polyelectrolytes which can subsequently be eluted, Altering the buffer pH so that the harge 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(depe on the type of ions which are to be adsorbed), are used in most ion-exch paration Liquids should penetrate the bed in plug flow, in either downward or 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 he 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 ommodities. The main areas of the food industry where the process is currently used or 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 (1) removal of minor components, e.g. deashing or decolorising (3)isolating valuable compounds very of protg of purified enzymes (2) enrichment of fractions, e.g. red om whey or bloo
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 161 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 biotechnoloy sis 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(HCO3)2-R-Ca-T+ 2NaHCO 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) Demineralisation Demineralisation using ion exchange is an established process for water 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"H++CaSO 4>(R")2Ca2++H2SO4 R-H++Na+ Na++HcL and the acids thus produced are fixed with an anion exchanger, e. g R+OH-+HtCl-→RCl+H2O 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, Ca, Mg+, CI", HPO4, citrate and lactate. lon-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 recurrent 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 HCO3 ions. Following this a weak cation column exchanges the
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+Cland 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 NH4. The whey salts are thus exchanged for ammonium bicarbonate which decomposes to NH3, CO2 and water during subsequent evaporation, the NH3 and CO2 being recovered. Jonsson and Arph(1987)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, 1971). 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%0). This gives rise to a very pure 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 alkali 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 imonin)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(rankin 1986: Bonorden et al., 1986)although this process is not permitted by other tradition wine producing countries. Acidification of milk to pH 2. 2, using ion-exchange during casein manufacture by the Bridel process, has also been described(Pierre and Douin 1984) Ion-exchange processes can be used to remove specific metals or anions from drinking ater 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
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 manufacture of sugar from either beet or cane, as well as for sugar solutions produced by hydrolysis 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