Chapter 6
Ion-exchange and electrodialysis
ALISTAIR S. GRANDISON, Department of Food Science and Technology, The
University of Reading, Reading RG6 6AP, UK
Ion-exchange and electrodialysis are distinct methods of separation, but can conveniently
be treated together, as the basic criterion for separation in both cases is the molecular
electrostatic charge. While ion-exchange involves retention of ionised solutes on a solid
support material, electrodialysis permits the separation of ions using selective ion-
exchange membranes.
6.1 ION-EXCHANGE
Ion-exchange methods can potentially be used for separations of many types of molecules
such as metal ions, proteins, amino acids or sugars. The technology is utilised in many
sensitive analytical chromatography procedures, frequently on a very small scale. On the
other hand industrial-scale production operations, such as demineralisation or protein
recovery, are possible. This chapter will consider only the larger-scale applications which
have current or potential use for production in the food and biotechnology industries.
6.1.1 Theory, materials and equipment
A brief summary of the theory of ion-exchange will be given here. More detailed ac-
counts can be found elsewhere (e.g. Vermeulen et al., 1984; Walton, 1983; Helfferich,
1962).
Solutelion-exchanger interactions
Ion-exchange could be defined as the selective removal of a single, or group of, charged
species from one liquid phase followed by transfer to a second liquid phase by means of a
solid ion-exchange material. In practice this involves the process of adsorption - the
transfer of specific solute(s) from a heterogeneous feed solution on to the solid ion-
exchanger. The mechanism of adsorption is electrostatic, involving opposite charges on
the solute(s) and the ion-exchanger. The feed solution is washed off, and this is followed
156 A. S. Grandison
by desorption, in which the separated species are recovered back into solution in a much
purified form.
The ion-exchange solids bear fixed ions which are covalently attached to a solid
matrix. There are two basic types of ion-exchanger:
(1)
(2)
Cation exchangers (sometimes called ‘anionic exchangers’) which bear fixed nega-
tive charges and are therefore able to retain cations, and
Anion exchangers (sometimes called ‘cationic exchangers’) which bear fixed posi-
tive charges.
Ion-exchangers can be used to retain simple ionised species, but may also be used in
the separation of polyelectrolytes which possess both positive and negative charges (i.e.
amphoteric molecules such as proteins) as long as the overall charge on the
polyelectrolyte is opposite to the fixed charges on the ion-exchanger. This overall charge
depends on the isoelectric point of the polyelectrolyte and the pH of the solution. At pH
values lower than the isoelectric point the net overall charge will be positive and vice
versa. In some circumstances it is even possible for ion-exchangers to retain macro-
molecules of like charge, presumably if a portion of the molecule carries a sufficient
opposite charge (Peterson, 1970). The main interaction is via electrostatic forces, and in
the case of polyelectrolytes the affinity is governed by the number of electrostatic bonds
between the solute molecule and the ion-exchanger. However, particularly with large
molecules such as proteins, multiple interactions may occur involving steric effects. Size
and geometric properties, and the degree of hydration of the ions may affect these
interactions, and hence the selectivity of the ion-exchanger for different ions. Charge
density may be more important than overall charge in determining the relative selectivity.
Figure 6.1 is a schematic diagram showing a generalised anion exchanger - i.e.
bearing fixed positive charges. To maintain electrical neutrality these fixed ions must be
balanced by an equal number of mobile ions of the opposite charge (Le. anions) which
are held by electrostatic forces. These mobile ions can move in and out of the porous
molecular framework of the solid matrix and may be exchanged stoichiometrically with
other dissolved ions of the same charge, and are termed counterions. Ion-exchange
systems can be considered to consist of two aqueous liquid phases - one confined within
Counter-ions
Imbibed solvent
Fig. 6.1. Schematic diagram of a generalised anion exchanger.
Ion-exchange and electrodialysis 157
the structure of the solid matrix in equilibrium with an outside phase. The interface
between the two phases acts as a semipermeable membrane which allows the passage of
any mobile ionic species depending on the Donnan equilibrium. This states that the
chemical potential of a salt must be the same inside and outside the ion-exchanger - e.g.
in the simplest case where the only mobile ions present are Na' and C1-, then at
equilibrium,
[Na'] [Cl-lInside phase = iNa'l [C1-lOutside phase
Thus a certain proportion of co-ions (mobile ions having the same sign - Na' in this
example - as the fixed ions) will be present even in the internal phase. Therefore, if an
anion exchanger (as in Fig. 6.1) is in equilibrium with a solution of NaC1, the internal
phase contains some Na' ions, although the concentration is less than in the external
phase because the internal concentration of C1- ions is much larger.
When an ion-exchanger is contacted with an ionised solution, equilibration between
the two phases rapidly occurs. Water moves into or out of the internal phase so that
osmotic balance is achieved. Counterions also move in and out between the phases on an
equivalent basis. If two or more species of counterion are present in the solution, they
will be distributed between the phases according to the proportions of the different ions
present and the relative selectivity of the ion-exchanger for the different ions. It is this
differential distribution of different counterions which forms the basis of separation by
ion-exchange. The relative selectivity for different ionised species results from a range of
factors. The overall charge on the ion and the molecular or ionic mass are the primary
determining factors, but selectivity is also related to degree of hydration, steric effects
and environmental factors such as pH or salt content.
