the major constituent of liquid foods and natural colours, contributes to the growth of the microorganisms. Removal of water helps to reduce the microbial load, thereby favouring an increase in the shelf-life of the liquid foods and natural colours. Hence, it is desirable to concentrate these liquid foods and natural colours to improve shelf-life, stability and to reduce storage/transportation costs (Philip, 1984; Petrotos and Lazarides, 2001). This chapter, besides briefly considering the existing concentration methods such as evaporative concentration, freeze concentration and membrane concentration discusses the newer athermal membrane processes like osmotic membrane distillation (OMD) and direct osmosis (DO) for the concentration of liquid foods and natural colours. Apart from merits and demerits, suggestions for future work and the possibilities of integrating the newer membrane processes (OMD/DO) with the existing processes are also addressed.

2 Existing methods The concentration of liquid foods constitutes the major aspect of the food processing industry. The following processes are currently in use for the concentration of liquid foods and natural colours.

2.1 Evaporative concentration Evaporation is one of the oldest methods employed for concentrating liquid foods and natural colours. Evaporation is defined as the removal of water by vaporization from the solution to produce a concentrated solution. Selection of the proper evaporator is necessary and depends upon many factors such as the properties of the feed material, quality of the product, operating conditions and operating economy. Some evaporators that are commonly used for the concentration of liquid foods and natural colours are discussed in following sections (McCabe et al., 2001). 2.1.1 Open pan evaporators

These are the simplest commercial available evaporators and their low cost makes them popular. Open pan evaporators consist of a container open to the atmosphere in which fluid is heated by a flame or by steam through a coil or external jacket. The pans may be closed to permit vacuum operation. Stirring increases the rate of heat transfer and reduces risk of product ‘burn on’. These are used in tomato pulp concentration, soup and sauce preparations and in jam and confectionary boiling. Small-jacketed pans are very useful, but with large capacities the ratio of heat transfer surface to liquid volume falls and the heating becomes less effective. Internal heating coils fitted in large units can interfere with the liquid circulation, so affecting the heat transfer rate. In general when larger capacities are required other types of evaporators are preferred.

Existing methods 253

2.1.2 Plate evaporators

This type of evaporator consists of a set of plates distributed in units in which vapour condenses in the channel formed between the plates. The heated liquid boils on the surface of the plates and forms a film. The good heat transfer and short residence time make the evaporator more useful to concentrate heat-sensitive products. Plate evaporators are mainly used to concentrate coffee, soup broth and citrus juices. 2.1.3 Rising film evaporator

These types of evaporator have tubes 3–10 m in length with diameters of 25–50 mm. Liquid preheated to near boiling is introduced at the bottom of the tube assembly. Expansion due to vaporization causes high velocity vapour bubbles and carries away the liquid which continues concentrating as it rises upward. Under optimum conditions the vapour lifts a thin film of rapidly concentrating liquid up the walls of the tubes. The leaving vapor–liquid mixture passes into a separator where vapour is removed. The concentrated liquid may be used directly, mixed with fresh feed and recirculated, or passed to a second evaporator for further concentration. Residence time in a climbing film evaporator is short which makes it useful for concentrating heat-sensitive materials. 2.1.4 Falling film evaporator

This is similar to a rising film evaporator but the pre-heated liquid feed enters at the top of the tube assembly. As the evaporation proceeds the vapour passes down the tubes as a central, high velocity core, dragging a film of liquid with it. Since a hydrostatic liquid head is absent, a uniform and low boiling temperature may be maintained. Residence times are short so this unit is excellent for the concentration of heat-sensitive materials. It is widely used with citrus fruit juices where high rates of evaporation are obtained at temperatures as low as 50–60°C under vacuum operation. 2.1.5 Agitated thin-film evaporators

These are essentially large diameter jacketed tubes in which the product is vigorously agitated and continuously removed from the tube wall by scraper blades (or wipers) with a shaft rotating inside the tube. Thus the material processed is continuously spread as a thin film. The horizontal, vertical and inclined type of agitated thin-film evaporators are more common. Concentration of liquid foods and natural colours by a thermal process like evaporation results in a loss of flavours/aroma, colour degradation and a ‘cooked taste’ leading to a low quality end product. Another serious drawback of the evaporation process is the high-energy requirement.

2.2 Freeze concentration Freeze concentration is another method employed for the concentration of liquid foods such as fruit juices. Freeze concentration involves partial freezing of the product and removal of ice crystals, thus leaving behind all the non-aqueous constituents in the

254 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

concentrated phase. In freeze concentration two distinctive steps are involved, i.e. ice crystallization and ice separation from the concentrate. In the former, fruit juice is super-cooled below its freezing point to allow water to form ice crystals. In the latter, the ice crystals are separated from the concentrated juice by centrifugation. The major advantages that the freeze concentration process offers over evaporation is that it can concentrate the fruit juices without appreciable loss in taste, aroma, colour and nutritive value. Furthermore, freeze concentration can eliminate self-oxidation problems and can produce higher quality juice than that obtained from an evaporation process. However, the major drawback of freeze concentration is the maximum achievable concentration, which is much lower than that achieved during the evaporation process. Because of low process temperatures and high viscosities of the fruit juices, most of the liquid foods cannot be concentrated beyond 40–45°Brix. The high viscosity of the fruit juice retards the rate of crystallization, furthermore, it makes the pumping of the juice concentrate and washing of ice crystals increasingly difficult. In addition, this technique is not suitable to handle liquid foods with a high pulp content. Moreover, the energy requirement for the formation of ice crystals during freeze concentration is high (Despande et al., 1982).

2.3 Membrane processes Membrane processing is a technique that permits concentration and separation of macro- and micromolecules based on molecular size and shape. Membrane processing is fast emerging among various unit operations available for separation processes, especially in the field of chemical engineering, biotechnology and food processing. Better process economy, higher yield, improved product quality, utilization of byproducts and a solution to some environmental problems, can all be achieved by using membrane processing. In recent years, membrane processes such as microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO) are gaining importance for processing liquid foods and natural colours. Concentration of liquid foods and natural colours has been widely explored after the discovery of asymmetric membranes by Loeb and Sourirajan in the early 1960s. These processes normally operate at ambient temperature, thereby reducing the thermal damage to the product and retaining the colour, flavour/aroma and nutritive components of the product. Studies have shown that membrane applications can be less energy intensive than evaporation and freeze concentration.

