5.5 The extraction and preparation of an enzyme

Enzyme recovery and purification are as relevant to the eco­nomics of production as the fermentation stage. Enzyme purification will be carried out only if the extra cost is justified by the intended application of the enzyme. The scale of the purification or downstream processing will dictate the choice of separation techniques as some are difficult to operate on a large scale. All microbial enzyme products that will be used in foods or medically related aspects are required to meet strict specifications with regard to toxicity (Table 5.6). At present, only a small number of microorganisms are used for enzyme production. Responsibility for the safety of an enzyme product remains with the manufacturer. In practice, a safe enzyme product should have low allergenic potential and be free of toxic materials and harmful microor­ganisms. Enzymes from animal and plant sources do not require toxicological studies to be performed. When enzymes are derived from microorganisms that are traditionally used in food or food processing, no testing is required (Table 5.6). Enzymes from other microorganisms may require extensive test­ing and also analysis for toxic metabolites such as exo- and endotoxins and mycotoxins. All bulk enzymes are supplied with a detailed Material Safety Data Sheet which covers potential dangers and also handling procedures for using the enzyme.

5.5 Immobilised enzymes

Almost 95% of all commercial enzymes are purchased in a soluble form, with the majority being used directly on a single-use basis in the areas listed in Table 5.1. The use of enzymes in a soluble or free form must be considered as very wasteful because the enzyme generally cannot be recovered at the end of the reaction. A new and valuable area of enzyme technology is that concerned with the immobilisation of enzymes on insoluble polymers, such as membranes and particles, which act as supports or carriers for the enzyme activity. The enzymes are physically confined during a continuous catalytic process and may be recovered from the reaction mixture and re-used over and over again, thus improving the economy of the process; this is merely a return to the natural immobilised state of most enzymes in living systems. Some enzymes that are rapidly inactivated by heat when in cell-free form can be stabilised by attachment to inert polymeric supports, while in other examples such insolubilised enzymes can be used in non-aqueous environments. Whole microbial cells can also be immobilised inside polyacrylamide beads and used for a wide range of catalytic functions. The variety of new enzymes and whole- organism systems that are likely to become cheaply available presents exciting possibilities for the future, especially in the pharmaceutical and diagnostic fields.

Table 5.8. The advantages of immobilised biocatalysts

1. Permits the re-use of the component enzyme(s)

2. Ideal for continuous operation

3. Product is enzyme-free

4. Permits more accurate control of catalytic processes

5. Improves stability of enzymes

6. Allows development of a multi-enzyme reaction system

7. Offers considerable potential in industrial and medical use

8. Reduces effluent disposal problems