In the adsorption stage, a negatively charged solute molecule (e.g. a protein P-) is
attracted to a charged site on the ion-exchanger (R') displacing a counterion (X-):
R+X- + P- -+ R'P- + X-
In the desorption stage, the anion is displaced from the ion-exchanger by a competing salt
ion (S), and hence is eluted:
R'P- + S- -+ R'S- + P-
Ion-exchangers may be further classified in terms of how their charges vary, with changes
in pH, into weak and strong exchangers. The terms strong or weak do not refer to the
strength of binding of the ions to the exchanger, or the mechanical strength of the matrix,
but to the pH range over which the materials are effective. Strong ion-exchangers are
ionised over a wide range, and have a constant capacity within the range, whereas weak
exchangers are only ionised over a limited pH range (e.g. weak cation exchangers may
lose their charge below pH 6 and weak anion exchangers above pH 9). Thus weak
exchangers may be preferable to strong ones in some situations, for example where
desorption may be achieved by a relatively small change in pH of the buffer in the region
of the pKa of the exchange group. Regeneration of weak ion-exchange groups is easier
than with strong groups, and therefore has a lower requirement of costly chemicals.
158 A. S. Grandison
Ion-exchange groups
Some common examples of cation exchangers are
-SO;H+ (strong - pK, 1-2)
--POi-(H+), (medium - pK, 2-3)
-COOH (weak - pK, 3.5-8)
Base function is almost invariably present as amines or imines. These are introduced
into the matries by chloromethylation, followed by reaction with the appropriate amine to
produce weakly to strongly basic ion-exchangers. Some common examples are
-O-CH2 -CH2 -NH+-(CH2 -CH3), (diethylaminoethyl - DEAE)
-0 - CH2 - CH2 - NH; (amino ethyl - AE)
- 0 - CH, - CH2 -N+(C2H5), -CH2 - CH(0H)-CH3 (quaternary
amino ethyl - QAE)
-CH2-N+(CH3), (quaternary amine - Q)
Q and QAE are strong anion exchangers while DEAE and AE are weak.
Ion-exchange materials
All ion-exchangers basically consist of a solid insoluble matrix to which are attached the
active, charged groups on which ion-exchange occurs. Various terms are used to describe
this material including resin, adsorbent, medium, or just ion-exchanger. There is no
general agreement on which is correct, and the usage is sometimes confusing - e.g. the
term ‘resin’ is sometimes used as a general term for ion-exchangers, or sometimes spe-
cifically for synthetic organic materials, while a resin is strictly a naturally occurring
organic compound (Kanekanian and Lewis, 1986).
The solid support must have an open molecular framework which allows the mobile
ions to move freely in and out, and must be completely insoluble throughout the process.
Most commercial ion-exchangers are based on an organic polymer network, although
inorganic materials may be used. The support material does not directly determine the
ionic distribution between the two phases, but it is a major factor in determining the
physical and chemical stability of the ion-exchanger. Hence this will determine factors
such as the capacity, the flow rate through a column, the diffusion rate of counterions into
and out of the matrix, the degree of swelling and the durability of the material. The
materials tend to be of two main types - xerogels or aerogels. Xerogels are insoluble
synthetic polymers containing a cross-linking agent. Their structure and porosity depends
on the solvent and degree of solvation and they are compressible to some degree.
Xerogels make up the majority of commercially available ion-exchangers including
polyacrylamides, polystyrene and dextrans. The pore size of these materials can be
controlled by the manufacturing conditions, especially the degree of cross-linking.
Aerogels have a much more fixed rigid structure (e.g. porous silica) and are therefore
incompressible, which has obvious advantages for production scale.
Ion-exchange and electrodialysis 159
As the adsorption is a surface effect, the available surface area is a key parameter. For
industrial processing the maximum surface area to volume should be used to minimise
plant size and product dilution. It is possible for a 1 ml bed of ion-exchanger to have a
total surface area >IO0 m2. The ion-exchange material is normally deployed in packed
beds, and involves a compromise between large particles (to minimise pressure drop) and
small particles to maximise mass transfer rates. Porous particles are employed to increase
surface area/volume. However, the surface must also be accessible to the solute
molecules, and hence materials with an enormous surface area due to the presence of
minute pores may be of very limited use, because much of this surface is inaccessible
even to small solute molecules. Manufacturers of ion-exchange materials generally quote
the exclusion limit of products with respect to molecular size. Particularly in the case of
biopolymers, the shape of the pores and the three-dimensional structure of the solute may
be a further consideration.
