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Bioethanol: Market and Production Processes 81

Physicochemical pretreatment—steam explosion or autohydrolysis,
ammonia fiber explosion (AFEX), SO explosion, and CO explosion
2
2
Chemical pretreatment—ozonolysis, dilute-acid hydrolysis, alkaline
hydrolysis, organosolvent process, and oxidative delignification
Biological pretreatment

However, not all of these methods may be technically or economically
feasible for large-scale processes. In some cases, a method is used to
increase the efficiency of another method. For instance, milling could be
applied to achieve better steam explosion by reducing the chip size.
Furthermore, it should be noticed that the selection of pretreatment
method should be compatible with the selection of hydrolysis. For exam-
ple, if acid hydrolysis is to be applied, a pretreatment with alkali may
not be beneficial [18]. Pretreatment methods have been reviewed by
Wyman [2] and Sun and Cheng [12].
Among the different types of pretreatment methods, dilute-acid, SO ,
2
and steam explosion methods have been successfully developed for pre-
treatment of lignocellulosic materials. The methods show promising
results for industrial application. Dilute-sulfuric acid hydrolysis is a
favorable method for either pretreatment before enzymatic hydrolysis
or conversion of lignocellulose to sugars.

3.6.3 Enzymatic hydrolysis
of lignocellulosic materials
Enzymatic hydrolysis of cellulose and hemicellulose can be carried out
by highly specific cellulase and hemicellulase enzymes (glycosyl hydro-
lases). This group includes at least 15 protein families and some sub-
families [15, 27]. Enzymatic degradation of cellulose to glucose is
generally accomplished by synergistic action of three distinct classes of
enzymes [2]:

1,4- -D-glucan-4-glucanohydrolases or Endo-1,4- -glucanases, which
are commonly measured by detecting the reducing groups released
from carboxymethylcellulose (CMC).
Exo-1,4- -D-glucanases, including both 1,4- -D-glucan hydrolases and
1,4- -D-glucan cellobiohydrolases. 1,4- -D-glucan hydrolases liberate
D-glucose and 1,4- -D-glucan cellobiohydrolases liberate D-cellobiose.
-D-glucoside glucohydrolases or -D-glucosidases, which release D-
glucose from cellobiose and soluble cellodextrins, as well as an array
of glycosides.
There is a synergy between exo–exo, exo–endo, and endo–endo enzymes,
which has been demonstrated several times.