The Preparation Of Bulk Enzymes For Food

Although the food industry accounts for a large proportion of industrial enzyme use, the majority of these enzymes carry out a narrow range of reaction types. By far the majority are hydrolases, which catalyse the breakdown of biological polymers such as proteins and polysaccharides (Table 1). As a consequence of their similar biological functions, they tend to share certain characteristics which make them particularly amenable to technological use on a large scale in food processes. Generally, they are secreted by the cell into the growth medium in order to hydrolyse polymers into smaller components which can then be transported into the cell for further metabolism. In commercial production, this overcomes the need for cell disruption. As these enzymes operate in a dilute environment compared to the cell cytoplasm, they are synthesised in large amounts. In addition, they are also in a relatively high state of purity at the start of downstream processing compared to cytoplasmic enzymes. The importance of this in maintaining low final product costs is illustrated by Figure 1 (Dwyer, 1984). Secreted enzymes also tend to be relatively stable as they have to operate in potentially harsh extracellular environments. The recovery rates from fermentation broths are therefore high and the losses of activity on storage and during commercial use are low.

The current applications of enzymes in the food industry do not require a high degree of enzyme purity. The main objective of downstream processing is therefore to obtain an active enzyme preparation in a form convenient for transport, storage and use. The purification of the specific active enzyme is not usually a consideration. Indeed, a typical industrial enzyme preparation may contain less than 5% enzyme protein (Cowan, 1991). In addition to fermentation residues which are not removed, other components may also be added for bulking and stabilization.

The main processes involved in the preparation of such commercial enzymes are the separation of the biomass from the fermentation medium and the removal of water to obtain the required product concentration. This can be achieved by low-selectivity separation processes such as centrifugation and filtration which are widely used in food processing for a variety of tasks. Figure 2 shows a typical processing stream for the preparation of bulk enzymes based on data from Atkinson & Mavituna (1991).

As enzymes are biological catalysts, a small amount may be used to carry out a biotransformation on a comparatively large amount of substrate. In the majority of current commercial uses in the food industry, enzymes are added in bulk directly to the substrate preparation. Using this method the enzyme cannot be recovered and reused and so its cost must be sufficiently low for it to be consumed during each process batch. Expensive enzymes which are produced in small amounts could not be used in this way in most food-related processes. Techniques such as encapsulation and immobilization can facilitate the reuse of the enzyme preparation over many substrate batches and can enable their application to more sophisticated and controlled processes. These require high-purity enzyme preparations if the maximum enzyme activity is to be achieved by the system. High resolution separation techniques are required to produce enzyme preparations which are free of most other contaminating proteins. The main areas covered by this chapter are therefore those recent developments in bioseparations which may contribute towards making the use of high-purity enzyme preparations more economically viable for food-related applications.


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