Capacity
The capacity of an ion-exchanger is defined as the number of equivalents of exchange-
able ions per kilogram of exchanger but is frequently expressed in meq/g (usually in the
dry form), and can be determined by titration of the charged groups with strong acid or
base. This property depends on the nature of the fixed ions as well as the available
surface area. Most commercially available materials have capacities in the range 1-10
equivalents/kg of dry material.
Blinding and fouling
The operational life of an ion-exchanger, or at least the time between major clean-up
campaigns, is limited by blinding or fouling. This is non-specific adsorption onto the
matrix surface, or within the pores, which effectively reduces the capacity, and certainly
affects the choice of ion-exchanger for a particular separation. The susceptibility of an
ion-exchanger to blinding or fouling with a particular feedstock may exclude its use for
that function despite having otherwise excellent binding capacity and specificity for the
molecules in question. For example, the presence of significant lipid levels in a feedstock
may exclude the use of some exchangers for protein separations.
Elution
The choice of method of elution depends on the specific separation required. In some
cases the process is used to remove impurities from a feedstock, while the required
compound(s) remains unadsorbed. No specific elution method is required in such cases,
although it is necessary to regenerate the ion-exchanger with strong acid or alkali. In
other cases the material of interest is adsorbed by the ion-exchanger while impurities are
washed out of the bed. This is followed by elution and recovery of the desired solute(s).
In the latter case the method of elution is much more critical - for example, care must be
taken to avoid denaturation of adsorbed proteins.
Elution of the adsorbed solute is effected by changing the pH or the ionic strength of
the buffer, followed by washing away the desorbed solute with a flow of buffer.
Increasing the ionic strength of the buffer increases the competition for the charged
sites on the ion-exchanger. Small buffer ions with a high charge density will displace
160 A. S. Grandison
polyelectrolytes which can subsequently be eluted. Altering the buffer pH so that the
charge on an adsorbed polyelectrolyte is neutralised or made the same as the charges on
the ion-exchanger will result in desorption.
Ion-exchange columns
Fixed bed operations consisting of one, or two columns connected in series (depending
on the type of ions which are to be adsorbed), are used in most ion-exchange separations.
Liquids should penetrate the bed in plug flow, in either downward or upward direction.
The major problems with columns arise from clogging of flow and the formation of
channels within the bed. Problems may also arise from swelling of organic matrices when
the pH changes.
Mixed bed systems
These may be used to avoid prolonged exposure of the solutions to both high and low pH
environments, as is frequently encountered when using anion and cation exchange
columns in series (e.g. during demineralisation of sugar cane juice to prevent hydrolysis
of sucrose as described below). Cation and anion exchangers are intimately mixed during
the adsorption phase so that the feed solution remains at high or low pH only for the time
required to pass from one particle to the next. Regeneration is possible on the basis that
the two exchange materials have different specific gravities, and thus separate into two
layers on backwashing. By the use of a regenerant distributor, strong acids and alkalis
may be used to regenerate the resins independently. After rinsing, the ion-exchangers are
remixed using compressed air.
Stirred tanks
The flow and swelling problems encountered with fixed beds are obviated by the use of
stirred tanks; however, these systems are less efficient and expose the ion-exchangers to
mechanical damage as there is a need for mechanical agitation. The system involves
mixing the feed solution with the ion-exchanger and stirring until equilibration has been
achieved (typically 30-90 min in the case of proteins - Kanekanian and Lewis, 1986).
After draining and washing the ion-exchanger, the eluant solution is then contacted with
the bed for a similar equilibration time before draining and further processing.
6.1.2 Applications of ion-exchange in the food and biotechnology industries
One method of classifying the applications of ion-exchange could be by industries or
commodities. The main areas of the food industry where the process is currently used or
is being developed are sugar, dairy and water purification, although sufficient
applications occur outside these to render this classification unsatisfactory. Ion-exchange
is widely employed in the recovery, separation and purification of biochemicals,
monoclonal antibodies and enzymes.
Another way of categorising the applications is by the type of separations attained, for
example:
(1)
(2)
(3)
removal of minor components, e.g. deashing or decolorising;
enrichment of fractions, e.g. recovery of proteins from whey or blood;
isolating valuable compounds, e.g. production of purified enzymes.
Ion-exchange and electrodialysis 16 1
Alternatively the chemical nature of the adsorbed ions could be used as a basis for
classification. Any ionisable component of a foodstuff can potentially be adsorbed on to
an ion-exchanger and thus separated.
The following is an attempt to classify applications in food and biotechnology on the
basis of the function of the process.
Softening
Softening of water and other liquids involves the exchange of calcium and magnesium
ions for sodium ions attached to a cation exchange resin, e.g.
R-(Na+)2 + Ca(HC03)2 -+ R-Ca2+ + 2NaHC03
The sodium form of the cation exchanger is produced by regenerating with NaCl
solution, Apart from the production of softened water for boiler feeds and cleaning of
food and processing equipment, softening may be employed to remove calcium from
sucrose solutions prior to evaporation (which reduces scaling of heat exchanger surfaces
in sugar manufacture), and from wine (which improves stability) (Cristal, 1983).
