Enzymes, classed as microbial or non-microbial, are used to catalyse chemical reactions but they are specific: proteases break down proteins, amylases break down starches and lipases break down lipids and fats. Exogenous enzymes are used to break down elements of plant tissues and cell-wall-degrading enzymes are present in fruits and vegetables. Pectic enzymes may be used to increase the efficiency of juice and colour extraction from grapes, citrus fruits and apples. Suspension of intact cells may be achieved with enzymes that promote cell separation without affecting the cell wall. Complete liquefaction of fruit tissues can be brought about by pectic and cellulolytic enzymes (Whittaker et al., 2002). Cellulase and proteinases have been used to hydrolyse the plant cell wall and allow extraction of proteins. Fermentation may be used to modify structure for a subsequent process such as extraction (Aguilera and Stanley, 1990).
More broadly, total liquefaction or partial solubilisation will enhance extractability of phytochemicals or biopolymers. Enzyme use can be before, during or after other processes.
An important factor determining the use of enzymes in a technological process is their expense; they also lose activity with time as a result of denaturation. When used in a soluble form, enzymes retain some activity that cannot always be economically recovered for re-use. This residual activity remains to contaminate the product and its removal may involve extra purification costs; the enzyme may need to be inactivated to halt the reaction.
A process window, defined in terms of pH and temperature, can be found where the reaction can be carried out with optimal yield and minimal biocatalyst costs. The maximal yield is then only defined by the catalysed reaction. Once the process conditions and the end-point have been chosen in the process window, the enzyme costs per kilogram of product are influenced by the type of reactor (batch, or continuous stirred tank or fixed-bed reactor) selected to carry out the process.
An alternative technique to conventional enzyme processing is enzyme immobilisation using a substrate, for example porous particles that can easily be filtered off at the end of the process. In these systems, the kinetics differ from those of systems with free enzymes, as the mass transfer inside and to and from the particles with the biocatalyst causes the formation of concentration and pH gradients that influence rates and yields. The plant size needed for continuous processes is two orders of magnitude smaller than that required for batch processes using free enzymes (Buchholz et al., 2005).
Table 8.1 describes some advantages and disadvantages of cells and enzymes as biocatalysts in comparison with chemical catalysts. In summary, the application of enzyme technology is diverse as a process adjunct in the food industry. Enzymes are associated with (Uhlig, 1998): (1) juice and wine production; (2) sterile filtration of plant extracts; (3) cheese ripening; (4) malting in beer production; (5) starch processing; (6) improved preservation of juice concentrates.
Table 8.1 Advantages and disadvantages of cells and enzymes as biocatalysts in comparison with chemical catalysts (from Buchholz et al. 2005)
• Low energy consumption
• Less by-products
• Non-toxic when correctly used
• Can be degraded biologically
• Can be produced in unlimited quantities
• Cells and enzymes are
- unstable at high temperatures
- unstable at extreme pH values
- unstable in aggressive solvents
- inhibited by some metal ions
- hydrolysed by peptidases
- are still very expensive
- require expensive co-substrates
• When inhaled or ingested enzymes are potential allergens
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