2.3.1 Microfiltration

In this process, large molecules (e.g. fat globules) and suspended particles are held by the membrane while the remaining components of the solution pass through the membrane. MF resembles conventional coarse filtration and can selectively separate particles with molecular weights greater than 200 kDa. The pore sizes of microfiltration membranes are in the range of 0.05–10
m and the porosity of the membrane is

Existing methods 255

about 70 per cent. Membrane thickness is in the range of 10–150
m. In microfiltration the applied pressure is in the range of 0.1–2 bar. Microfiltration finds application in cell harvesting, clarification of fruit juice, wastewater treatment, separation of casein and whey protein and separation of oil–water emulsions (Petrus and Nijhuis, 1993).

2.3.2 Ultrafiltration

This process is mainly used for clarification, concentration and purification of fruit juices, natural colours etc. The membranes are made up of polysulphone, polyvinyldene fluoride and cellulose acetate of pore size 1–100 nm. The applied pressure is in the range of 1–10 bar (Mulder, 1998). Ultrafiltration is also used for the separation of high molecular components from low molecular components having applications in the food, dairy, pharmaceutical, textile, chemical, metallurgy, paper and leather industries. The various applications in the food and dairy industry are concentration of milk, recovery of whey proteins, recovery of potato starch and proteins, concentration of egg products and clarification of fruit juices and alcoholic beverages (Mulder, 1998).

2.3.3 Reverse osmosis

In this process the concentration of solute in the solution (feed) will increase by the flow of water (or solvent) across the membrane to a dilute solution. This can be accomplished by applying the pressure in excess of the osmotic pressure (10–100 bar) of the solution. RO removes most of the organic compounds and up to 99 per cent of all ions. This process achieves rejection of 99.9 per cent of viruses, bacteria and pyrogens and was the first cross flow membrane separation process to be widely commercialized. The RO involves dense membranes having pore size of 2 nm. The porosity of the membrane is about 50 per cent. The separation mechanism is based on solution diffusion across the membrane. Membranes are made up of cellulose triacetate, polyether urea and polyamide. RO finds application in the concentration of liquid foods such as fruit juices, milk, etc. and desalination of brackish and seawater (Girard and Fukomoto, 2000). The limitations of these membrane processes (MF, UF, RO) are concentration polarization, membrane fouling, shear damage (in proteins) and constraints on the maximum attainable concentration (only up to 25–30°Brix). Even the new membrane process, membrane distillation (MD), is not without limitations as it suffers from membrane wetting, temperature polarization and loss of volatiles (Lee et al., 1982; Lawson and LLoyd, 1997; Girard and Fukomoto, 2000). Recently, technological advances related to the development of new membrane processes and improvements in process engineering have enabled the above limitations to be overcome to a large extent. Newer membrane processes such as osmotic membrane distillation (OMD) and direct osmosis (DO) have the potential to concentrate liquid foods and natural colours at ambient temperature and pressure without product deterioration and are discussed in detail in the following section.

256 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

3 Osmotic membrane distillation 3.1 Fundamentals of osmotic membrane distillation Osmotic membrane distillation (OMD) is a novel athermal, non-pressure driven membrane process capable of concentrating liquid foods (fruit and vegetable juices, non-food aqueous solutions) and natural colours under ambient temperature and pressure, thus avoiding thermal degradation of the product. OMD is a separation process in which a microporous hydrophobic membrane separates the two aqueous solutions (feed and osmotic solution) having different solute concentrations. The driving force in OMD is vapour pressure difference across the membrane generated as a result of a difference in concentration. Water evaporates from the surface of the solution having higher vapour pressure (feed); the vapour passes through the pores of the membrane and condenses on the surface of the solution of the lower vapour pressure (osmotic agent, OA) as shown in Figure 10.1. This results in the concentration of the feed and dilution of the OA solution. OMD is also known as osmotic evaporation, membrane evaporation and isothermal membrane distillation or gas membrane extraction. It can be employed to achieve maximum concentration of up to 70°Brix without product damage (Hogan et al., 1998). In OMD, the vapour pressure of flavour/fragrance components due to low concentration (relative to that of water) is substantially depressed, thereby reducing the driving force for transmembrane transport of these solutes. The solubilities of these lipophilic solutes are substantially lower in concentrated saline (OA) solutions than in pure water. As a consequence, the vapour pressure of these solutes, when present even in trace concentration in such solutions, is much higher than that over water at the same concentration. Thus, the vapour pressure driving force for vapour phase transfer of these solutes from feed to the strip is far lower. Furthermore, due to the higher molecular weights of these solutes, their diffusive permeabilities through the membrane are Feed

Membrane

Pore

Figure 10.1

Mechanism of osmotic membrane distillation.

OA

Osmotic membrane distillation 257

lower. The overall result of all these factors makes OMD an attractive complementary or alternative process for the concentration of liquid foods with high flavour retention. Some of the OMD processes which have been carried out by various researchers are listed in Table 10.1. Furthermore, OMD can also be employed as a pre-concentration step prior to lypholization (freeze drying) of temperature-sensitive biological products such as vaccines, hormones, enzymes and proteins to obtain the product in powder form without product deterioration.

