Fundamental Food Microbiology, Third Edition - Fuad Fathir

FOOD INGREDIENTS AND ENZYMES OF MICROBIAL ORIGIN 251
Figure 17.1 Schematic presentation of four methods of immobilization of enzymes.
resins) and washing away the unattached molecules. The association is very weak,
and the molecules can be desorbed and removed.
2. Covalent Bonding
The enzyme molecules are covalently bound to a solid surface (such as porous
ceramics) by a chemical agent. The enzyme molecules are accessible to the substrate
molecules. The enzymes are more stable.
3. Entrapping
The enzyme molecules are enclosed in a polymeric gel (e.g., alginate) that has an
opening for the substrate molecules to come in contact with the catalytic sites. The
enzymes are added to the monomer before polymerization.
4. Crosslinking
Crosslinking is achieved by making chemical connections between the enzyme
molecules to form large aggregates that are insoluble. This is a very stable system.
There are several disadvantages in enzyme immobilization. Immobilization can
reduce the activity of an enzyme. Substrate molecules may not be freely accessible
to the immobilized enzymes. The method may not be applicable if the substrate
molecules are large. a-Amylase may not be a good candidate for immobilization
because starch molecules, its substrate, are fairly large. However, glucose isomerase
can be immobilized, as its substrate is small glucose molecules. The supporting
materials can be contaminated with microorganisms that are difficult to remove and
can be a source of contamination in food. The materials to be used as support should
not be made of substances that are unsafe and should be approved by regulatory
agencies. Some of the immobilized enzymes currently used are glucose isomerase,
b-galactosidase, and aminoacylase.
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