Environmental Engineering Reference
In-Depth Information
Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI), has the goal
of advancing the efficacy and knowledge base of pretreatment technologies (3), reviewed in
[16, 17].
Mechanical lignin disruption effectively hydrolyzes a significant fraction of the
hemicellulose, but is less effective in hydrolyzing cellulose. This difference is caused by the
different structures of the two polymers: hemicellulose is a highly branched, typically
amorphous polymer that is therefore relatively easy to hydrolyze into its component sugars
(pentoses D-xylose and L-arabinose; hexoses D-galactose, D-glucose, D-mannose; and uronic
acid; all highly substituted with acetic acid). Hemicelluloses from hardwoods are typically
high in xylose, while those in softwoods contain more hexoses [3].
Cellulose, in contrast to hemicellulose, is a semicrystalline polymer of pure glucose
linked by beta-glucoside bonds. The beta linkages form linear strands that establish extensive
H-bonds between them, leading to a highly stable structure that is quite resistant to
degradation. Many chemical approaches have been explored to hydrolyze cellulose, though
none are completely satisfactory; currently, dilute acid hydrolysis procedures are being
proposed for several near-term commercialization efforts until more effective technologies
are available [5].
2.1.3. Enzymatic cellulose hydrolysis
. Enzymatic hydrolysis by cellulases is the ultimate
goal in biomass processing for fermentation: this method has the advantages of reduced sugar
loss through side reactions and it is less corrosive of process equipment [18]. In addition, the
hydrolyzed product requires no neutralization prior to fermentation [19].
Cellulases consist of multicomponent enzyme complexes acting synergistically: complete
cellulose hydrolysis requires the activity of an endoglucanase, which cleaves interior regions
of cellulose polymers; an exoglucanase, which cleaves cellobiose units from the ends of
cellulose polymers; and a beta-glucosidase, which cleaves cellobiose into its glucose subunits
[20]. Because of the complexity and insolubility of the substrate, cellulase catalysis is not
only relatively slow, but it is also understood much less completely than other enzymes,
despite over four decades of cellulase research [8]. One of the most important organisms in
the development of cellulase enzymes is Trichoderma reesei, the “ancestor of many of the
most potent enzyme- producing fungi in commercial use today”[3]. By 1979, genetic
enhancement had produced mutants with up to 20 times greater cellulose productivity than
the original organisms found in World War II; today, surprisingly, the most lucrative cellulase
market is in the manufacture of stone-washed jeans [3].
2.1.4. Fermentation
. Fermentation of glucose to ethanol is performed by numerous
bacteria, yeasts, and other fungi, and several yeasts have also been identified that can convert
xylose to ethanol. Pentose fermentation to ethanol does not commonly co-occur with hexose
fermentation to ethanol, however, spurring efforts to combine these two fermentation
pathways into single organisms. Genetic engineering has since provided both bacteria and
yeasts capable of fermenting both 5-carbon and 6-carbon sugars [21, 22].
Although hydrolysis of biomass cellulose by cellulases was once performed as a distinct
step between pretreatment and fermentation, fermentation can begin as soon as glucose
subunits are released from cellulose. The realization of this led to the development of the
Simultaneous Saccharification and Fermentation (or Co-Fermentation) Process (SSF or
SSCF), which now provides the great advantage of simultaneous cellulose hydrolysis and
glucose fermentation [13]. This process enhances cellulase activity by relieving the product
inhibition of beta-glucosidase by glucose, since the products are consumed as soon as they are