3.2 Mathematical models 3.2.1 Mass transfer

In OMD, water transport across the membrane involves evaporation of water at the surface of the solution, diffusion of water vapour through the membrane and condensation of the water vapour on the osmotic agent (OA) side. The basic model used to describe the water transport in the system that relates the mass flux (J) to the driving force is given by: J  K P b

(1)

where K is the overall mass transfer coefficient which accounts for all three resistances (feed, membrane and OA side) for water transport. 3.2.2 Mass transfer through the membrane

Mass transfer in OMD occurs by diffusive transport of water vapour across the microporous hydrophobic membrane. The mode of diffusion for water vapour through the stagnant gas phase of the membrane pore can be described either by the Knudsen diffusion or molecular diffusion mechanism depending on the pore size (Geankoplis, 1993). When the membrane pore size is lower than the mean molecular free path, the molecules tend to collide more frequently with the pore wall. Under these conditions, the mode of diffusion is by Knudsen diffusion and the equation for water flux can be written as (Schofield et al., 1987): ⎡ re JK  ⎢⎢1.064  ⎢ ⎣

⎛ M ⎞⎟0.5 ⎤⎥ ⎜⎜ ⎟ ⎥ (P  P ) ⎜⎝ RT ⎟⎟⎠ ⎥ 1 2 ⎦

(2)

where the first term of RHS in Equation (2) is the Knudsen diffusion coefficient corrected for membrane porosity as well as pore tortuosity and the second term accounts for the driving force. When the membrane pore size is relatively large, the collisions between the gas molecules themselves are more frequent and the mode of diffusion is called molecular diffusion. Water flux across the membrane is represented by (Sherwood et al., 1975): ⎡ 1 De M ⎤ ⎥ (P  P ) Jm  ⎢⎢ ⎥ 1 2  Y RT ⎦ ⎣ ln

(3)

258 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

Table 10.1 Work carried out by various researches on osmotic membrane distillation Type

Juices/water/ Colorants

Osmotic agent

Membranes and their operating conditions

Fluxes (l/m2/h)

References

OMD

Orange juice, seawater

Seawater MgSO4

Module: Hollow fibre PP membrane; r 700 A; S 0.18 m2,  50%

0.4–7.9

Lefebvre (1988)

OMD

Orange, apple, NaCl grape juices

Module: Syrinx plate and frame PTFE membrane; r 0.2
m; S 0.7 m2;  100
m Temperature: 29–40°C

0–2.2

Sheng et al. (1991)

OMD

Water

NaCl

Module: Lewis cell (stirred cell) a) Millipore PVDF (GVHP) r 0.2
m;  70%;  125
m; S 0.00275 m2 b) Millipore PTFE (FHLP) r 0.2
m;  70%;  175
m; S 0.00275 m2 c) Gelman PTFE (TF-1000) r 1
m;  80%;  178
m; S 0.00275 m2 d) Gelman PTFE (TF-450) r 0.45
m;  80%;  178
m; S 0.00275 m2 e) Gelman PTFE (TF-200) r 0.2
m;  80%;  178
m; S 0.00275 m2

0–0.5

Mengual et al. (1993)

OMD

Water

NaCl, MgCl2

Module: Capillary modules (LM2P06, MD020CP2N) in shell-tube configuration a) Accurel PP Q3/2; r 0.2
m;  70%; SI 0.04 m2 b) Accurel PP S6/2; r 0.2
m;  70%; SI 0.104 m2 Temperature: 25–50°C

0.2–2.5

Gostoli (1999)

OMD

Water

CaCl2

Module: Flat sheet membrane (Co-current) a) Pall-Gelman TF200 r 0.2
m;  165
m:  60%; S 0.004 m2 b) Pall-Gelman TF450 r 0.45
m;  178
m:  60%; S 0.004 m2 Temperature: 25°C

4–12

Courel et al. (2000a)

OE

Passion fruit juice

CaCl2

Module: Hollow fibres module PP membrane; r 0.2
m; S 10.2 m2 Temperature: 30°C

0.5–0.75

Vaillant et al. (2001)

OMD

Water

Glycerol, NaCl, CaCl2

Module: Stirred cell PP membrane; r 0.1
m;  90 mm;  55%; S 0.00113 m2 Temperature: 20–45°C

0.4–3.2

Alves and Coelhoso (2002)

OMD

Water, sugarcane juice

NaCl, K2HPO4, CaCl2

Module: Flat membrane test cell a) PP membrane; r 0.05
m;  90
m; S 0.045 m2 b) PP membrane; r 0.2
m;  150
m; S 0.045 m2 c) PTFE membrane; r 0.025
m; S 0.045 m2 Acoustic field: 1.2 MHz Temperature: 25–60°C

0.4–0.93

Narayan et al. (2002)

OMD

Phycocyanin

CaCl2, K2HPO4

Module: Flat membrane test cell a) PP membrane; r 0.05
m;  90
m; S 0.0115 m2 b) PP membrane; r 0.2
m;  150
m; S 0.0115 m2 Temperature: 25°C

1.4–1.9

Naveen et al. (2003a)

OE

Water

Brine

Module: Ceramic tubular membrane r 0.2 and 0.8 106 m Temperature: 25–35°C

0.15–1.4

Brodard et al. (2003)

Osmotic membrane distillation 259

Both these models are useful for predicting the mass transfer through the membrane, each of them having its own limitations. The Knudsen model requires details of membrane pore geometry (pore radius, membrane thickness and tortuosity), whereas the molecular diffusion model is not valid at low partial pressure of the air (as Yln tends to zero) (Schofield et al., 1987).

3.2.3 Mass transfer through the boundary layers

The boundary layers of concentrated feed and dilute brine solution are present on either side of the membrane. This results in significant resistance to mass transfer which cannot be neglected. Mengual et al. (1993) proposed a model to study the influence of the boundary layer on transmembrane flux during OMD in a stirred cell, which explains the dependency of flux on bulk concentration and on stirring rate. In the case of a specially designed cross flow/stirred membrane cell, the liquid mass transfer coefficient in the boundary layer (ki) has been obtained by using the following correlations (Courel et al., 2000b; Alves and Coelhoso, 2002; Naveen et al., 2003b): Sh  b1Reb2 Scb3

(4)

where Sh 

k iL , Dw

Re 

uL




and

Sc 




Dw

(5)

where Dw is the water diffusion coefficient and Ki is liquid mass transfer coefficient.

3.2.4 Heat transfer

The water transport in OMD is a simultaneous heat and mass transfer process. Evaporation cools the feed and condensation warms up the brine (OA). The resultant temperature gradient across the membrane translates into a lower vapour pressure gradient, which in turn results in reduction of the driving force. The total heat transferred across the membrane is given by: Q  HT

(6)

where H is the overall heat transfer coefficient which accounts for all three resistances (feed, membrane, and OA) (Courel et al., 2000b).

