Environmental Engineering Reference
In-Depth Information
usually refers to ethanol made from the breakdown and fermentation of whole-plant biomass. Most
of the dry biomass of a plant consists of the cell walls, which are composed mainly of cellulose,
hemicellulose, lignin, and a few other minor components, collectively known as lignocellulosic
biomass. Cellulose is composed of long chains of glucose molecules bound by β-glycosidic bonds,
which comprise between 30% and 40% of the dry mass of the cell wall (Vermerris 2008). Other
structural carbohydrates are collectively referred to as hemicelluloses, which are highly varied
among the plant kingdom. In grasses most of the hemicellulose consists of glucuronarabinoxylans,
complex polymers of primarily xylose molecules, which also contain glucuronic acid and arabinose
residues (Vermerris 2008). Lignin is a complex polymer of phenolic molecules derived from the
phenylpropanoid pathway, which has an important function in providing strength and flexibility to
plant tissues and plays a role in plant reactions to diseases and insect attacks.
The glucose subunits in the cellulose are of primary interest in the production of ethanol from
biomass. In cellulosic ethanol production, enzymes such as cellulase break down the cellulose
chains into glucose molecules that can then be fermented into ethanol by yeast. However, cellulose
is difficult to extract from the cell wall because it is embedded in a cross-linked matrix with the
other components, hemicellulose, lignin, pectins, and proteins. It is thought that lignin physically
limits the access of cellulase enzymes to interact with the cellulose chains and can thus decrease
the overall conversion efficiency. Using a transgenic approach, Chen and Dixon (2007) down-
regulated six different genes in the lignin biosynthesis pathway in alfalfa (
Medicago sativa
L.).
They observed that enzymatic saccharification efficiency of acid-pretreated stem biomass was
directly related to lignin content. Composition of the lignin subunits also appears to influence
conversion efficiency, but in general lignin is very resistant to chemical breakdown and is thus
difficult to eliminate in the conversion process. One of the goals in developing biofuel crops is to
select species or cultivars with reduced lignin content, or to modify the structural components of
the lignin, without rendering plants susceptible to lodging, diseases, or insect pests.
1.3.1 c
rop
r
ESiduES
aS
a
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ourcE
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iofuElS
After crops such as wheat and maize are harvested, a significant amount of crop residue is usually
left in the field. With the advent of cellulosic ethanol technology, it has become possible to harvest
and utilize this material to make fuel, although how much residue can be removed without negative
environmental and agronomic effects is still being debated. In addition to grain yield and quality,
it has now become increasingly important for maize breeders to consider the properties of leaves
and stems. Fortunately, the maize plant is also used as forage and silage, so some knowledge of its
fiber properties already exists. Many of the characteristics important in forage digestibility are also
important in conversion of lignocellulosic biomass into fuel.
Four low-lignin mutants of maize have been identified,
bm1
(Halpin et al. 1998; Vermerris et al.
2002),
bm2
(Vermerris and Boon 2001),
bm3
(Miller et al. 1983; Vignols et al. 1995), and
bm4
, all
of which are associated with a visual phenotypic marker—brown pigmentation in the stalks and
midribs. These mutations—blocks in the lignin biosynthesis pathway—are recessive, and the brown
coloration is caused by the accumulation of phenylpropanoid lignin precursors. In maize, these low-
lignin mutants generally have very good forage digestibility, but the severe decrease in lignin tends
to be associated with lower grain yields, lodging, and disease susceptibility (reviewed by Pedersen
et al. 2005). Although some brown-midrib (
bm3
) maize hybrids are available for forage use (de
Leon and Coors 2008), it is not likely that a dual-purpose grain and biomass type will be developed
using this mutation. The other three
bm
loci have not been studied as extensively and could poten-
tially be useful in biomass improvement of maize stover. In addition to lignin content, the structure
and composition of the lignin is also known to affect cell wall digestibility (reviewed by Barrière et
al. 2009). Thus, altering lignin structure, rather than severely reducing the lignin content, appears
to be the best strategy for improving grain maize stover as a biomass source.