Deminerulisution
Demineralisation using ion exchange is an established process for watsr treatment, but
over the last 20 years it has been applied to other food streams. Typically the process
employs a strong acid cation exchanger followed by a weak or strong base anion
exchanger. The cations are exchanged with H+ ions, e.g.
2R-Hf + CaS04 -+ (R-)2Ca2+ + H2S04
R-H' + Na'C1- + R-Na' + H+Cl-
and the acids thus produced are fixed with an anion exchanger, e.g.
R'OH- + H'Cl- -+ R'CI- + H20
Demineralised cheese whey is desirable for use mainly in infant formulations, but also in
many other products such as ice cream, bakery products, confectionery, animal feeds etc.
The major ions removed from whey are Na', K+, Ca2+, Mg2+, C1-, HPO,, citrate and
lactate. Ion-exchange demineralisation of cheese whey generally employs a strong cation
exchanger followed by a weak anion exchanger (Houldsworth, 1976). This can produce
more than 90% reduction in salt content, which is necessary for infant formulae. Lower
levels of demineralisation, obtained using a by-pass system, may be adequate for other
applications. Due to the high salt content of whey, the system must be regenerated after
the treatment of 10-15 bed volumes of whey. This is achieved, following rinsing, by the
treatment of cation and anion exchangers separately with strong acids and alkalis
respectively. Typically a cycle is about 6 h, of which 4 h are required for regeneration,
therefore two or three parallel systems may be necessary. The use of countercurrent
regeneration reduces the consumption of regeneration chemicals.
Jonsson (1984) described the SMR (Swedish Dairies Association) process for whey
demineralisation, in which the whey first enters a weak anion column in which the whey
anions are exchanged for HCOT ions. Following this a weak cation column exchanges the
162 A. S. Grandison
whey cations for NH;. The whey salts are thus exchanged for ammonium bicarbonate
which decomposes to NH3, C02 and water during subsequent evaporation, the NH, and
C02 being recovered. Jonsson and Arph (1 987) compared conventional ion-exchange
demineralisation of cheese whey to the SMR process and concluded that the requirement
for regeneration chemicals and production of waste chemicals are much reduced in the
SMR process.
Demineralisation by ion-exchange resins is used at various stages during the manufac-
ture of sugar from either beet or cane, as well as for sugar solutions produced by hydroly-
sis of starch. In the production of sugar from beet, the beet juice is purified by liming and
carbonatation and then may be demineralised by ion-exchange (McGinnis, 197 1). The
carbonated juice is then evaporated to a thick juice prior to sugar crystallisation.
Demineralisation may, alternatively, be carried out on the thick juice which has the
advantage that the quantities handled are much smaller, but is limited by the fact that
diffusion rates are low at high sugar concentrations. To produce high-quality sugar the
juice should have a purity of about 95%. Rousseau (1984) described the ‘new
demineralisation/demi’ process which utilises a mixed bed of weak cationic and weak
anionic resins in a batchwise process to treat the thick juice (dry matter 70%). This gives
rise to a very pure thick juice with minimum dilution, with the bonus of a decolorisation
at no extra cost. A further application in beet sugar production is the Quentin process by
which the sugar level of molasses can be decreased. This is achieved by exchanging
potassium and sodium ions of the juice prior to the final crystallisation, for magnesium
using a strongly acidic cation exchanger. Magnesium is less molassigenic than alkaline
ions.
Ash removal or complete demineralisation of cane sugar liquors has been described by
Chen (1985). The process is carried out on liquors that have already been clarified and
decolorised, so the ash load is at a minimum. The use of a mixed bed of weak cation and
strong anion exchangers in the hydrogen and hydroxide forms, respectively, reduces the
prolonged exposure of the sugar to strongly acid or alkali conditions which would be
necessary if two separate columns were used. Destruction of sucrose is thus minimised.
The cation and anion resins are sometimes used in their own right for dealkalisation or
deacidification, respectively. Weak cation exchangers may be used to reduce the
alkalinity of water used in the manufacture of soft drinks (Carney, 1988) and beer (Cristal,
1983), while anion exchangers can be used for deacidification of fruit and vegetable
juices (Lue and Chiang, 1989; Dechow et al., 1985). In addition to deacidification, anion
exchangers may also be used to remove bitter flavour compounds (such as naringin or
limonin) from citrus juices (Johnson and Chandler, 1985). Anion or cation exchange
resins are used in some countries to control the pH or titratable acidity of wine (Rankine,
1986; Bonorden et al., 1986) although this process is not permitted by other traditional
wine producing countries. Acidification of milk to pH 2.2, using ion-exchange during
casein manufacture by the Bride1 process, has also been described (Pierre and Douin,
1984).
Ion-exchange processes can be used to remove specific metals or anions from drinking
water and food fluids, which has potential application for detoxification or radioactive
decontamination. For example, procedures have been described for the removal of lead
(Brajter and Slonawska, 1986), barium and radium (Snoeyink et al., 1987), aluminium
Ion-exchange and electrodialysis 163
(Pesavento et al., 1989), uranium (Sorg, 1988) and nitrates (Lauch and Guter, 1986) from
drinking water. Removal of a variety of radionuclides from milk has been demonstrated.