3.2.5 Heat transfers through boundary layers

Heat transfer across the boundary layer influences the rate of mass transfer and mainly depends on the physical properties as well as the hydrodynamic conditions of the solution. So far, no study has been published regarding the magnitude of the heat transfer through the boundary layers. However, the boundary layer heat transfer coefficients

260 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

can be estimated from empirical correlations involving dimensionless numbers, like Nusselt (Nu), Reynolds (Re) and Prandtl (Pr) numbers and are given by: Nu=b1Reb2Pr b3 where Nu 

hLL kT

and

Pr 

(7)
CP kT

3.3 OMD membranes The membranes are made up of synthetic polymers such as polytetrafluoroethylene (PTFE), polypropylene (PP) and polyvinylidene difluoride (PVDF), which are hydrophobic in nature and can be employed for OMD processes (Kunz et al., 1996). The membrane employed in the OMD process should be highly porous (60–80 per cent) and as thin as possible (0.1–1
m) since the flux is directly proportional to the porosity and inversely to the membrane thickness (pore length). Furthermore, it should be highly conductive so that the energy of vaporization of the feed can be supplied by conduction across the membrane at a low temperature gradient, thereby making the process essentially isothermal. The hydrophobicity of the membrane is a decisive parameter to make the OMD process viable. However, quantifying this parameter on porous material is not easy, as it is not supported by any theory. The method of estimating the contact angle by accounting the surface energy of smooth dense material does not apply for porous membranes. The pressure variable can be included in the wettability definition via the liquid entry pressure represented by the Laplace equation (Courel et al., 2001): Pentry 

2BL Cos max

(8)

where Pentry is the liquid entry pressure, B is geometric factor, L is liquid surface tension, is liquid–solid contact angle and max is the largest pore radius. Once pressure drop across the vapour–liquid interface Pinterface exceeds penetration pressure Pentry, the liquid can penetrate into the membrane pores and the membrane is termed ‘wetted’. Hence, wettability of OMD membranes can be better defined by a critical surface tension combined with operating pressure conditions rather than by contact angle measurements. Development work is currently underway to attempt to produce hollow fibre microporous membranes from more hydrophobic membranes such as PTFE or PVDF or to make laminate membranes that prevent liquid intrusion without impeding vapour transport (Hogan et al., 1998; Michaels, 1999). Recently, it has been found that an amorphous copolymer of a certain perfluorinated dioxole monomer, namely perfluoro2,2-dimethyl-1,3-dioxole (PDD), can be formed into non-porous gas membranes, which provide acceptable transmission rates for OMD. These types of membranes can concentrate pulpy fruit juices and limonene-containing juices, such as orange juice, at high flux for long durations between membrane cleanings. Additionally, less

Osmotic membrane distillation 261

contamination of the OA solution into the feed side occurs, thus providing a high quality concentrate (Bowser, 2001). The membrane having relatively larger pore sizes at the surface showed higher organic volatiles retention per unit water removal than those with smaller openings. Accordingly pores with larger diameters at the membrane surface allow greater intrusion of the feed and OA streams, which provides an extended boundary layer. This extended layer offers extra resistance through which the diffusion of volatile components occurs. The above study helps to understand the utilization of membranes with larger surface pore diameters when the retention of volatiles and flavour/fragrance components are desirable for product quality (Barbe et al., 1998). More recently, Brodard et al. (2003) successfully employed hydrophobic ceramic tubular membranes in the osmotic evaporation process. Ceramic tubular membranes have been obtained by grafting siloxane compounds on alumina porous supports. Ceramic membranes have the advantage of physical and chemical stability when compared with polymeric membranes.

3.4 Effect of various process parameters The effect of various process parameters, such as type of osmotic agent, concentration, flow rate, temperature and membrane pore size, on transmembrane flux are discussed in the following sections. 3.4.1 Type of osmotic agent

The OMD process involves the use of a concentrated osmotic agent (OA) solution at the downstream side of the membrane. The rate of water (solvent) transport increases as the solvent vapour pressure on the OA side is reduced. In order to maintain the required driving force (vapour pressure difference), generally salts of high water solubility and low equivalent weights such as NaCl, CaCl2, MgCl2, MgSO4, K2HPO4, KH2PO4 are suitable as OAs in OMD. Potassium salts of ortho- and pyrophosphoric acid offer several advantages, including high water solubility, low equivalent weight, steep positive temperature coefficients of solubility and safe use in foods and pharmaceuticals. 3.4.2 Concentration

The feed and osmotic agent concentrations influence the OMD flux. The effect of OA concentration on transmembrane flux was studied in considerable detail for a model system (water as feed) as well as for real systems (fruit juices) (Mengual et al., 1993; Courel et al., 2000a; Alves and Coelhoso, 2002; Narayan et al., 2002; Ravindra Babu, 2003). It was observed that the transmembrane flux was increased with an increase in OA concentration in all cases. This is mainly due to an increase in vapour pressure difference (driving force) across the membrane. The transmembrane flux decreases with an increase in feed concentration and it strongly depends on the osmotic pressure difference between the two aqueous solutions (feed and OA). When osmotic pressure difference is decreased from 416 atm (dilute feed) to 280 atm (concentrated juice), a fivefold decline in flux was observed (Sheng et al., 1991).

262 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

3.4.3 Flow rate

The effect of OA flow rate on transmembrane flux was observed for model as well as for real systems (Courel et al., 2000a, b; Naveen et al., 2003b). In all these cases, transmembrane flux was increased with an increase in OA flow rate, which can be attributed to reduction in the concentration polarization layer. 3.4.4 Temperature

The effect of temperature on transmembrane flux has been studied by various researchers (Mengual et al., 1993; Gostoli, 1999; Courel et al., 2000a; Alves and Coelhoso, 2002; Narayan et al., 2002). The transmembrane flux increased with an increase in temperature. The rise in temperature provides an additional driving force that works synergistically with the driving force generated due to the concentration gradient. It is not difficult to appreciate the strong dependence of flux on temperature, which follows the Arrhenius dependency. 3.4.5 Membrane pore size

The effect of membrane pore size on transmembrane flux was studied (Mengual et al., 1993) and not much change in flux was observed. Recently, Brodard et al. (2003) have employed ceramic (inorganic) membranes made up of alumina having pore sizes of 0.2 mm and 0.8 m. The water transport was independent of pore size and followed molecular diffusion. Furthermore, the water fluxes obtained were much lower than those obtained by Courel et al. (2000b).

