Biomedical Engineering Reference
In-Depth Information
the lignified lignocellulose cannot be degraded by the cellulase produced by white-
rot fungi. Only after the lignin and hemicellulose are removed can the cellulose be
degraded. Other types of microorganisms can degrade all the components of the
cell wall at the same speed, and the cell wall is gradually rarefied. Cavitary-type
decomposition generally occurs in the lignocelluloses, which have a high content
of lignin and are difficult to invade. Erosion-type decomposition generally occurs in
materials with less lignin that are easily penetrated. For example, white-rot fungus
generally exists in broad-leaved wood.
Some microorganisms, such as
Panus conchatus,
invade the cell cavity through
the stomata on the surface of grass straw, and then they grow in the cavity and
parenchyma cells. Parenchyma cells are first disintegrated.
Panus conchatus
first
degrades regions of the fiber secondary cell wall and intercellular layer, keeps the
fibers intact and makes the fiber bundles separated with each other.
Because of the poor venting quality of cell walls, many fungi grow rapidly in the
surface of raw cellulose materials, and they could secret enzymes extracellularly.
At the same time, part of the mycelia grows within the cell cavities and cell
walls. Generally, in solid-state fermentation, the mycelia are hard to overgrow
in the materials. If the ventilation is strengthened using periodic pressure, it will
make the mycelia overgrow internal materials, and the fermentation period will be
significantly shortened.
In summary, the action mode of microorganisms on the raw lignocellulosic
materials is closely related not only to the types of raw cellulose materials but also
to the types of microorganisms and culture conditions.
3.7.3
Mechanism of Microbial Degradation of Lignocellulose
3.7.3.1
Degradation Mechanism of Filamentous Fungi
The strains of
Trichoderma
spp.,
Aspergillus
spp.,
Penicillium
spp.,
Rhizopus
spp.,
Cladosporium
spp., and in particular
Trichoderma viride
and its closely
relative strains, are able to produce the enzymes capable of decomposing cellulose,
hemicellulose, and pectin. Therefore, small filamentous fungi are mainly used
for the production of cellulolytic enzymes. Most studies showed that the purified
cellulase can digest pure cellulose or amorphous cellulose but has low digestibility
of the crystalline form or a combination of cellulose and lignin. There are fewer
enzymes to decompose the ester bond between cellulose, hemicellulose, and lignin
and the wax substances on the straw surface and lignin [
102
]. The mechanism of
degradation by filamentous fungus is commonly regarded as synergism. Fungal
cellulase has two forms of synergistic effect. One is synergy between endoglucanase
and exoglucanase. Endo-1,4-
-D-glucanase first inner cuts amorphous cellulose
to produce a new end (the reducing end or nonreducing end); then, exo-1,4-
“
-D-
glucanase from the reducing end or the nonreducing end cutting cellulose chain
produces cellobiose (or glucose). Another is synergy between exonuclease and
“
