Page 336 - Advances in Textile Biotechnology

Hydrolysis of regenerated cellulose fi bres for textiles 317

chains. As the activity of the enzymes is directly related to the accessibility
of a certain part of the polymer chain, the action of EGs is preferably
observed in the non-crystalline regions. Exocellulases or CBH attack the
polymer chain ends, producing cellobiose as the primary product. Depend-
ing on the cellulase formulation, cellobiose and small chain oligomers are
released into solution and are hydrolysed by β-glucosidases to glucose,
which is released into the solution as a major product of the cellulase
activity.
To identify the site of action of the different enzymes in a total crude
cellulose, the relative sizes of enzymes, glucose units, fi brils in the fi bre and
pores have to be considered (Bredereck and Gruber, 1995; Lee et al., 2000).

In Table 13.2, average dimensions of elements of fibre architecture and of
proteins have been summarised.
As shown Table 13.2, the enzymes are too large to enter into the micro-
pore system of the fibres and, thus, primary action of cellulases occurs at

the surface of the fibres. Mechanical agitation can support the action of the
enzymes by formation of macropores e.g. owing to fibrillation of the fi bre.

Progressive degradation removes the more reactive amorphous parts of the

fibres resulting in formation of indentations, which enable enzymes to

access to inner parts of the fibre. Figure 13.2 gives a schematic presentation
of the action of CBH, EG and β-glucosidase on an insoluble cellulose
substrate.
Both EG and CBH reactions with the insoluble cellulose substrate are
heterogeneous chemical reactions whereas the β-glucosidase reaction with
the dissolved short-chain oligomers and cellobiose is a homogenous process
in the liquid phase. In total crude cellulase mixtures, this homogeneous
reaction can be supposed to be more rapid and less rate determining than
the heterogeneous reactions of EG and CBH.
In addition to the specific activity of the enzymes applied, the heteroge-

neous reactions of EG and CBH will directly be influenced by the ‘surface’
properties of the insoluble cellulose substrate treated. Thus, besides the

Table 13.2 Representative average dimensions of fibre elements and cellulases
(Bredereck and Gruber, 1995; Lee et al., 2000; Öztürk et al., 2009)
Structural element Dimension (nm)

Length of unit cell in crystalline cellulose I 1.0
Elementary fibril (36 cellulose chains) 3.5

Average pore diameter of viscose, modal fi bre (ISEC) 2.3–2.9
Average pore diameter of cotton fi bre (ISEC) 4.1
Microfi brils 10–30
Macrofi brils 60–360
Fibre 10 000–30 000
Cellulase 2 × 11 to 4 × 25

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