Radiostrontium and radiocaesium can be removed using a strongly acidic cation ex-
changer (Tait et al., 1989; Koga et al., 1968), while I*31 can be adsorbed on to a variety
of anion exchangers (Barth et al., 1970). The production of low sodium milk, with
potential dietetic application, has been demonstrated (Nakazawa and Hosono, 1989).
Decolorisation
Demineralisation processes may have the added benefit of colour removal. There are,
however, other cases where colour removal is required without demineralisation.
Sugar liquors from either cane or beet contain colourants such as caramels,
melanoidins, melanins or polyphenols combined with iron. Many of these are formed
during the earlier refining stages, and it is necessary to remove them in the production of
a marketable white sugar. The use of ion-exchangers just before the crystallisation stage
results in a significant improvement in product quality. It is necessary to use materials
with an open, porous structure to allow the large colourant molecules access to the
adsorption sites. Chen (1985) described the use of strongly basic resins operated in the
chloride cycle for decolorisation during cane sugar refining. These are sometimes the
only decolorising systems used, but in other cases complement the use of carbon
adsorbents. Bohm and Schafer (1969) described the decolorisation of beet sugar juice on
an industrial scale using ion-exchange resins.
A new approach to the use of ion-exchange for decolorisation of sugar solutions is the
application of powdered resin technology. Finely powdered resins (0.005-0.2 mm diam-
eter) have a very high capacity for sugar colourants due to the ready availability of
adsorption sites. The use of such materials on a disposable basis eliminates the need for,
and the accompanying disposal problems of, chemical regenerants, as well as removing
the problem of sugar dilution which occurs during column operation. However, the
advantages must be weighed against the added expense of discarding expensive resins
after a single use (Chen, 1985). Colour reduction of fermentation products such as wine
has also been described. Brown et al. (1988) used a strongly basic anion exchanger to
remove colouring matter, followed by a strong cation exchanger to restore the pH. It is
claimed that colour reduction can be achieved without substantially deleteriously affect-
ing the other wine qualities.
Protein purification
Ion-exchange can be used successfully in many protein purification processes in the food
and pharmaceutical industries. High purity protein isolates can be produced in a single
step from dilute solutions containing other contaminating materials. The process
compares favourably with competing techniques in terms of cost and efficiency. The
amphoteric nature of protein molecules permits the use of either anion or cation exchang-
ers, depending on the pH of the environment. Elution takes place by either altering the
pH or increasing the ionic strength. The eluate can be a single bulk, or a series of
fractions produced by stepwise or linear gradients, although fractionation may be too
complex for large-scale industrial production. Separation of a single protein may take
place on the basis that it has a higher affinity to the charged sites on the ion-exchanger
164 A. S. Grandison
compared to other contaminating species, including other proteins present in the feed. In
such cases, if excess quantities of the feed are used, the protein of interest can be
adsorbed exclusively, despite initial adsorption of all the proteins in the feed (Kanekanian
and Lewis, 1986). Alternatively it may be possible to purify a protein on the basis that it
has a much lower affinity for the ion-exchanger than other proteins present in the feed,
and thus the other proteins are removed, leaving the desired protein in solution.
One limitation of the process for protein treatment is that extreme conditions of pH,
ionic strength and temperature must be avoided to prevent denaturation of the protein.
An area of great potential is the recovery of proteins from whey. It is estimated (van
Hoogstraten, 1987) that about 110 million tonnes of whey are produced each year as a
by-product of the manufacture of cheese and related products such as casein. Typically
whey contains 0.648% protein, which is highly nutritious and also displays excellent
physical properties, yet the vast majority of this is wasted or under-utilised. The Vistec
protein recovery process employs carboxymethyl cellulosic anion-exchange materials to
produce high purity functional protein from cheese whey (Jones, 1976; Palmer, 1977).
The system uses a stirred tank reactor into which the whey is introduced at low pH.
Following rinsing of non-adsorbed material, the protein fraction is eluted at high pH, and
further purified by ultrafiltration so that the final protein content is approximately 97%
(on a dry matter basis). The product is commercially exploited by the Bio-isolates com-
pany (Fig. 6.2). Ayers and Petersen (1985) have described a similar process based on
sulphopropyl cellulosic materials which is also used for commercial recovery of cheese
whey protein. Silica-based ion-exchangers such as Spherosil have the advantage that they
are rigid and do not swell or contract when the pH or ionic strength of the environment
Fig. 6.2. Ion-exchange recovery of food proteins (with permission of Bio-isolates plc).