3.5 Process design and economics The essential design parameters which affect the OMD process performance for the concentration of liquid foods and natural colours are: 1 2 3 4 5

plant capacity solute concentrations in feed and concentrate water vapour pressure/concentration relationship for the feed stream water vapour pressure/concentration relationship for the OA solution and intrinsic water vapour permeability of the OMD membrane.

The potential and compatibility of OMD for concentrating liquid foods and natural colours have been proved beyond doubt. OMD like any other membrane process has low flux and production cost is higher than the thermal evaporation process. In order to overcome this problem, attempts have been made to enhance the transmembrane flux by the application of an acoustic field in a lab-scale membrane cell (Figure 10.2). The application of an acoustic field disturbs the hydrodynamic boundary layer of the solution, thereby reducing the effect of concentration polarization (Narayan et al., 2002). Another serious problem associated with commercial application of OMD is management of the diluted osmotic agent solution. It is essential to reuse the OA solution for better economics of the process. Corrosion and scaling make it expensive to

Direct osmosis 263

Cover Amicon cell (50 ml capacity)

Stirrer guide Vent Strip solution Teflon tube

Feed inlet Membrane (hydrophobic)

Feed solution (pure water)

Acoustic waves

Transducer Magnetic stirrer

Figure 10.2

Application of an acoustic field in membrane cell (lab-scale).

reconcentrate the diluted OA solutions. Evaporation, solar ponds and reverse osmosis could be used for re-concentration of OA solutions.

4 Direct osmosis 4.1 Fundamentals of direct osmosis Direct osmosis (DO) is another non-pressure driven membrane process capable of concentrating liquid foods and natural colours at ambient conditions without product deterioration. The concept is similar to that used by Eastern European farmers for the concentration of fruit juices, wherein a bag filled with juice was immersed in a brine solution (Cussler, 1984). Initially this process could not be exploited commercially due to low flux. In recent years, DO is gaining importance for the concentration of liquid foods and natural colours (Popper et al., 1964; Bolin et al., 1971; Loeb and Bloch, 1973; Rodriguez et al., 2001) and desalination of seawater (Kravath and Davis, 1975). DO, which is also known as direct osmosis concentration (DOC), uses a semipermeable dense hydrophilic membrane which separates two aqueous solutions (feed and OA solution) having different osmotic pressures (Figure 10.3). The driving force is the difference in osmotic pressure across the membrane (Beaudry and Lampi, 1990). The transfer of water occurs from lower to higher solution concentration until the osmotic pressures of both the systems become equal. DO also offers similar advantages as OMD with respect to energy and thermoliable component retention during the concentration of liquid foods and natural colours and concentration up to about 45–60°Brix could be achieved (Wong and Winger, 1999). Some of the DO processes as carried out by various researchers are shown in Table 10.2.

264 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

4.2 Mathematical models Various models have been proposed to explain mass transport through the membrane in the DO process. 4.2.1 Mass transfer through the membrane

The water, along with solute, tends to diffuse through the porous support of the membrane and solute concentration increases at the surface of the active membrane skin Feed

OA

Membrane

Pore Figure 10.3

Mechanism of direct osmosis.

Table 10.2 Work carried out by various researchers on direct osmosis Type

Juices/water/ colorants

Osmotic agent

Membranes and their operating conditions

Fluxes (l/m2h)

References

DOC

Grape juice

NaCl

2.5

Popper et al. (1964)

DO

HFCS

0.9–1.4

Wrolstad et al. (1993)

5–6

Herron et al. (1994)

DOC

Red raspberry juice Orange, raspberry, tomato juices Tomato juice

0.37–3.1

Petrotos et al. (1998)

DOC

Tomato juice

NaCl CaCl2 Ca(NO3)2 Sucrose PEG Brine

4.5

Petrotos and Lazarides (2001)

DOC

Red radish

HFCS

Module: Plate and frame membrane module Cellulose acetate membrane Module: Osmotek DOC cell S 0.14 m2 Module: Osmotek DOC module Cellulose triacetate membrane MWCO 100 Da,  90
m Module: Tubular membrane module AFC99 aromatic polyamide thin film composite reverse osmosis membrane  500 and 600
m Module: Flat sheet membrane module Commercial reverse osmosis membrane,  260
m Module: Pilot plant (Osmoteck Inc., Corvalis OR)

0.5–2

Rodriguez-Saona et al. (2001)

DOC

PEG, HFCS

Direct osmosis 265

layer and it continues to rise until a steady state is reached resulting in internal polarization. The increase in solute concentration can cause back diffusion of the solute. Hence, the water flux will be more when the solute back diffusion rate is high, in other words the solute resistivity in the membrane porous substructure is low. Thus, in direct osmosis the porous substructure has a large significance on water flux and is given by (Loeb et al., 1997): Jw 

1 ⎛⎜ OA ⎞⎟⎟ Ds ⎜ln ⎟ t K ⎜⎜⎝ Feed ⎟⎠

⎛ ⎞⎟ ⎜⎜ OA ⎟ ⎟⎟ ⎜⎜ln

⎜⎝ Feed ⎟ ⎠

(9)

where K is the resistivity of the porous substructure to water transfer, DS is the diffusivity of the solute, t is the thickness of the porous substructure, Feed and OA are the osmotic pressure of the feed and OA solutions, respectively. The preferential sorption capillary flow (PSCF) model offers a better visualization of the factors implied in transport across a reverse osmosis (RO) membrane and the same was preferred to explain the transport through a DO membrane (Ghiu et al., 2002). This model considers the surface layer of the membrane to be microporous and heterogeneous. The mechanism of separation is dictated by surface phenomena and pressure driven transport through capillary pores. When there is a pressure difference, the solute and solvent tend to permeate through the membrane, but water is adsorbed into the pores, whereas solute is rejected (due to physiochemical nature of the surface layer). However, due to the difference in chemical potential, eventually the solute is transported by diffusion through the pores and the flux is proportional to the concentration difference across the membrane. The solute flux through the membrane is given by: dm (10) Js  dt  S ⎛D K ⎞ Js  ⎜⎜⎜ SM S ⎟⎟⎟ (C SM  C SDI) ⎝  ⎟⎠

