Biomedical Engineering Reference
In-Depth Information
cellulosic raw material can spread to the interior of the raw material to decompose
cellulose depends not only on the raw material structure itself but also on the decom-
position ability of the different strains. Some basidiomycetes, including brown
pathogens and white pathogens, are the origin to initiate the decomposition of lig-
nocellulosic biomass. Brown pathogens mainly degrade cellulose and hemicellulose
without impact on lignin. They contain a strong hydrolytic enzyme to decompose
cellulose, demethylate, and destruct the aromatic ring. White pathogens first affect
the lignin, almost without the decomposition of cellulosic polysaccharides. They
destroy the lignin to form a white substance, removing the methyl group through the
oxidation process, and forming diphenol, which is then decomposed by oxygenases.
Soft-rot fungi, such as
Cheatomium globosum,
decompose both lignin and cellulose
.
In addition, ascomycetes, such as
Penicillium
and
Aspergillus
, and
Adelomycete
,
such as
Fusarium altornaria
, have the same degradation ability. Other microbes
also participate in the degradation of lignocellulose, such as
Streptomyces
spp.
and
Thermomonospora
spp. of actinomycetes
, Achromobacter
spp.,
Agrobacterium
spp.,
Acinetobacter
spp.,
Nocardia
spp.,
Pseudomonas
spp.,
Xanthomonas
spp., and
Micrococcus
spp., which belong to bacteria. Their abilities to decompose the lignin
are weak, but the colony effect is huge [
88
,
100
].
3.7.1
Nutritional Properties of Lignocelluloses
Carbon-containing substances in raw cellulose materials are structurally stable and
hardly decomposable materials (i.e., cellulose, hemicellulose, and lignin). Easily
degrading organic materials such as starch, soluble sugars, lipids, pectins, amino
acids, and so on are low in content. A nitrogen source is one of the important
nutrients for microorganisms. Usually, C/N in microbial cells is 8:1-12:1. In the
growth of the microorganism, 50 % of the carbon supplies the breathing energy,
and 50 % of the carbon composes the microbial cells; thus, an ideal C/N in growth
medium is 16:1-24:1, making sure that there is enough carbon for the biological
use of nitrogen [
101
]. Overall, the nitrogen content of natural cellulose material is
low. The C/N of natural lignocellulose means the ratio of an available carbon source
to the nitrogen source. In natural lignocellulose, because of the compact structure
constructed by cellulose, hemicellulose, and lignin, most of the carbon source is
difficult to utilize. If only an easily degradable carbon source and nitrogen source
are considered, the C/N of natural lignocelluloses is estimated as significantly low.
In the process of degradation of natural lignocellulosic materials, the C/N
keeps changing. During the microbial degradation process of natural lignocellulosic
feedstock, many carbon sources are released in the form of CO
2
into the atmosphere
through microbial respiration. The nitrogen source experiences constant enrichment,
and inorganic nitrogen is converted into organic nitrogen, so that a limited source of
nitrogen in the debris of plants is utilized. Thus, the C/N of natural lignocellulose
decreases gradually, dropping to about 25:1 eventually.