Ion-exchange and electrodialysis 165
are altered, and are used commercially for whey protein recovery in the mne-Poulenc
process (Mirabel, 1978). However, the capacity may be lower than the cellulose-based
materials. Skudder (1983, 1985) has demonstrated the use of Spherosil QMA to produce
fractions of total whey protein as well as further fractionating the proteins into their
separate components or groups of components. This approach has the potential of
producing protein fractions with a range of functional properties which could be
extremely valuable for use in the food industry. The method is not yet carried out
commercially to the author's knowledge, possibly due to the complex operating
procedures required, and the relatively low capacity of Spherosil. Another application of
adsorption of whey protein by ion-exchangers could be to improve the heat stability of
milk (Kelly, 1982). The use of ion-exchange to recover or separate the caseins in milk is
not carried out commercially, although it has been shown to be feasible - e.g. Ng-Kwai-
Hang and Pelissier (1989).
Jones (1976) has described the use of the Vistec system (see above) for continuous
recovery of food protein from protein-containing waste streams other than whey. A
10 000 gal d-' pilot plant system, recovering protein from abattoir effluent, had been run
continuously for 3 months. The recovered protein could then be incorporated into animal
feeds. The same author has also demonstrated the use of this system for recovery of food
proteins from waste streams resulting from the processing of soya, fish, vegetables and
gelatine production. Such protein fractions could be used as functional proteins in the
food industry.
Howell and Lawrie (1983) employed anion-exchange using DEAE-Sephadex to
fractionate porcine plasma proteins in an attempt to maximise use of by-products of the
meat industry. While the properties of such fractions would clearly be desirable for their
use as functional food proteins, the stepwise elution may be too complex for commercial
application.
Various other food proteins have been purified or fractionated by ion-exchangers,
including pea globulins (Gueguen et al., 1984), gliadin from wheat flour (Charbonnier
and Mosse, 1980), egg protein (Parkinson, 1967) and groundnut and soya protein
(Satyanarayana et al., 1981).
Purification of proteins from fermentation broths usually involves a series of separa-
tion steps and frequently includes ion-exchange. Hammond and Scawen (1989) reviewed
the use of ion-exchange in the high-resolution fractionation of proteins in downstream
processing. Werner and Berthold (1988) discussed the purification by ion-exchange of a
range of recombinant DNA-derived proteins, produced by fermentations, which can be
used as active ingredients in pharmaceuticals. The industrial production of recombinant
insulin from Escherichia coli fermentation involves an ion-exchange purification (Prouty,
1989).
Large-scale purification of a variety of enzymes has been described. For example,
a-amylase recovery from Aspergillus awamori (Bhella and Altosaar, 1985) or L-leucine
dehydrogenase from Bacillus cereus (Schuette et al., 1985) may be achieved by ion-
exchange. Porter et al. (1991) described the large-scale purification of P-galactosidase
from soybean meal using a combination of strong anion- and strong cation exchangers.
Ion-exchange is used in the recovery, separation and purification of monoclonal anti-
bodies which are used for high-resolution diagnostic purposes. Duffy et al. (1988)
166 A. S. Grandison
describe the purification of kilogram quantities of monoclonal antibodies by ion-
exchange.
Purification of other compounds
Ion-exchange has been used for numerous other separations involving food and bio-
chemicals, which do not fit into the above categories.
Fructose production is of great interest as it is considerably sweeter than sucrose and
glucose, and can be used as a natural sweetener at reduced caloric intake. Although
present in many natural sources, it is produced commercially from corn starch by
hydrolysis to dextrose, which is then partially converted to fructose using the enzyme
isomerase. The resulting high fructose corn syrup may be deionised by ion-exchange and
then a pure fructose fraction can be recovered with a sulphonic cation exchanger (such as
Amberlite IR-140). The separation is based on the fact that such resins, in the divalent
salt form, exhibit a slightly higher affinity for fructose than glucose (Kunin, 1979). In
practice a small volume of syrup containing both sugars is placed on top of a bed of the
ion-exchanger and slowly displaced with water so that the glucose travels more rapidly
down the column than fructose. By alternating the feed of syrup with eluting water, a
series of ‘cuts’ of fructose and glucose may be collected. Although the mechanism is
complex, the process is carried out successfully in a number of commercial plants.
A further potential application is the production of lactose-free milk. A process using
sulphonated cation exchangers has been used to reduce the lactose level of skim milk to
<lo% of that in the feed, while retaining >90% of protein, minerals and citrate (Harju,
1987).
Many products derived from fermentation processes are purified by ion-exchange. The
following are some examples. Kunin (1974) described the recovery of the aminoglycoside
antibiotics streptomycin and neomycin directly from a fermentation broth using an ex-
panded bed which permits particulate matter to pass through without clogging the bed.
Purification of fermentation-derived ethanol for use as fuel is possible using a combina-
tion of strongly basic and strongly acidic ion-exchangers (Rohm and Haas (patent),
1988). The purification of phenylalanine, which may be used in sweetener production,
from fermentation broths using cationic zeolite material, has been patented (UOP (pat-
ent), 1990).
Ion-exchange may also be used to purify enzymic reaction products, e.g. Heinzler et
al. (1987) incorporated the process in a system for recovery of flavour constituents from
the enzymic degradation of fruit wastes.
