Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 40

industry due to the energy consumption and sugar loss associated with

the operation.

4.1.2 Simultaneous Saccharification and Co-Fermentation

For ethanol fermentation from starch-based feedstocks, the mash is liquefied at
elevated temperatures of 90-110C by thermo-tolerant amylase, the endo-
enzyme hydrolyzing starch randomly into dextrins, and further hydrolyzed by
glucoamylase, the exoenzyme hydrolyzing the dextrins from the non-reducing
end to release glucose at 60-62C for 20-30 min to achieve the dextrose
equivalent of 15-20 only, which is then cooled down to 30-32C and pumped
into fermentors to initiate ethanol fermentation. Since most dextrins are
hydrolyzed into sugars during the fermentation, the process is termed simul-
taneous saccharification and fermentation (SSF), and has been widely practiced
in the industry. When a similar strategy is applied to ethanol production
from ligonocellulosic biomass, the term simultaneous saccharification and
co-fermentation (SSCF) is used, taking into account the unique characteristics
of the hydrolysate that includes both C5 and C6 sugars. However, the
saccharification of the dextrins/pretreated cellulose and the fermentation/
co-fermentation of glucose/C5 and C6 sugars are by no means simultaneous,
but sequential in nature.
The SSCF process is simple in design and easy to operate. Most importantly,
higher ethanol yields can be achieved due to the alleviation of product inhibition
in cellulases, which results in more complete hydrolysis of the cellulose com-
ponent [ 53 ]. However, temperatures for the enzymatic hydrolysis and ethanol
fermentation are significantly different, making the simultaneous optimization of
the two unit operations impossible, and the SSCF process must be operated at
lower temperatures to accommodate microbial growth and ethanol fermentation,
normally at 30-35C. Thus, the rate of the enzymatic hydrolysis is inevitably
compromised, and a much longer time is needed to complete the hydrolysis.
Moreover, lignin cannot be separated from cellulose prior to fermentation, which
makes the fermentation broth extremely viscous, and the mixing and heat and
mass transfer performance is correspondingly affected. Therefore, the SSCF
process cannot operate under HG conditions, and energy consumption is high for
the distillation of the fermentation broth with low ethanol concentrations as well
as for the treatment of distillage since the amount of the discharge is much
larger. For example, a time as long as 96 h was reported for the fed-batch SSCF
system to convert pretreated wheat straw with 11% water insoluble solids and
produce only 3.3% (w/v) ethanol [ 54 ].
A hybrid process like the SSF process practiced in ethanol fermentation from
starch-based feedstocks can be developed, in which a pre-hydrolysis under opti-
mum temperature conditions is applied to the enzymatic hydrolysis of cellulose,
followed by the SSCF process to shorten the time required by the hydrolysis and