Biomedical Engineering Reference
In-Depth Information
the pretreatment. Despite intensive R & D worldwide for decades, two barriers still
remains to be overcome for developing viable processes to make bioethanol
economically competitive.
Unlike amylases and glucoamylases that are available at low prices for com-
mercial production of various bulk products including ethanol from starch-based
feedstocks, cellulases to liberate glucose from cellulose for bioethanol production
are more expensive due to the difficulty of their fermentation production
as well as the heterogeneous characteristic of the enzymatic hydrolysis which
significantly compromises the reaction rate and increases the enzyme dosage [
47
].
generation Bioethanol Production
'
' for details. On the other hand, the ethanolo-
genic species, either S. cerevisiae which has been used for ethanol production from
sugar- and starch-based feedstocks since the establishment of the industry, or
Z. mobilis which has been intensively studied over the years due to its unique
Entner-Doudoroff (ED) pathway for ethanol production with less biomass accu-
mulation [
48
], cannot ferment pentose sugars in the hydrolysates into ethanol at
rates and yields that are acceptable from the viewpoint of industrial production.
Although the pentose sugars can be converted into other products like furfural
through intramolecular dehydration of xylose by chemical catalysis [
49
], and
xylitol, lactic acid and 2,3-butanediol by fermentations [
50
], all these processes
seem not to be economically competitive at present, and most effort is still focused
on the co-fermentation of the pentose and hexose sugars for bioethanol production
by engineered strains.
4.1 Strategies for Hydrolysis and Fermentation
Based on the considerations of cellulase production and the process configu-
rations of cellulose hydrolysis and ethanol fermentation, separate hydrolysis and
co-fermentation, simultaneous saccharification and co-fermentation and consol-
idated bioprocessing have been developed, and are illustrated schematically in
Fig.
8
.
4.1.1 Separate Hydrolysis and Co-Fermentation
For the separate hydrolysis and co-fermentation (SHCF) process, cellulose is
completely hydrolyzed to glucose by cellulases under optimum conditions, par-
ticularly temperatures around 50C that facilitate the enzymatic hydrolysis, and
correspondingly reduce the enzyme dosage, but cannot be tolerated by microor-
ganisms performing ethanol fermentation at temperatures around 35C. After
complete hydrolysis of cellulose, lignin is left, which can be recovered by a filter
and processed as value-added by-products. In the meantime, the viscosity of the
hydrolysate is very low, which is suitable for high gravity (HG) fermentation to
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