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Bioethanol: Market and Production Processes 95

3.12.5 Strategies for fermentation of
enzymatic lignocellulosic hydrolyzates
The cellulose fraction of lignocelluloses can be converted to ethanol by
either simultaneous saccharification and fermentation (SSF) or separate
enzymatic hydrolysis and fermentation (SHF) processes. A schematic of
these processes is shown in Fig. 3.9. It is also possible to combine the
cellulase production, enzymatic hydrolysis, and fermentation in one
step, called direct microbial conversion (DMC). There are cost savings
because of the reduced number of required vessels. However, there is less
attention to DMC for industrial purposes because of the low ethanol
yield in DMC, formation of several by-products, and low ethanol toler-
ance of the microorganisms used [2].

3.12.6 Separate enzymatic hydrolysis
and fermentation (SHF)
In SHF, enzymatic hydrolysis for conversion of pretreated cellulose to
glucose is the first step. Produced glucose is then converted to ethanol
in the second step. Enzymatic hydrolysis can be performed in the opti-
mum conditions of the cellulase. The optimum temperature for hydrol-
ysis by cellulase is usually between 45 C and 50 C, depending on the
microorganism that produces the cellulase. The major disadvantage of
SHF is that the released sugars severely inhibit cellulase activity. The
activity of cellulose is reduced by 60% at a cellobiose concentration as

Cellulolytic enzymes Microorganisms

Pretreated Enzymatic Liquid
lignocellulosic Enzymatic Fermentation Distillation
materials hydrolysis hydrolyzate
Solid residue Ethanol
A: Separate enzymatic hydrolysis

Cellulolytic
enzymes Microorganisms

Pretreated Simultaneous Liquid
lignocellulosic saccharification and Distillation
materials fermentation

Ethanol
Solid residue
B: Simultaneous saccharification and fermentation
Figure 3.9 Main steps in SSF or SHF for ethanol production.