6.2 ELECTRODIALYSIS
Electrodialysis (ED) can be used to separate ionic species in the food and biotechnology
industries. The process permits the separation of electrolytes from non-electrolytes, con-
centration or depletion of electrolytes in solutions, and the exchange of ions between
solutions.
Ion-exchange and electrodialysis 167
6.2.1 Theory and equipment
Separation occurs due to electromigration of ions through membranes, which depends on
the electrical charge on the molecules, combined with their relative permeability through
membranes. Separations are based on the use of ion-selective membranes which are
effectively sheets of ion-exchange resins. The membranes are composed of polymer
chains which are cross-linked and intertwined into a network, and bear either fixed
positive or fixed negative charges. These may be heterogeneous membranes which
consist of ion-exchange resins dispersed in a polymer film, or, more commonly,
homogeneous membranes in which the ionic groups (-NHi or -SOT) are attached
directly to the polymer. Counterions are freely exchanged by the fixed charges on the
membranes and thus carry the electric current through the membranes, while co-ions are
repelled by the fixed charges and cannot pass through the membrane. Therefore cation
membranes allow the passage of positively charged ions, while anion membranes allow
the passage of negatively charged ions.
In practice the cation and anion membranes are usually arranged alternately with
plastic spacers (Fig. 6.3) to form thin solution compartments as shown schematically in
Fig. 6.4. In commercial practice 100-200 membranes may be assembled to form a
membrane stack (Fig. 6.5), and an ED system may be composed of one or more stacks.
Commercial ED membranes may be as large as 1-2m2. The spacers must maintain a
constant spacing between the membranes, but must not cover a large fraction of the
membrane surface or cause stagnation of the fluids. This is usually achieved by either
using a netting material which also promotes turbulence, or the use of a tortuous path
arrangement which forces the liquid stream into a long pathway before leaving the cell.
The basic unit of a membrane stack is called a cell pair and comprises a pair of
membranes and spacers as illustrated in Figs. 6.3 and 6.4. A positive electrode at one end
Fig. 6.3 Electrodialysis membranes and spacers (with permission of Ionics Inc.).
168 A. S. Grandison
Concentrated Demineralized
effluent product
cg;:; <1 c l*j A r :igt
Cathode - 1 , [n:[l 0 -0-0 7 1 Anode +
1 7 1 1 i 1 7 Anode
feed
Cathode
feed
r------
I
Brine Feed
stream
Fig. 6.4. Schematic diagram of electrodlalysis process
Fig 6 5. Electrodialysis membrane stacks (with permission of Ionics Inc.).
and a negative electrode at the other permit the passage of a d.c. current. The electrical
potential causes the anions to move towards the anode and the cations to move towards
the cathode. However, the ion-selective membranes act as barriers to either anions or
cations. Hence, anions migrating towards the anode will pass through anion membranes,
but will be rejected by cation membranes, and vice versa. The membranes, therefore,
form alternating compartments of ion-diluting (even numbered compartments in Fig. 6.4)
Ion-exchange and electrodialysis 169
and ion-concentrating (odd numbered) cells, If a feed stream containing dissolved salts
(e.g. cheese whey) is circulated through the ion-diluting cells and a brine solution through
the concentrating cells, free mineral ions will leave the feed and be concentrated in the
brine solution. Demineralisation of the feed is, therefore, achieved. Note that charged
macromolecules, such as proteins, will attempt to migrate in the electrical field, but will
not pass through either anion or cation membranes due to their molecular size. The
efficiency of electrolyte transfer is determined by the current density and the residence
time of the solutions within the membrane cells. The energy required to produce a certain
separation can be calculated from Faraday’s law, as described by Lopez Leiva (1988a).
The electrodes are bathed in a solution of an electrolyte which is circulated to remove
gases produced by the discharge of ions, and other ionised species. In practice the dem-
ineralisation is limited by the decreasing electrical conductivity of the feed as the process
proceeds - e.g. 90% de-ashing is considered to be the practical limit for whey, and much
greater production capacity is possible if lower levels of de-ashing are acceptable (de
Boer and Robbertsen, 1983). To obtain greater levels of de-ashing it is possible to use
combined ion-exchange/ED plants.
Alternative configurations of ion-exchange membranes are possible. Ion replacement
can be achieved using either cation- or anion-exchange membranes only. The example
shown in Fig. 6.6(a) is a process where cation-exchange membranes are used to replace
X+ ions with Y+ ions, A more efficient substitution of ions is possible using a
configuration as shown in the example for cations in Fig. 6.6(b). In this case three distinct
streams are used - donor, product and acceptor. Very high degrees of substitution are
possible if strong brine solutions are employed. One particular advantage of this approach
is that pH adjustment can be made without increasing the salt level as would occur by
addition of acid - Le. H+ ions are added to the solution without the addition of an anion.