(11)

where Js is solute flux through the membrane, S is membrane surface area, CSM is solute concentration of feed at the membrane surface, CSDI is solute concentration in feed solution,  is the membrane thickness, DSM is diffusion coefficient of the salt in the membrane and KS is the solute partition coefficient between solution and membrane. The first term in equation (10) is known as the salt permeability and it is ideally the same for both RO and DO and was simplified as (DSM KS/) in order to calculate the salt permeability, by making the assumption that the solute concentration of feed at the membrane surface is the same as the concentration in the feed solution and is given by: ⎤ ⎛ DSMK S ⎞⎟ ⎛ m ⎞⎟ ⎡ 1 ⎜⎜ ⎥ ⎟⎟  ⎜⎜ ⎟⎟ ⎢ ⎜⎝  ⎟⎠ ⎜⎝ S ⎟⎠ ⎢⎢ 0.5(A  B)T 2  C SO T ⎥⎥ ⎦ ⎣

(12)

where m is the moles of solute transported through the membrane at time T, A and B are changes in solute concentration on the OA and feed side respectively.

266 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

4.3 DO membranes DO uses membranes similar to those used for RO, however, the osmotic pressure difference across the membrane is the driving force in the former, while it is hydraulic pressure in the latter (Beaudry and Lampi, 1990). The DO membranes are generally hydrophilic and asymmetric in nature, having a low molecular weight cutoff (100 Da). The hydrophilic nature provides a continuous water link between feed and OA. An asymmetric membrane structure is one that has a very thin active layer (skin), which facilitates the rejection of smaller polar compounds, and a much thicker and porous layer that provides strength to the membrane. The most suitable membranes for the DO process are cellulose acetate, cellulose diacetate, cellulose triacetate, polyamide and polysulphone membranes. Asymmetric cellulose-based membranes have shown better performance due to their water absorbing characteristics. Furthermore, these membranes are more resistant to fouling than many commercially available dense RO membranes (Herron et al., 1994; Wong and Winger, 1999).

4.4 Effect of various process parameters 4.4.1 Type of osmotic agent

The selection of the OA is one of the most important parameters, as it affects the DO performance. An osmotic agent should be highly soluble in water, hygroscopic, nontoxic, inert towards the flavour, odour and colour of the foodstuffs and should not pass through the membrane. Generally, the higher the concentration of the dissolved solids and the lower the molecular weight of the dissolved solids, the higher is the osmotic pressure. The most commonly employed osmotic agents are sodium chloride, sucrose, glycerol, cane molasses and corn syrup. Studies show that the transmembrane flux obtained for polyethylene glycol 400 (PEG 400) and carbohydrate solutions were less when compared to those of other solutions such as sodium chloride (NaCl), calcium chloride (CaCl2) and calcium nitrate (CaNO3). This is mainly due to the physical properties of the OA solutions such as viscosity (high in the case of PEG 400 and carbohydrate) and diffusivity. The lower viscosity offers less resistance to mass transfer through the concentration polarization layer (Petrotos et al., 1998). 4.4.2 Concentration

The extent of the concentration achievable in the DO process depends on the osmotic pressure of the osmotic agent (OA). Also, higher OA concentration results in higher flux and better relative rejection of salt ions (which otherwise leads to cross contamination). It is desirable to have the highest possible OA concentration, however, physical limits usually constrain achieving this. The OA solutions must have an osmotic pressure greater than that of the concentrated feed. For example, the osmotic pressure of a 74°Brix high fructose corn syrup is about 270 bar which is greater than the 90 bar for 42°Brix pulpy orange juice. The transmembrane flux decreases with an increase in feed side concentration due to the decrease of the osmotic pressure difference between the OA solution and the feed.

Direct osmosis 267

4.4.3 Temperature

An increase in temperature increases the osmotic pressure difference, which in turn increases transmembrane flux. This is mainly due to the reduction in viscosity and increase in diffusion coefficients. The increase in temperature reduces the mass transfer resistance during DO. 4.4.4 Flow rate

The flux in the DO process increased with an increase in the flow rate of the solutions. This increase in flux is due to a reduction in the resistance offered to mass transfer by the concentration polarization layer adjacent to the membrane. However, the effect of flow rate on transmembrane flux is less in DO when compared to OMD. Petrotos et al. (1998) reported that an increase in flow rate (4.6 fold) resulted in a marginal flux increase (only 32 per cent) during concentration of tomato juice. 4.4.5 Membrane thickness

The transmembrane flux increases with a decrease in membrane thickness. Petrotos et al. (1998) observed that the fluxes increased exponentially with a decrease in thickness of the backing material (600–400
m). Meanwhile, Beaudry et al. (1990) observed it to be linear. Even Loeb et al. (1997), who have studied permeation fluxes in DO process for non-food systems, observed that the porous fabric of the membranes clearly decreased the osmotic permeation. Some further detailed studies are required in this regard.

4.5 Process design and economics The design parameters which affect the DO performance are: 1 2 3 4 5

volume of feed to be concentrated final concentration required osmotic pressure difference between feed and OA physical properties of feed and OA and characteristics of membrane (thickness, water permeability, etc).

Even though DO can be employed for the concentration of dilute liquid foods, there are still some constraints which limit its full commercial application. Water flux is only one-fifth when compared to RO, but the ability to concentrate without prefiltering provides an advantage in DO. DO has a wide range of applications for the concentration of various liquid foods and natural colours. The economics of DO operation is dependent on effective OA management. The cost of DO operation depends mainly on the re-concentration method chosen. Re-concentration processes for the OA include solar evaporation, thermal evaporation and RO, but salts at high concentrations are corrosive to the metals which are used in the evaporators. Sugar solutions employed as OA can be concentrated by thermal evaporation or RO.

268 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

Figure 10.4

Stirred membrane cell (controlled conditions).

