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Bioethanol: Market and Production Processes 97

Cellulase is inhibited by ethanol. For instance, at 30 g/L ethanol, the
enzyme activity was reduced by 25% [2]. Ethanol inhibition may be
a limiting factor in production of high ethanol concentration. However,
there has been less attention to ethanol inhibition of cellulase, since
practically it is not possible to work with very high substrate con-
centration in SSF, because of the problem with mechanical mixing.
Another problem arises from the fact that most microorganisms used
for converting cellulosic feedstock cannot utilize xylose, a hemicellu-
lose hydrolysis product [8].

3.12.8 Comparison between enzymatic and
acid hydrolysis for lignocellulosic materials
The two most promising processes for industrial production of ethanol
from cellulosic materials are two-stage dilute-acid hydrolysis (a chemi-
cal process) and SSF (an enzymatic process). Advantages and disad-
vantages of dilute-acid and enzymatic hydrolyses are summarized in
Table 3.3. Enzymatic hydrolysis is carried out under mild conditions,
whereas high temperature and low pH result in corrosive conditions for
acid hydrolysis. While it is possible to obtain a cellulose hydrolysis of
close to 100% by enzymatic hydrolysis after a pretreatment, it is diffi-
cult to achieve such a high yield with acid hydrolysis. The yield of con-
version of cellulose to sugar with dilute-acid hydrolysis is usually less
than 60%. Furthermore, the previously mentioned inhibitory compounds
are formed during acid hydrolysis, whereas this problem is not so severe
for enzymatic hydrolysis. Acid hydrolysis conditions may destroy nutri-
ents sensitive to acid and high temperature such as vitamins, which may
introduce the process together with the lignocellulosic materials.

TABLE 3.3 Advantages and Disadvantages of Dilute-Acid and Enzymatic
Hydrolyses
Dilute-acid Enzymatic
Parameters hydrolysis hydrolysis
Rate of hydrolysis Very high Low
Overall yield of sugars Low High and depend upon
pretreatment
Catalyst costs Low High
Conditions Harsh reaction conditions Mild conditions (e.g.,
(e.g., high pressure and 50 C, atmospheric
temperature) pressure, pH 4.8)
Inhibitors formation Highly inhibitory Noninhibitory
hydrolyzate hydrolyzate
Degradation of sensitive High Low
nutrients such as vitamins