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 bloo160 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