Environmental Engineering Reference
In-Depth Information
O
O
O
Fermentation
products
OH
HO
OH
OH
HO
NH
2
O
O
O
O
O
O
Dehydration
products
OH
HO
O
Saccharides
e.g.
glucose,
cellulose (C6),
hemicellulose (C5)
OH
OH
OH
OH
Reduced
sugars
HO
OH
OH
HO
OH
OH
OH
O
O
O
O
Anhydrosugars
HO
O
HO
OH
Figure 4.3
Example platform molecules derived from saccharides.
chemical treatment to produce platform molecules, the polysaccharides (e.g. starch
and cellulose) may require prior hydrolysis to their monomer sugars (e.g. glucose,
fructose and xylose). Saccharification of polysaccharides (i.e. formation of sugars
from polysaccharides) is particularly important when targeting platform molecules
derived via fermentation, such as succinic and itaconic acid, since the bacterium
or yeast used in these fermentations requires carbohydrates in the form of simple
sugars [34]. On an industrial scale, glucose is currently produced by the enzymatic
hydrolysis of starch from corn, wheat, potato and tapioca [35]. In the short to
medium term, starch hydrolysis will continue to be the major industrial route to
glucose for use in platform molecule production.
Utilisation of cellulose and hemicellulose represents a greater challenge for
platform molecule production, but the successful implementation of a biorefinery
converting cellulose or hemicellulose into platform molecules also offers poten-
tially greater rewards. There is variation in the amounts of cellulose and hemicel-
lulose found within plant matter, and this variation is linked to species diversity,
specific sections of a plant and seasonal changes. Nevertheless, cellulose and
hemicellulose combined typically represent 70 wt% of lignocellulosic biomass.
Cellulose and hemicellulose represent a far greater proportion of global biomass
composition compared to starch; these polysaccharides are therefore likely to
become the major constituent of the biomass feedstock for platform molecules in
the long term. Additionally, neither cellulose nor hemicellulose derived from lig-
nocellulose would be in competition for direct or land use in food, as is the case
for starch and sugars. Utilisation of the polysaccharides in lignocellulose for etha-
nol production ('cellulosic ethanol') has been extensively investigated and shows
promise [36, 37]. Significant developments are still required and diversity in the
nature of the lignocellulose continues to cause challenges when developing suit-
able pretreatment methods [38]. These pretreatments are vital as the recalcitrant
nature of plant biomass, borne out from the structural integrity of the lignocellulose
matrix, means the efficacy of any hydrolysing enzyme is reduced until the structure is