5 Membrane modules The membrane configurations normally employed for operating OMD/DO are stirred membrane cell, plate and frame, tubular, spiral wound and hollow fibre. The stirred membrane cell (Figure 10.4) is suitable for bench-top feasibility studies. However, the flux data obtained in stirred cells are not a good indication of the flux that can be obtained with larger modules. A majority of the laboratory scale modules are designed for use with flat sheet membranes (Figure 10.5), as these membrane modules are more versatile when compared with tubular or hollow fibre membrane modules. Flat sheet membranes are easier for examination and cleaning. As a result the same membrane module can be used to test many different types of membranes. Tubular membrane modules are usually operated under turbulent flow conditions, which help in the reduction of the concentration polarization effect. Tubular membranes do not require a support, which results in lower boundary layer resistances compared to flat sheet membrane modules. However, the main disadvantage of the tubular unit is the low surface area to volume ratio and hence the requirement for high floor space. Another disadvantage is the high hold-up volume within these units. The hollow fibre membrane configuration consists of a membrane in the form of a self-supporting tube and has the advantage of a ‘back-flushing’ provision. However, the disadvantage of the hollow fibre module is the cost of membrane replacement. This is mainly because even if one

Applications 269

Figure 10.5

Flat membrane module.

single membrane fibre ruptures, the entire membrane cartridge needs to be replaced. The spiral wound module is one of the most compact and inexpensive designs available today. These modules are basically flat sheets arranged in parallel to form a narrow slit to fluid flow. The main advantage of the spiral wound module is its surface area to volume ratio, which is fairly high and results in a lower floor area. The other advantages of spiral wound module are low capital cost and low power consumption (Herron et al., 1994; Kunz et al., 1996; Lawson and Lloyd, 1997; Hogan et al., 1998; Petrotos et al., 1999; Wong and Winger, 1999; Shaw et al., 2001; Vaillant et al., 2001).

6 Applications 6.1 OMD Most of the work in OMD has been carried out at lab scale to concentrate numerous fruit juices, vegetable juices, natural food colours, proteins and other aqueous solutions. Only a few reports are available at pilot scale. OMD can be employed as a pre-concentration step prior to relatively costlier processes such as lypholization, in the case of thermally-sensitive products like enzymes/ proteins, natural food colours, etc. Another potential application is de-alcoholization of fermented beverages (wine or beer). The most common application of the OMD process is to concentrate fruit juices up to 70°Brix without product damage. The use of pressure driven membrane processes such as UF, RO, NF for the concentration of phycocyanin (natural food colourant/protein) may result in shear damage to

270 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

the product as well as membrane fouling (Jaoquen et al., 1999). In order to overcome these drawbacks, experiments using OMD have been carried out at CFTRI, Mysore, India. C-phycocyanin obtained from freshly harvested biomass (initial concentration of 0.9 mg/ml) was concentrated by employing the OMD process in a flat membrane module using a hydrophobic polypropylene membrane. The concentration of C-phycocyanin increased by around 220 per cent without product damage (confirmed by spectrophotometric analysis). Furthermore, the concentrated C-phycocyanin was lyophilized to obtain it in a powdered form, which can be readily used for food applications (Naveen et al., 2003a). The newer membrane OMD processes suffer from low flux, which limits their full commercial application. Also, using the RO process, the maximum achievable concentration is only up to 25–30°Brix due to osmotic pressure limitation. In order to overcome these drawbacks and to improve the product quality and process economics, concentration of liquid foods and natural colours by integrated membrane processes appears very attractive. Therefore, an integrated membrane process involving clarification by MF/UF, pre-concentration by RO and final concentration by OMD yields a high quality product with a significant reduction in production cost. This area has scientific potential and can provide an efficient scalable alternative athermal process for the concentration of liquid foods and natural colours without product deterioration. Table 10.3 summarizes some of the integrated membrane processes employed for clarification and concentration of various liquid foods (such as orange juice, grape juice, passion fruit juice, carrot juice and coconut water) and some of them are discussed below. Many researchers have carried out concentration of fruit juices (orange, passion fruit and grape) involving microfiltration (MF)/ultrafiltration (UF) followed by an OMD process on a pilot scale level. These studies demonstrated the feasibility of integrating OMD with MF to concentrate fruit juices to an intermediate concentration degree with high flavour quality (Bailey et al., 2000; Shaw et al., 2001). More recently, a three-stage hybrid membrane process for the concentration of ethanol-water extracts of the Echinacea plant (which is used as immunostimulant) has been investigated. This resulted in a highly concentrated product suitable for marketing in capsule form (Johnson et al., 2002). Table 10.3 Summary of integrated membrane processes Feed

Integrated processes

References

Grape juices

UF/RO and OD are integrated and fresh fruit juices were concentrated up to 65–70°Brix without product deterioration or loss of flavours

Hogan et al. (1998)

Orange and passion fruit juices

A pilot scale process involving MF and OE. The juices (orange and passion) were concentrated up to 33.5°Brix and 43.5°Brix, respectively. The quantitative analysis shows about 32–36% average loss of volatile components

Shaw et al. (2001)

Citrus (orange and lemon) and carrot juices

The integrated membrane process (UF, RO and OMD) was used. The citrus and carrot juices were concentrated up to 60–63 gTSS/100 g. Total antioxidant activity, aroma, colour and quality of the juices were better preserved during concentration

Cassano et al. (2003a)

Coconut water

The RO concentrated coconut water (20–25°Brix) was further concentrated up to 56°Brix by using OMD. The sensory analysis shows that there is not much significant difference between fresh coconut water and final concentrate

Rastogi et al. (2003)

Suggestions for future work 271

An integrated membrane process for the production of concentrated kiwi fruit juice has been evaluated. Fresh kiwi fruit juice, after enzymatic treatment was subsequently clarified/concentrated by UF and OD. The UF clarified juice was concentrated by an OD process up to about 60°Brix. A small reduction of total antioxidant (TAA) was observed and the vitamin C content was well preserved in the final concentrate (Cassano et al., 2003b). Possible integration of aqueous two-phase extraction (ATPE) with membrane processes such as OMD/DO is being explored for the purification and concentration of food colours (especially when there are proteins). The use of ATPE will enable desired products (enzyme/protein) to partition to one of the phases, thus purifying and reducing the volume of the process stream to be handled. Furthermore, the OMD/DO process can be used as pre-concentration step to reduce the water load on subsequent processing steps such as freeze drying or subsequent purification steps such as electrophoresis, chromatography, etc.

