Biomedical Engineering Reference
In-Depth Information
Microorganisms need to obtain carbon and energy to grow from insoluble natural
lignocelluloses. Thus, the accessibility of substrate is an important parameter for
microbial degradation of natural lignocelluloses. For natural lignocelluloses, the
accessibility of the matrix depends on their particle surface area and porosity.
The second limiting factor that affects microbial nutrients obtained from natural
lignocellulose is that insoluble substances must be transformed first by microbes.
The conversion process includes depolymerization, hydrolysis, and other chemical
reactions. The crystallinity of the lignocellulose and the mass transfer characteristics
of the surface of natural lignocellulose affect the transformation process.
3.7.2
Invasion of Microbes into Lignocellulose
The invasion means of microorganisms into natural lignocelluloses are generally
classified into the following two kinds: The first is to invade into lignocellulose
through the intrinsic orifices. For example, in wood, the radial direction is arranged
by ray parenchyma cells, in which the fiber direction is arranged by the tracheid
(softwood), catheter (hardwood), or pits of plant cell walls. The invasive capacity of
most filamentous fungi to the plant cell wall is lower than for the basidiomycete.
Generally, they invade into the cell through pits. The second way is the direct
penetration of plant cell walls. For example, basidiomycetes act just like pathogenic
fungi; the mycelia contact the cells and hydrolyze the secondary wall by cellulase
and hemicellulase to make it more susceptible to microbial decomposition.
Most research showed that the microbes decompose layer S2 of the secondary
wall, which is much easier decomposed in the plant cell wall. Layer S2 is often
decomposed by the wood-rotting fungi in precedence. According to the differences
of invasion of the plant cell wall, it can be divided into the cavitary and erosion types.
When the mycelia reach the S2 layer, they grow parallel to the long axis of cells,
that is, in the direction of the fiber axis. They form clusters in the cell wall with the
S3 layer of the secondary wall not decomposed. Some microorganisms grow in the
cell wall and secrete enzymes. They first break down the S2 and S1 layers through
the S3 layer by diffusion, and then the S3 followed. Generally, the middle layer,
which is highly lignified (intercellular layer on both sides of the primary wall), is
not decomposed. Although the mycelial invade the cell wall, they are rarely found
in the cell wall. For example, the mycelia can be found during the decay process of
wood but cannot be found later. There is no necessary correlation between the degree
of wood decay and hyphae. Cavitary-type decomposition is usually the way of
degradation of cell wall by soft-rot fungi and brown-rot fungi. Generally, cellulose
and hemicellulose are degraded, while lignin is just chemically modified, such as by
demethoxylation. Erosion-type decomposition is usually the degradation that first
decomposes the S3 layer of the cell wall from the cell cavity, followed by the S1
and S2 layers and finally the middle layer. At the beginning of the white-rot fungus
decomposition process, lignin and hemicellulose in the cell wall are first degraded,
with the cell wall thickness completely unchanged. Before the remove of lignin,
