Environmental Engineering Reference
In-Depth Information
plant cell wall that could serve as a potential substrate for the production of value-
added products under optimized conditions [4]. In general, the secondary cell walls
of plants contain cellulose (40-80%), hemicellulose (10-40%), and lignin (5-25%).
The arrangement of these components allows cellulose microfibrils to be embedded
in lignin, much as steel rods are embedded in concrete to form reinforced concrete
[5]. The composition of hemicellulosic fractions from different natural sources is
summarized in Table 1.
The carbohydrate fraction of the plant cell wall can be converted into fermentable
monomeric sugars through acidic and enzymatic (hemicellulase/cellulase) reactions,
which have been exploited to produce ethanol, xylitol, and 2, 3-butanediol via
microbial fermentation processes [1, 4, 12]. In the hemicellulosic fraction of the
plant cell wall, xylan is the major backbone, linking compounds like arabinose,
glucose, mannose, and other sugars through an acetyl chain [4]. They can be char-
acterized as galactomannans, arabinoglucuronoxylans, or glucomannans based on
their linkage with the main xylan backbone [13].
Thermal, chemical, and enzyme-mediated processes and combinations thereof
are being explored in order to obtain monomeric components of hemicellulose with
maximum yield and purity. The depolymerization of hemicellulose by chemical
or enzyme-mediated processes yields xylose as the major fraction and arabinose,
mannose, galactose, and glucose in smaller fractions [12]. This sugar syrup can be
converted into ethanol; xylitol; 2, 3-butanediol (2, 3-BD); and other compounds [4].
The use of hemicellulose sugar as a primary substrate for the production of multiple
compounds of industrial significance is summarized in Fig. 1.
A wide variety of microorganisms are required for the production of metabo-
lites from hemicellulosic-derived sugar syrup. The ability to ferment pentoses is not
widespread among microorganisms and the process is not yet well-established in
Ta b l e 1
Cell wall composition among various lignocellulosic sources considered for biofuel
(% of dry material)
Hemicellulose
∗
Cellulose
Lignocellulosic source Glucan
Xylan Arabinan Mannan Galactan Lignin References
Sugarcane bagasse
40.2
22.5
2.0
0.5
1.4
25.2
[6]
Wheat straw
32.1
19.5
2.8
0.6
1.1
20
[7]
Corn stover
37.5
21.7
2.7
0.6
1.6
18.9
[8]
Switch grass
34.2
22.8
3.1
0.3
1.4
19.1
[7]
Pine wood
44.8
6.0
2.0
11.4
1.4
29.5
[9]
Aspen wood
48.6
17.0
0.5
2.1
2.0
21.4
[9]
Spruce wood
41.9
6.1
1.2
14.3
1.0
27.1
[10]
42.6
26.4
0.5
1.8
0.6
18.9
[9]
Birch wood
41.5
15.0
1.8
3.0
2.1
25.2
[9]
Douglas fir wood
46.1
3.9
1.1
14.0
2.7
27.3
[11]
∗
Tot a l
hemicellulose
amount present in lignocellulosics on the basis of % of dry material-
Sugarcane bagasse, 27.5; Switch grass, 30; Corn stover, 26.8; Wheat straw, 50; Pine, 26; Aspen,
29; Spruce, 26; Birch wood, 23; Salix wood, 21.7; Douglas fir wood, 20.3.
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