6.2 DO DO can be employed to improve the quality of grape wines by concentrating a low quality grape juice into a product with increased soluble solids. It can be used to reduce the alcohol content in wine or beer products. Speciality products and pharmaceutical products such as flavouring agents and aloe vera, which are used in many cosmetic applications, have been concentrated using DO (Wong et al., 1999). Anthocyanin extracted from red radish extract was concentrated by conventional evaporative technology followed by direct osmosis (Rodrigrez-Saona et al., 2001). Another novel application (integrated with electrodialysis and RO) has been in the recovery of about 97 per cent water from wastewater. The relative non-fouling behaviour of DO allows processing of wastewater containing oils and soap scum that quickly foul the RO system (Beaudry and Herron, 1997). Currently, one plant in the USA has gone commercial to concentrate vegetable juices (cited by Wong et al., 1999).

7 Suggestions for future work In this chapter, the mechanism of water transport, effect of process and membrane related parameters in both OMD/DO processes have been discussed along with the advantages and potential applications of these processes both at lab scale and pilot scale. However, there is still ample scope for future research and development. Efforts are required to develop and evaluate hybrid processes on a larger scale involving MF/UF, RO and OMD/DO (Figure 10.6). If OMD/DO is to be made a commercially viable option, development of suitable membranes with improved diffusional characteristics, selectivity, better pore geometry and stability and membranes with longer life cycles needs to be undertaken at affordable costs. Another major constraint for the wide spread commercial application of OMD/DO process is the management of spent OA solution. An effective and

272 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

Feed reservoir

Reconcentrated OA solution

OA solution reservoir OMD/DO module

Condenser

MF/UF module

RO module

RO concentrate Heat exchanger

Condensate To waste or recovery RO permeate OMD/DO concentrate

Evaporator

Lypholization (in case of natural colours)

Figure 10.6

Integrated concentration process for liquid foods and natural colours.

environmentally benign re-concentration technique for spent OA needs to be developed. Like any other membrane processes OMD/DO have the problem of low flux which needs to be addressed by the application of an acoustic field on a larger scale. Plant/marine algae constitute the major source of different natural colourants that differ in their colour and stability. Hence, efforts are required to carry out detailed studies in order to develop simple, efficient and economic methods for the processing of these natural colours and to develop possible process integration involving aqueous two-phase extraction (ATPE) with OMD/DO. In the view of the scientific and industrial potential of OMD/DO, even if some of these aspects, namely development of tailor-made membranes, optimization of process parameters and integration of process steps are addressed in greater depth by future researchers, it is certain that both OMD/DO processes will find their applications in the food and allied industry in the years to come.

8 Conclusions The potential advantages of athermal membrane processes (osmotic membrane distillation/direct osmosis) for the concentration of liquid foods and natural colours have

Nomenclature 273

been successfully demonstrated, with respect to maximum achievable concentration, improved product quality, ease of scale up and low energy consumption. Furthermore, efforts are required in the development of suitable membranes with improved diffusional characteristics, selectivity and with longer life cycles in order to make osmotic membrane distillation/direct osmosis viable options. Improvements of process engineering in terms of module design as well as process design and optimization are required in order to overcome the drawbacks like low flux. Integrated membrane processes involving microfiltration/ultrafiltration, reverse osmosis and osmotic membrane distillation/direct osmosis are expected to gain prominence in the near future for processing of liquid foods and natural colours. The integrated membrane processes allow improvements in the process efficiency and economy. Efforts are required also to develop possible integration of osmotic membrane distillation/direct osmosis with aqueous two-phase extraction for extraction, purification and concentration of natural colours and biomolecules. Due to the ever increasing cost of energy, it can be easily anticipated that the athermal membrane processes will be the technology of the future in the food and allied industries.

Acknowledgements The authors thank Dr V Prakash, Director, CFTRI, Mysore, for his encouragement and keen interest in the research work on athermal membrane processes. Financial assistance from the Department of Science and Technology, New Delhi, is gratefully acknowledged. Naveen Nagaraj and Ganapathi Patil thank the Council of Scientific and Industrial Research for Senior Research Fellowships.

Nomenclature B cp CSDI CSM D Dm h H J k K K KS kT

pore shape geometry factor heat capacity (J/kg/K) solute concentration in feed tank (mol/l) solute concentration in the membrane (mol/l) diffusion coefficient (m2 /s) moles of solute transported through the membrane in time dt (mole) liquid heat transfer coefficient (W/m2/K) total heat transfer coefficient (W/m2/ K) flux, mass (kg/m2/h), molar (mol/m2/s), volume (m3/m2/s) mass transfer coefficient in boundary layer (m/s) resistivity of the membrane (m2/h/kg) overall mass transfer coefficient (kg/m2/h/Pa) solute partition coefficient between adjacent solution and membrane thermal conductivity (W/m/K)

274 Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours

L M P Q R r S t T u xs Yln    




length of the membrane (m) molecular weights of constituents (kg/mol) vapour pressure (Pa) heat flux (W/m2) gas constant (kJ/mol/K) membrane pore radius (m) membrane surface area (m2) thickness of porous substructure (m) temperature (K or °C) velocity of the fluid (m/s) osmotic agent molar agent mole fraction of air (log-mean) [] porosity [] membrane thickness (m) difference surface tension (N/m) viscosity of the fluid (Pa/s) tortuosity factor [] contact angle (ds) density of the fluid (kg/m3) osmotic pressure (bar)

Hydrodynamic dimensionless numbers Nu Nusselt Pr Prandtl Re Reynolds Sc Schmidt Sh Sherwood Subscripts 1 feed side 2 OA side k knudsen diffusion L liquid M molecular diffusion S solute W water or vapour Abbreviations ATPE aqueous two-phase extraction DO direct osmosis DOC direct osmosis concentration MD membrane distillation MF microfiltration OA osmotic agent

References 275

OE OMD PP PTFE PVDF RO TSS UF

osmotic evaporation osmotic membrane distillation polypropylene polytetrafluoroethylene polyvinylidenedifluoride reverse osmosis total soluble solids ultrafiltration

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