Carrier-type immobilization

Enzyme immobilization techniques can be roughly divided in three main categories related to the presence, or absence of carrier, and a hybrid third one (Figure 10.1). In the carrier-type immobilization, the enzyme can be linked to the surface carrier, or entrapped... 

The wide spectrum of external conditions which can influence the conformational state of charged gels (the variation of these conditions can induce collapse or decollapse transition), makes these gels possible materials for data control devices of different types, absorbers, reactors and catalysts with regulated diffusion characteristics, carriers of immobilized enzymes, etc. The networks synthesized at high dilution are also new mechano-chemical systems which show very high sensitivity to external actions. 

Carrier type conditions Compression in immobilized enzyme... 

Cell immobilization has been used in all types of alcoholic beverages production and various other biotechnological processes, and therefore, many such techniques have been developed, which can be grouped into the following four major categoriesi (1) immobilization on a solid carrier surface, (2) immobilization by entrapment in a porous matrix, (3) carrier-free immobilization, and (4) containment behind barriers. 

Figure 1 (A) Carrier-bound immobilized enzymes of defined size and shape. Insoluble carriers vary in iheir geometric parameters. Different shapes and types of enzyme carrier are illustrated (a) bead, (b) fiber, (c) capsule, (d) film, and (e) membrane. (B) Methods used for immobilizing enzymes onto a spherical solid support matrix 1, physical absorption 2, covalent binding 3, electrostatic binding 4, intermolecular cross-linking 5, gel entrapment 6, chelation and/or metal binding. E, enzyme M, metal.

Studied for many years, and various carriers have been used. However, only a few types of carrier and immobilization processes have been commercialized. 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Two types of immobilization are used for immobilizing glucose isomerase. The intracellular enzyme is either immobilized within the bacterial cells to produce a whole-ceU product, or the enzyme is released from the cells, recovered, and immobilized onto an inert carrier. An example of the whole-ceU process is one in which cells are dismpted by homogenization, cross-linked with glutaraldehyde, flocculated using a cationic flocculent, and extmded (42). 

In a second example, a cell—gelatin mixture is cross-linked with glutaraldehyde (43). When soluble enzyme is used for binding, the enzyme is first released from the cell, then recovered and concentrated. Examples of this type of immobilization include binding enzyme to a DEAE-ceUulose—titanium dioxide—polystyrene carrier (44) or absorbing enzyme onto alumina followed by cross-linking with glutaraldehyde (45,46). 

In supported liquid membranes, a chiral liquid is immobilized in the pores of a membrane by capillary and interfacial tension forces. The immobilized film can keep apart two miscible liquids that do not wet the porous membrane. Vaidya et al. [10] reported the effects of membrane type (structure and wettability) on the stability of solvents in the pores of the membrane. Examples of chiral separation by a supported liquid membrane are extraction of chiral ammonium cations by a supported (micro-porous polypropylene film) membrane [11] and the enantiomeric separation of propranolol (2) and bupranolol (3) by a nitrate membrane with a A/ -hexadecyl-L-hydroxy proline carrier [12]. 

Like enzymes, whole cells are sometime immobilized by attachment to a surface or by entrapment within a carrier material. One motivation for this is similar to the motivation for using biomass recycle in a continuous process. The cells are grown under optimal conditions for cell growth but are used at conditions optimized for transformation of substrate. A great variety of reactor types have been proposed including packed beds, fluidized and spouted beds, and air-lift reactors. A semicommercial process for beer used an air-lift reactor to achieve reaction times of 1 day compared with 5-7 days for the normal batch process. Unfortunately, the beer suffered from a mismatched flavour profile that was attributed to mass transfer limitations. 

---