factor regeneration. The cofactor molecules, e.g., NADH are prevented from pene-
trating the membrane by enlarging their molecular weight via attachment to poly-
ethylene glycol (Kula and Wandrey, 1987; Wandrey, 1987). Immobilization methods
for lipases and reactor configurations used for the modification of fats and oils are
reviewed by Malcata et al. (1990).
13.3.2 Carrier materials
Among theinorganic carrier materials, SiO 2 should be mentioned especially, as this
can be manufactured as spherical beads of uniform diameter and pore size (con-
trolled pore glass) (Figure 3). The surface of these particles can be activated by
reaction with organosilicon compounds such as 3-aminopropyltriethoxysilane, fol-
lowed by the immobilization via glutaraldehyde or other bifunctional reagents (Ja-
nasek and Spohn, 1998). Functionalized porous SiO 2 has often been used for the
covalent fixation of phospholipases (see Sections 2.4.1 and 2.4.2). Further inorganic
materials applied for immobilization include metal oxides and hydroxides of Al, Zr,
Ti, etc. and, as shown recently, mesoporous phases (Gimon-Kinsel, 1998).
Organic carriersmust to subdivided into natural and synthetic polymers. Materi-
als based on hydrophilic polysaccharides such as cellulose, crosslinked dextrans or
agarose belong to the first group, and are modified for example with BrCN or 2-
(diethylamino)-ethanol (DEAE-cellulose). Other naturally occurring polysacchar-
ides mainly applied to the immobilization of micro-organisms include Na-algi-
nate, which is transferred from a sol to a mechanically stable gel through the ex-
change of Na+ions by Ca2+ions (Klein et al., 1983),j-carrageenan (ter Meer,
1984), or chitosan (Ziehr et al., 1987), the primary amino groups of which can
also be used for coupling reactions. Shinonaga et al. (1996) successfully applied
crosslinked chitosan beads (Fuji Spinning Co.) as a porous support to immobilize
13.3 Immobilization of biocatalysts 269
Table 2.A selection of commercially available carrier materials and their properties, used for enzyme
immobilization.
Carrier Active group/
Concentration
(lmol g–1)
Surface size
(m^2 g–1)Pore diameter
(nm)Particle size (lm)Eupergit C epoxy/> 600 (–) 2001 150
Eupergit C250L epoxy/> 200 (–) 10001 250
VA-Biosynth epoxy/300 140 30 50–2000
Deloxan DAP III -NH-R-NH 2 /(–) 400 25 ...100^2400
Trisoperl -NH 2 /80–330 30–120 31–126 100–200
Fractogel (EMD) epoxy/1500 (–) 80 40–90
azlacton/100 (–) 80 40–90(^1) Exclusion range (kDa).
2 Maximum of pore size distribution.