Commercial applications of ED have depended on the development of membranes of
high mechanical strength, low electrical resistance and high ion-selectivity. As with any
membrane process ED membranes are subject to concentration polarisation and
fouling. This limits the rate of demineralisation, so that the process cannot be accelerated
at will by increasing the current density. In particular, the increased concentrations of
salts near the membrane/brine interface may lead to the precipitation of scale. Suitable
process design, especially to minimise boundary layers, and appropriate cleaning regimes
must be employed. The most serious fouling problem in conventional ED is frequently
fouling of the anion exchange membrane by negatively charged, colloidal organic matter.
One possible solution to this is to employ ‘transport depletion’ in which the anion-
exchange membranes are replaced by neutral membranes (Lopez Leiva, 1988a) as shown
in Fig. 6.6(c). Longer processing times, easier cleaning and higher current densities may
result, but effectiveness of demineralisation is reduced as only one set of membranes is
selective.
6.2.2 Applications of ED in the food and biotechnology industries
The largest application of ED has been in the desalination of brackish water to produce
potable water. In Japan, all the table salt consumed is produced by ED of sea water
(Lopez Leiva, 1988a).
Anode
-
Cathode
-
> whey powder
(95-98% T.S.)
tanks
25%
T.S. whey
,,
Reduced mineral
172 A. S. Grandison
Removal of calcium from milk has the benefit of improving the stability of the casein
during freezing, Improvement of the protein stability of frozen milk following ED has
been demonstrated (Lonergan et al., 1982). Removal of calcium from buffalo milk by ED
has also been carried out as part of a process to simulate human milk for infant nutrition
(Kuchroo and Ganguli, 1980). As with ion-exchange, ED can also be used to remove
radioactive metal ions from milk (Thiele, 1969).
Improved flavour and textural quality of fermented milk products following ED
demineralisation and deacidification has been described (Bodor et al., 1987).
ED can be used to extract salts from grape musts and wine and hence improve their
stability. Relatively modest reductions in potassium are sufficient to prevent precipitate
formation, the precise amounts being dependent on the type of wine (Lopez Leiva,
1988b). It is not desirable to process for longer than necessary as ED is associated with
small losses of alcohol or sugar. If concentrated grape musts are used as sweetening
agents in wine it is necessary to reduce levels of potassium and tartaric acid substantially
in order to prevent formation of precipitates. This has been achieved successfully by ED
(Audinos et al., 1985; Escudier et al., 1989). A further application in vinification could
be to control the sugar/acid ratio in wine by either deacidification of the grape musts by
ion substitution ED using anionic membranes, or acidification using cationic membranes
(Wucherpfenning and Keding, 1982).
The process can be used to produce deacidified fruit juices, either to reduce the
sourness of the natural juices, or possibly for the health food market. A system using only
anion-exchange membranes is required, in which the citrate ions of the juice are replaced
by OH- ions. Pilot plant studies have shown that acidity of apple juice could be reduced
from 1.5 to 1.1% w/v, and lemon juice from 5.8 to 4.8% w/v (Lopez Leiva, 1988b).
However, Adhikary et al. (1987) reported that such products displayed adverse
organoleptic properties.
ED could potentially be employed in the refining of sugar from either cane or beet. In
fact commercial applications in these industries are limited by the severe membrane
fouling problems caused by the presence of a range of organic compounds. However, a
Japanese plant for the demineralisation of cane sugar syrup, following chemical
pretreatment, has been described (Yamauchi et al., 1985). Incorporation of ED in a
process replacing carbonatation with membrane processes for the purification of sugar
beet juices has been described (Hanssens et al., 1984), but this has not been used com-
mercially.
A further application of ED in the food industry is desalination of spent pickling brine
(Wan Der Pan et al., 1988).
ED has found a variety of applications in biotechnology. It is particularly applicable as
a means of process control (e.g. altering the pH of fermentation media or enzymic
reactors), or as a component of downstream processing. The process can be integrated
into continuous fermentation or reactor designs.
In fermentation technology, ED has been used to isolate inhibitive metabolites to
allow continuous fermentation. The possibility of increasing the production rate of lactic
acid by up to six times using ED has been demonstrated (Murdia et al., 1990). Similar
applications include the preparation of acetic acid (Rehmann and Bansch, 1989) and
propionic acid (Boyaval and Corre, 1987). Also Nomura et al. (1988b) have used ED for
Ion-exchange and electrodialysis 173
continuous product removal, and hence increased fermentation rates, during acetic acid
production. Similarly Prigent and Franco (1984) used ED for continuous extraction of
sodium lactate during the fermentation of lactose. Wang et al. (1991) used a cation
exchange membrane in the electrochemical production of L-cysteine, which has potential
use in the food and pharmaceutical industries. The production of a range of amino acids
using an integrated downstream process incorporating ED has also been described (Tichy
et al., 1990). Reed (1984) described the use of ED for the purification of genetically
engineered proteins, while Aretz and Sauber (1990) have incorporated the process in the
purification of enzymes. Heinzler et al. (1987) designed a membrane reactor incorporat-
ing ED for enzymic degradation of pectin in waste fruit products (such as apple pomace
or citrus peel). The reaction products - D-galacturonic acid, oligogalacturonic acid - can
be used as flavourings or pharmaceuticals.
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