Biomedical Engineering Reference
In-Depth Information
exists in leaves. It is related to physiological processes such as fruit ripening and
softening and leaf abscission. Pectinase is also the earliest enzyme secreted when a
fungus invades the plant cell wall, which plays a key role in the utilization of natural
fibrous materials by microorganisms [
59
].
2.8
Pigment
Pigment is also an important substance of lignocellulosic materials. Wood rays exist
in the cambium, which is the meristematic tissue of wood existing between timber
and bark. The function of cambium is to transport horizontally and store tissues,
which makes the wood have various color changes because of the susceptibility
to colored bacteria. Up to 1 year of age, tissues near the center of the xylem
would be gradually replaced by branches, tannins, pigments, and other substances
and then degrade to dead cells with the lost of physiological activity. Many types
of heartwood have special pigments; for example, ebony heartwood is black,
mahogany is red, and pine is brown [
2
].
Plant pigment is generally generated from lignin and some lignin degradation
products with benzene ring. Lignin contains the chromophore and auxochrome, but
the current understanding of the lignin chromophore is not uniform. It is generally
believed that the lignin chromophore mainly contains quinone structures and
conjugated systems of carbonyls and double bonds. The results of studies showed
that the content of the lignin quinoid structure is less than the content of catechol.
However, as the optical absorption coefficient of the quinone is higher than that of
'
kg
1
, respectively), the quinoid structure has great
optical absorption; the optical absorption in the visible region accounts for 35-60 %
of the total absorption. Therefore, Kringstad et al. reported that the quinoid structure
was the most important chromophore in lignin. The formation and degradation of
lignin are involved with formations of paraquinones and
O
-quinone. The
O
-quinone
is red, and the paraquinones are yellow; they are the major chromogenic structures
of lignin. Paraquinone structures are mostly dissolved in the unit form instead of
condensed units form except for a few condensed units, but
O
-quinone structures
mainly remain in the lignin macromolecules without dissolution [
60
,
61
].
Plants also contain small amounts of flavonoids and anthocyanin. In the transfor-
mation of plant materials, these pigment-like substances will be extracted, affecting
the further conversion of lignocellulosic materials. In the pulp and papermaking
process, the blue stain of pine would exacerbate the wood color and having adverse
impacts on wood products and the papermaking process. The presence of pigments
would cause a certain degree of inhibition of the properties of enzymolysis and
fermentation. Therefore, pretreatments of lignocellulosic feedstock usually need to
remove pigments in the hydrolysate; these treatments include macroporous resin
adsorption, ion exchange, electrodialysis, and precipitation.
-carbonyl (680,000 and 800 m
2
